Download Evolutionary Consequences of Skeletal

AMER. ZOOL., 15:329-350 (1975).
Evolutionary Consequences of Skeletal Differentiation
BRIAN K. HALL
Department ofBiology, Life Sciences Centre, Dalhousie University, Halifax, N.S., Canada
SYNOPSIS. Some aspects of the differentiation, growth, and morphogenesis of the tissues
within the skeleton are discussed and related to the evolution of the vertebrate skeleton. The
tissues considered are bone, cartilage, dentine, and enamel.
The histology of the skeletal tissues of the Ordovician agnatha is reviewed with the
conclusion that the skeletal tissues of the first vertebrates were as diverse and as specialized as
are those of present-day vertebrates. Phylogenies of skeletal tissues cannot be established.
The trend during evolution appears to have been toward reduction in amount of skeletal
tissue and in the number of types of tissues present.
The factors which determine when and where a skeletal element develops ontogenetically
are reviewed and used to discuss the origin and evolution of jaws, the evolution of membrane bones and the origin of such structures as sesamoid bones. Special importance is
attached to epithelial-mesenchymal interactions.
The factors which determine what particular skeletal tissue will form at a particular site
within the body are reviewed with especial reference to modulation, germ layer derivation,
and the role of epigenetic factors.
The factors which determine size and shape of the skeleton, both ontogenetically and
phylogenetically, are reviewed and the directive role of adjacent tissues emphasized.
INTRODUCTION
This paper will discuss the limitations
placed on the skeleton during its evolution
by examining knowledge of the way in
which the tissues of the skeletons of present
day vertebrates and invertebrates develop,
differentiate, and react to environmental
stimuli.
This problem could be examined from
the vantage point of the evolutionary
biologist or palaeontologist who would use
the fossil record as a baseline for understanding the skeletal tissues of present day
species. Alternatively, it could be studied
from the standpoint of the developmental
biologist or "skeletal biologist" who could
project the results of experimental analysis
back in time so as to put the flesh of cause
and effect on the bones of the fossil record.
The express request of the organizers of
the symposium to "get participants (in this
case a developmental biologist-skeletal
The author's original research has been supported
by National Research Council of Canada grant A-5056
and by the Dalhousie University Research and Development Fund. The manuscript was critically read
by Dr. E. T. Garside and by Dr. P. Person.
biologist) to contribute the sort of papers
they do not usually write but can write,"
coupled with my own background, bias,
and inclinations, dictates that I be backward
rather than forward looking.
I am going to be primarily concerned not
with the skeleton as an organ system, nor
with the elements of this organ system as
individual organs, but with the dynamics of
the cells and tissues which make up the
skeleton. In order to be able to make meaningful projections as to the evolutionary
consequences resulting from the limitations (or potentials) of skeletal development and differentiation the following
points must hold true:
1) Recent and fossil skeletal tissues are
equivalent tissues produced by equivalent
cells.
2) The basic plasticity (and perhaps
function) of these tissues has remained the
same during evolution.
3) The methods of forming these tissues
(the developmental processes) have not
substantially altered over geological time.
4) The chemical composition of these
tissues (the lowest evidence of gene expression preserved in the fossil record) has not
329
330
BRIAN K. HALL
markedly changed with time.
The possibility of correlating recent and
If these hold true we may justifiably fossil skeletal tissues has recently been
begin to examine experimental results per- greatly enhanced, because fossil skeletal tistaining to skeletal differentiation and plas- sues can now be decalcified and sectioned,
ticity and apply these to interpretations of both for the light and for the electron microscope (0rvig, 1951, 1957, 1965; Enlow
the evolution of the vertebrate skeleton.
and Brown, 1956; Moss, 19616; Halstead,
19696) and because chemical analyses can
TYPES OF SKELETAL TISSUES
now be carried out on both the inorganic
I should perhaps define the skeletal tis- and the organic components of these tissues which will be discussed, for I shall be sues (Mathews, 1966, 1967; Ho, 1967; Biltz
concerned primarily with the hard and Pellegrino, 1969; Kobayashi, 1971;
(mineralized) tissues of the skeleton. Other Hancox, 1972). The latter studies have intissues of the musculo-skeletal system, such dicated essential similarity in the chemistry
as ligaments, tendons, fibrous and other of the skeletal tissues of fossil and recent
connective tissues, vascular, muscular, and vertebrates.
hemopoietic elements, will take second
place in the discussion. This will be done, SKELETAL TISSUES OF ORDOVICI AN VERTEBRATES
not because these other tissues are unimportant, for they obviously are important,
A brief review of the skeletal tissues of
especially in the context of the evolution of the early vertebrates will serve to illustrate
complex, multi-tissue functional units (see their homology with the tissues of the
Bock and von Wahlert, 1965; Moss, 1968<2; present-day vertebrates, their general
Alexander, 1975), but because they are not complexity and high degree of specializausually preserved in the fossil record.
tion, and their approximate times of apThe mineralized skeletal tissues may be pearance in the fossil record. I will consider
broadly classified into four types (see 0rvig, the members of the Class Agnatha (the jawless vertebrates), the earliest vertebrates for
1967, for a detailed classification).
1) Bone: intramembraneous and en- which fossil skeletal material is available.
dochondral, cellular and acellular, cancel- The major orders within the Agnatha and
lous and lamellar (see Smith, 1947, for an their distribution within the geological time
excellent summary of the classification of scale are summarized in Table 1. All except
the Cyclostomata are extinct. The members
bone).
of
the extinct orders were all small (most 50
2) Cartilage: hyaline, elastic, fibrocartito 300 mm in length, some Osteostraci atlage, calcified, chondroid.
taining 1 m in length) and are known cnly
3) Dentine: cellular and acellular.
from fossilized cephalic shields.
4) Enamel.
The notochord is assumed to have been
5) Tissues intermediate between the
present as a supporting soft skeleton in all
above.
The basic cell type which produces these of these specimens. The presence or abmineralized tissues has been termed the sence of uncalcified cartilage can be argued
Scleroblast by Moss (1964a), implying that only by inference (i) because cartilage, unthe tissues represent a family or race of like the notochord, is not a universal diagtissues with close affinities one to another. nostic vertebrate tissue, and (ii) because if
Such cells may be mesodermal, ectodermal, uncalcified it is unfossilizable. Its inferred
or ectomesenchymal (from the neural presence or absence in the early vertecrest). Thus, the osteoblast (mesodermal, ec- brates, especially whether it antedated the
tomesenchymal) produces bone; the chon- appearance of bone, as it does in ontogenetdroblast (mesodermal, ectomesenchymal— ic endochondral ossification, has led to
ectodermal?—) produces cartilage; the numerous ingenious but speculative
Odontoblast (mesodermal) produces den- theories of vertebrate evolution, adaptive
tine; and the ameloblast (ectodermal) pro- radiation, and the origin of skeletal tissues
(Romer, 1942, 1963, 1964; Smith, 1947;
duces enamel.
331
SKELETAL DIFFERENTIATION AND EVOLUTION
TABLE 1. Classification and geological distribution of the jawless vertebrates (Agnatha).
Subphylum: Vertebrate
Class: Agnatha
Subclass: Cephalaspidomorphi (Monorhina)
Order: Osteostraci (middle ordovician to late devonian)
Anaspida
(middle ordovician to late devonian)
Cyclostomata
(Lower devonian to recent)
suborder: Petromyzontia
: Myxinoidei
Subclass: Pteraspidomorphi (Diplorhina)
Order: Heterostraci (early ordovician to late devonian)
Coelolepida (middle ordovician to late devonian)
Berrill, 1955; Jarvik. 1959; Urist, 1962; (globular, calcospheritic) calcification has
Denison, 1963; Moss, 1961*, 1964a, been observed. Similar cartilage is seen in
1968o,c; 0rvig, 1967, 1968; Halstead, the mandible and palatoquadrate of mod1969a).
ern sharks (Applegate, 1967) and has been
The skeletal tissues which have been pre- observed in homografted hyaline cartilage
served in the fossil record are summarized in man (MacConaill, 1973). Perichondral
in Table 2 along with their time of earliest bone was also present in some of the members of this group in association with a
appearance.
The oldest vertebrates known (Archodus, neurocranium presumed to have consisted
Palaeodus), species of unknown affinity of uncalcified cartilage. The Osteostraci
from the early Ordovician of Russia, pos- (Alaspis, Cephalaspis, Hemicyclaspis) had a
sessed a dermal skeleton which consisted of dermal exoskeleton of cellular bone capped
isolated denticles composed of dentine by dentine and enamel, and a pharyngeal
(Denison, 1963). The Anaspida (Anaspis, and cranial endoskeleton of endochondral
Birkenia, Pterolepis) possessed isolated der- bone (Fig. 1). Spherulitic and globular calmal scales composed of acellular bone, the cified cartilage was also present. The earso-called aspidin(e). The Heterostraci (As- liest jawed vertebrates, the Acanthodii
traspis, Pycnaspis, Eriptychius), from the early (middle Silurian to Permian) possessed a
and middle Ordovician, had a solid dermal similar range of tissues, except that their
armor of acellular, coarse-fibered aspidin, bone was always cellular.
covered by a layer of dentine which in some
Thus, during the early Ordovician two
was in turn covered by a layer of enamel major combinations of skeletal tissues were
(enameloid) (Denison, 1963; Halstead, present within the Agnatha: acellular bone,
1969ft; 0rvig, 1951). In one species, Erip- dentine, enamel, and calcified cartilage in
tychius, cartilage exhibiting spherulitic the Heterostraci, and cellular bone, dentine,
TABLE 2. A summary of the occurrence of mineralized skeletal-tissues in the vertebratefossil record. First appearance of major
groups (classes) is also shown.
Ordovician
Cambrian
Pre Cambrian
1
Mammals
Birds
Note that acellular bone reappeared in teleosts.
re
U
bo
a
a.
<
,-
re
ichoiidr
•p
V
peril
c
-a
rm
ha)
W
lage
Amphibia
Chondrichthyes
Placoderms
Osteichthyes
Agnatha
0
X
tine
Reptila
c
idi
Devonian
Silurian
re
-5
mel
Quaternary
Cretaceous
Jurassic
Triassic
Permian
Pennsylvanian
Mississipian
£
V
c
CQ
CO
332
BRIAN K. HALL
FIG. 1. Bone containing globular structures (chondrocytes?) from (A) a cephalaspid, and (B,C) an antiarch. (From 0rvig, 1968.)
in structure than the bone described previously. Some intermediate tissues were indeed present in the Agnatha, however they
do not represent a phylogenetic trend, but
rather intermediates between two
specialized mineralized tissues. 0rvig
(1951) has made a very extensive study of
three principal types of dentine, of tissues
intermediate between dentine and bone,
and of tissues intermediate between bone
and calcified cartilage, in the early vertebrates. The considerable debate over
whether aspidin was dentine or bone,
partly a debate over whether aspidin was
cellular or not, is in itself good evidence for
the presence of intermediate tissues among
the early vertebrates (reviewed by
Halstead, 1969<2,6). The holostean fish,
Amia calva, possesses cellular dentine in
which the process leading to the persistence
of the odontoblasts is indistinguishable
from that leading to the persistence of osteocytes in bone (Moss, 19646). 0rvig's
(1965) contention that the basic distinction
between bone and dentine "is one of
scleroblast behavior rather than one of
scleroblast activity" would seem to be a
good summary of the interrelations between the two tissues. There were then less
clear cut distinctions between the skeletal
tissues of the Ordovician vertebrates than
exists between the tissues of present-day
vertebrates.
enamel, and calcined cartilage in the Osteostraci. The replacement of cartilage by
the formation of endochondral bone appears to have been a slightly later development in the middle Ordovician Osteostraci
(Romer, 1964). With the possible exception
of aspidin whose homology with bone, and
especially with the acellular bone of the
teleosts, has been questioned by some, all of
the skeletal tissues of the Agnatha may be
readily equated with and appear to be just
as complex and "advanced" as those of the
living vertebrates.
One might expect to find many skeletal
tissues in these, the first vertebrates, which
were "missing links"—tissues intermediate
between one recognizable tissue such as
bone and another such as cartilage, or tissues which were, for example, bone, but
bone that was simpler and more primitive
An oft-debated question is that of the
relationship between cellular and acellular
bone. Some of the teleosts possess acellular
bone, a consistently cellular bony skeleton
not having been universally attained until
the evolution of the tetrapods. It seems to
have been well established that in the evolution of the teleosts from the actinopterygians, acellular bone was derived secondarily from cellular bone (Kolliker, 1859; Denison, 1963). The relationship of Agnathan
aspidin to cellular bone has been more
hotly debated, all possible views having
been proposed: that the two evolved independently (0rvig, 1965; Moss, 1968a); that
aspidin evolved from cellular bone (0rvig,
1957, 1968); and/or that cellular bone
evolved from aspidin (Denison, 1963). The
whole question has been reviewed by Tarlo
(1964), 0rvig (1965), Halstead (1969a) and
333
SKELETAL DIFFERENTIATION AND EVOLUTION
Hancox (1972). The fact that the two tissues
appeared contemporaneously in the Ordovician in two distinct groups of Agnathans indicates that the two must have
arisen earlier, either independently from
less-specialized tissues or from a single
primitive skeletal tissue.
A further question which has been debated back and forth for decades is: "which
of these skeletal tissues appeared first in the
vertebrate skeleton during its evolution?"
usually reduced to a question of "which
came first, cartilage or bone?" This considerable debate was, to a large extent, fostered by the application of embryological
theory to the study of skeletal evolution and
illustrates the need for caution when making such applications. Because the vertebrate embryo was found to have a predominately cartilaginous skeleton, because
cartilage precedes bone in endochondral
bone formation, and because the cartilaginous fishes (the Chondrichthyes) were
considered primitive, cartilage was assumed to have evolved before bone and
phylogenies of the vertebrates were constructed accordingly (Fig. 2) (Romer 1942,
1963). However, as the data summarized in
Table 2 indicate, enamel, dentine, calcified
cartilage, and dermal bone (both cellular
and acellular) share equal honors in terms
of first appearance in the ancient verte-
AMPHIBIA"
Crossopterygii*
Dipnoi* . 4 ^ .
^SARCOPTERYGir
Jp
Teleosts*
^
Actinopterygii*
OSTEICHTHYES*
Chondrichthyes^. .^.
Cyclostomes
PLACODERMS*
QstArms'
AGNATHA*
FIG. 3. The evolution of the lower vertebrates assuming bone (*) appeared early in evolution. (Modified from Romer, 1964.)
brates. Endochondral bone seems to have
arisen later (first in the Middle Ordovician
Osteostraci, then in the Silurian acanthodians, and in some of the placoderms) and
to have come to prominence in those
groups from which later groups evolved
(the rhipidistian sarcopterygians which
gave rise to the land vertebrates, and the
actinopterygian fishes). Thus, the first vertebrate hard tissues were histologically diAMPHIBIA*
verse and specialized. A phylogeny which
takes these facts into account is shown in
DIPNOI
Figure 3.
The question of the adaptive role played
Crossopterygians*
by the skeletal tissues in the early vertebrates has received considerable attention
with the assumption that the factors reOSTEICHTHYES
sponsible for the adaptations were the same
factors as were responsible for the initial
Placoderms"
-^fcOstracoderms* evolution of the skeletal tissues. The following possible functions have been proposed:
^
CHONDRICHTHYES
^
Bone served a mechanical supporting function as it does today (Berrill, 1955; Schaeffer, 1961; Denison, 1963). The dermal
skeleton served both as a reservoir and as a
CYCLOSTOMATA
permeability barrier to conserve calcium
and
especially to conserve phosphorus.
FIG. 2. The evolution of the lower vertebrates assuming bone (*) appeared late and off the main The ultimobranchial gland and, in later
evolutionary line. (Modified from Romer, 1964.)
groups, the parathyroid gland aided in this
334
BRIAN K. HALL
function (Berrill, 1955; Smith, 1961; Urist,
1962, 1963, 1964; Moss, 1964a; Tarlo,
1964; McLean and Urist, 1968). This view
has been denied in general by Denison
(1963) and for acellular bone in particular
by Moss (1963). The dermal armor may
also have provided a defensive outer shield
(Denison, 1963; Romer, 1963). Cartilage
was thought to have served both for
mechanical support and for rapid growth,
especially embryonic growth and development (Romer, 1942, 1963, 1964; Berrill,
1955; Denison, 1963).
a restriction of the types of tissues produced from many specialized tissues in
lower groups to few tissues, no more
specialized, in the higher groups. Moss
(1963, p. 339) states: "It is almost as if in an
essentially nonweight bearing skeleton
(that of fishes) no selective advantage is
found in one histological type of osseous
tissue or another."
The transition from the reptiles to the
mammals did involve fundamental changes
in the associations and positions of skeletal
elements, e.g., the evolution of the new jaw
articulation (du Brul, 1964); the growth of
the jaw (the sutural growth of the multiSKELETAL TISSUES DURING EVOLUTION
boned reptilian mandible vs. the apical
Enlow and Brown (1956, 1957, 1958) growth of the mammalian dentary—
have provided a very detailed comparative Sicher, 1966); the attachment of the teeth
survey of the histology of both fossil and to the jaw (ankylosed in reptiles and a synrecent bone, and more recently Enlow desmosis in mammals). These involved
(1969) has reviewed the bone of the reptiles changes in the developmental processes,
and Ricqles (1968, 1969, 1972) that of the but the tissues produced were fundamentetrapods. No advance in the structure of tally the same as were those of the Ordovibone was associated with the evolution of cian vertebrates. Thus, bone histology canthe reptiles from the amphibia or of the not be used to identify individuals of a particular species and often not members of
mammals from the reptiles. Thus:
the same class (Enlow, 1966).
" . . .while much structural variation of bone tissue is found (throughout evolution), these differThe conclusions to be drawn from an
ences usually involve variation in the arrangements examination of the histology of the bones of
of fundamental components and not major differences in the structure of the components fossil vertebrates would appear to be as folthemselves" (Enlow and Brown, 1958, p. 212). lows: The skeletal tissues of the early OrAgain (p. 220) " . . .in the history of bone tissue, a dovician vertebrates were essentially the
single evolutionary line cannot be recognized. It same as are those of present-day verteis not possible to trace a precise series of progres- brates, although more intermediate tissues
sive, increasingly complex developmental stages
were present then than are present now.
from ancient fish to modern animals."
Again: "Anatomical specializations are not per The first mineralized vertebrate tissues
se necessarily correlated with phyletic modifica- known from the fossil record were complex
tions in the histological structure of hard tissues" and specialized (implying earlier unfos(0rvig, 1965, p. 554).
silized and perhaps simpler (?) skeletal tisFor example the degree of mineralization sues). Skeletal evolution in the vertebrates,
of the skeleton, as indicated by its density, at least since the Ordovician, has not indoes not show a phylogenetic trend, but volved major changes in cell or tissue orindicates local adaptation to environmental ganization but rather has involved adaptive
conditions such as mechanical stress, responses of already specialized and plastic
equilibrium, buoyancy, protection, tem- tissues to new local environmental changes.
perature, conservation of energy, as has
been documented for the Ostracoderms THE BORDERLAND BETWEEN EMBRYOLOGY AND
(Schaeffer, 1961) and for the Cetacea (Felts
PALAEONTOLOGY
and Spurrell, 1966). There is very considAs it can be maintained that skeletons of
erable variation in the histological structure
of fish bone and cartilage (explosive radia- the earliest vertebrates were highly
tion) which is not seen in the Amphibia or specialized structures whose tissues were as
the Reptilia (Moss, 1964a, 1969), indicating complex (advanced) as are those of present-
SKELETAL DIFFERENTIATION AXD EVOLUTION
day vertebrates, it seems reasonable to conclude that the genetic machinery required
to produce such tissues was established
equally early. The subsequent evolution of
the hard tissues of the skeleton did not involve major, progressive changes in their
fundamental structure and so may not have
necessitated major changes in the genome.
If it can also be shown, as will be attempted
later, that skeletal tissues are especially susceptible to modulations from environmental factors, then experimental studies on
skeletal development and differentiation
become extremely valuable as tools for understanding the evolution of the skeleton.
This view has been aptly synopsized by
Moss (1964a): " .. .intrinsic embryological
phenomena, as modified by the extrinsic
factors of mechanical function, forms the
basis of a meaningful discussion of the
evolution of skeletal tissue type."
We might study the interaction between
these two bodies of knowledge by examining available information on the developmental processes involved in producing the
skeleton. This we shall do by following the
life history of a "typical bone," an approach
used previously in reviewing the histogenesis and morphogenesis of bone
(Hall, 1971). We will ask a series of questions about skeletal development, present
the current status of knowledge on that topic, and then determine whether the information is of any value in understanding
aspects of the evolution of the skeleton.
335
bone. The cells which condense either arise
locally in the position which the bone will
subsequently occupy, as in the formation of
the vertebrate, limbs, or elements of the
skull (Hall, 1971), or they migrate from
elsewhere in the body to the site at which
skeletogenesis is to occur, as in the migration of mesodermal cells into the lower jaw
of the chick (Jacobson and Fell, 1941), or
the migration of the ectomesenchymal cells
from the neural crest into the skull, mandible, or pharynx (Johnson and Listgarten,
1972). The factors responsible for condensation are largely unknown, although cell
adhesion may play a role (Ede, 1971; Moss,
1972ft). In the case of the ectomesenchymal
cells of the neural crest which form the
cartilage of the head, the stimulus for condensation results from interaction with the
pharyngeal endoderm (Holtfreter, 1968).
Epithelial-mesenchymal interactions
The position of the condensed
mesoderm or ectomesenchyme, and the
shape, size, and rate of growth of the condensed cells, are determined by interaction
with adjacent ectoderm (epithelia), the socalled epithelial-mesenchymal interactions.
These interactions may involve the two-way
interchange between a stationary layer of
ectoderm and local mesoderm, as in the
formation of the amniote limb, or they may
involve establishment of an association between mobile (migrating) mesoderm
and/or ectoderm, as in tooth or middle ear
formation. Other mesodermal cells ajacent
INITIATION OF SKELETOGEXESIS
to these specialized epithelia, but outside
the condensation, can form skeletal tissues,
The first question is a double-barreled but only the tissues within the condensation
one: How does the development of a bone do form skeletal tissues (reviewed by Hall,
commence, and what determines where 1971). A good example is the formation of
that bone will develop?
the cartilaginous primordia of the long
bones in the limb of. the embryonic chick.
Mesenchymal condensation
Any mesodermal cell in the limb bud is
capable
of producing either cartilage or
The first sign that skeletogenesis is immuscle
up
to Hamilton-Hamburger stage
minent is condensation of mesodermal (or
24
(4-'/2
days
of incubation). Thereafter
ectomesenchymal) cells to form the anlage
only
the
cells
in the central condensed
(primordium) of the bone. The position of
mesoderm
form
cartilage and these are the
the condensation within the embryo
cells most closely associated with the
defines the subsequent position of the bone specialized ridge of ectoderm—the apical
within the adult; the shape of the condensa- ectodermal ridge. An extra ridge grafted to
tion defines the future basic shape of the
336
BRIAN K. HALL
the limb bud will induce cells outside the
condensation to condense and to form additional skeletal tissue, indicating that the
ectoderm influences the fate of the mesodermal cells. Thus, one can propose, as I and
others have done in the past (Hall, 1970,
1971), that potentially any mesenchymal
cell is capable of producing more than one
skeletal tissue (such as cartilage and bone)
and that during development, local environmental factors stabilize the genome of
cells in specific sites for skeletogenesis and
further influence what type of skeletal tissue will form.
Teeth form at the junction between the
buccal epithelium and those ectomesenchymal cells which have migrated in from
the neural crest. Cells from both germ
layers are required before tooth formation
can be initiated. The odontoblast arises
from the ectomesenchyme and produces
dentine only when it is in contact with adjacent oral epithelium (Horstadius, 1950).
The ameloblast arises from the ectoderm,
produces enamel, and, in coordination with
the ectomesenchymal odontoblast, leads to
the formation of a normal tooth (Fig. 4).
It has been shown by Goedbloed (1964)
that the development of the mouth cavity,
the middle ear cavity and the external auditory meatus, and their associated tissues,
involve epithelial-mesenchymal interactions brought about by the movement of
epithelial borders into association with new
ECTODERM
neural tube
neural crest=buccal epithelium
PHARYNGEAL
ENDODERM
odontoblast = = s ameloblast
dentine
enamel
TOOTH
FIG. 4. The major ectodermal-mesodermal interactions involved in the formation of the tooth. Double
arrows indicate inductive interactions. (Modified from
Koch, 1972.)
mesenchyme. For example, the mesenchyme which produces the tissues of the
middle ear arises from several regions and,
depending on the nature of the associated
epithelium, produces cartilage, muscle, or
soft connective tissue. The same holds true
for the origin of the tissues of the lower jaw
in birds, except that it is the mesoderm (ectomesenchyme?) which shifts in position.
Cells destined to form cartilage, bone, or
muscle arise in three separate mesenchymal centers outside the mandible (each
associated with a "transitory epithelial
thickening") and then migrate into the
mandible where they differentiate (Jacobson and Fell, 1941).
There is then experimental evidence for
the shifting of epithelial-mesenchymal
borders during development. These shifts
are brought about either by movement of
site and tissue-specific epithelia or by
movement of tissue-specific mesenchyme.
Can we use this knowledge to understand
the initiation of new skeletal elements during evolution? Two obvious areas to consider are the origin of the jaws during
evolution of the gnathostomes and the
evolution of the mammalian jaw articulation, both of which involve the shifting in
position, and modification of function, of
pre-existing skeletal elements.
In the evolution of the gnathostome vertebrates from the Agnatha, the skeleton of
the first apparent gill arch was extended
anteriorly to form the skeleton of the jaws
(Fig. 5). The upper half of the arch formed
the upper jaw (the palatoquadrate); the
lower half of the arch formed the lower jaw
(the mandible). The skeleton of the arches
consists of cartilage and bone derived, in
part, from ectomesenchyme of neural crest
origin. The skeletal tissues of the lower jaw
include a cartilaginous rod (Meckel's cartilage), also probably of neural crest origin,
and a number of membrane bones, to be
discussed later. The simplest developmental model which would explain this
evolutionary sequence and which would be
consistent with the experimental data
summarized above would be as follows: as
the mouth enlarged, the mesenchyme of
the gill arch region extends anteriorly as
the primordium of the jaw, taking with it its
SKELETAL DIFFERENTIATION AND EVOLUTION
337
which invest the cartilaginous rod. These
membrane bones were not present in the
gill arch and therefore had to be produced
de novo, and will be discussed below.
Before discussing the origin and development of membrane bones, I will turn
to the evolution of the mammalian jaw articulation. The reptilian jaw articulates
through the articular of the lower jaw and
the quadrate of the upper jaw. In the transition to the mammals these two endochondral bones migrated posteriorly to form, respectively, the malleus and the incus of the
middle ear. A new jaw articulation developed between a newly formed condyle
of the now greatly expanded dentary
(which comprises the lower jaw) and the
squamosal of the skull. Ample evidence illustrating the gradual migration of these
bones in the synapsid reptiles is provided
from the fossil record, and the concept of
shifting of epithelial-mesenchymal borders
would seem to provide an adequate
mechanism to explain the developmental
processes involved. The search for the
FIG. 5. The origin of the jaws, /f .jawless Agnatha.B, stimulus which initiated the movements inGnathostome. First gill arch modified as jaws. C, Os- volves consideration of the skull as a functeichthyes. Jaws braced by second gill arch. (From du
tional unit and the changing conformation
Brul, 1964.)
of the skull base, changes in muscle mass,
associated specific epithelium, and produc- new directions of stress (see below and also
ing in the new site, a cartilaginous and bony Noble, 1973). It may be that no fundamenrod as it would have done in the gill arch. tal changes in the genome were required to
The alternatives of establishment of a new bring about the development of this new
pathway of migration of neural crest di- articulation.
rectly from the neural tube to the anterior
segment of the lower jaw and of subsequent
establishment of new site and tissue-specific The origin of membrane bones
epithelia to induce the now adjacent
mesenchyme to produce Meckel's cartilage
The question of the origin of the memappears less likely (i) because several de- brane bones of the lower jaw, or indeed, of
velopmental processes are involved, and (ii) membrane bones in general, irrespective of
because the pre-existing cartilage- their location, is one to which we shall now
determined tissues within the gill arch turn. The factors which induce mesencould be transposed more readily. How- chymal cells to modulate to osteoblasts and
ever, in the avian embryo, neural crest cells to deposit bone matrix in vivo have not
do follow such a route along a cleavage been determined, except for those memplane between the ectoderm or pharyngeal brane bones surrounding the brain and
endoderm and the adjacent mesoderm forming the dermocranium and for the
(Johnson and Listgarten, 1972).
periosteal bones associated with the cartilage
models of long bones.
In addition to Meckel's cartilage and an
The intramembraneous (perichondral,
endochondral bone(s) which develops in
Meckel's cartilage (see below), the lower jaw periosteal bone) which forms around the
contains a number of membrane bones shafts of endochondral bones has been
338
BRIAN K. HALL
shown to form under the influence of an
induction from the associated hypertrophic
chondrocytes (Mareel, 1967). These chondrocytes are in turn either replaced by, or
converted into, endochondral bone (Holtrop, 1966; Crelin and Koch, 1967; Hall,
1970). A similar situation has been postulated for the initiation of membrane bone
formation around Meckel's cartilage, although here the evidence is not as good.
Frommer and Margolies (1971), from
studies of normal development of the
mandible in the mouse, maintain that the
close spatial relationship between initial
chondrogenesis of Meckel's cartilage and
initial intramembraneous ossification in the
mandible indicates induction of membrane
bone by Meckel's cartilage. Further, they
feel that the membrane bone so induced in
turn enables the adjacent areas of Meckel's
cartilage to undergo endochondral ossification. One could suggest that the membrane
bones surrounding Meckel's cartilage (and
this is a topographic arrangement not duplicated elsewhere in the body) were originally a single perichondral bone developed
in the perichondrium of the skeleton of the
gill arch. If during transformation into the
lower jaw, parts of Meckel's cartilage subsequently failed to undergo hypertrophy,
subdivision of the single bone could have
occurred allowing it to loosen its previously
close connection with the cartilage. Friant
(1959, 1964, 1966, 1968, 1969) has published an extensive series of observations
indicating that Meckel's cartilage is completely replaced by endochondral bone in
some mammalian species, incompletely replaced in others, and that in some it remains as a cartilaginous rod, the chondrocytes failing to hypertrophy. In those where
replacement is partial, bone forms only adjacent to hypertrophicchondrocytes. Other
areas of the cartilage fail to undergo hypertrophy and remain cartilaginous.
is activated by influences from the brain
and from the notochord. Of potential use
in the study of the evolution of the dermal
skeleton is the work of Schowing
(I968a,b,c.) He has shown that, in the embryonic chick, the brain, nerve cord, and
notochord induce overlying mesoderm to
form the intramembraneous bones of the
skull and to form them in the appropriate
spatial relationships to one another (Fig. 6).
The strong development of the dermal
head skeleton in the Ordovician agnathous
vertebrates indicates that this mechanism
may have been established at the outset of
vertebrate evolution. The ventral and
ventro-lateral components of the tetrapod
neurocranium, e.g., the occipital and the
basisphenoid (Fig. 6), are all endochondral
in origin, whereas the more dorsal elements
are of course intramembranous. Have we
here a mechanism for distinguishing what
tissue type a given mesodermal condensation will initially form? Could one speculate
that it is a combination of inductive
influences emanating from both the
notochord and the central nervous system
which determine endochondral ossification, whereas neural influences alone determine intramembranous ossification?
The somatic mesoderm in which the vertebrae develop also requires an induction
from the notochord before chondrogenesis
can commence (Lash, 1968). As in the development of the endochondral bones of
the skull, the endochondral vertebrae receive a dual inductive stimulus both from
the notochord and from the spinal cord. In
fact in some vertebrates, e.g., the salaman-
Many membrane bones do not develop in
proximity to primary cartilage. Urist (1962,
1970) maintains that these membrane
bones are induced by the adjacent fibrous
connective tissue, although experimental
6. The inductive interactions between brain,
evidence is lacking. The membrane bones FIG.
notochord and overlying mesenchyme responsible for
surrounding the brain develop from formation of cranial dermal bones in embryonic chick.
mesenchyme whose skeletogenic potential (From Schowing, 1968c.)
SKELETAL DIFFERENTIATION AND EVOLUTION
ders, cartilage develops within the
notochord (Wake and Lawson, 1973).
To summarize the answer to the
double-barreled question, the site and timing of skeletogenesis depends primarily on
ectodermal-mesodermal interactions, a
concept which can be very usefully applied
to the study of the evolution of skeletal tissues and of the skeleton.
TYPES OF SKELETAL TISSUES
This leads into a discussion of the second
question: What determines the type of
skeletal tissue that will develop?
Modulation and the scleroblast
It is only after the mesenchymal cells
have condensed that skeletogenesis begins.
Depending on the site within the body and
on local epigenetic factors, one of the following types of scleroblasts will form:
chondroblast, chondrocyte (cartilage); osteoblast, osteocyte (bone); fibroblast (ligament, tendon); odontoblast (dentine);
ameloblast (enamel). Each is characterized
by a particular extracellular product and by
a particular structure. Along with the concept that any mesenchymal cell has the potential to become a scleroblast given the
appropriate stimulus is the concept that the
various types of scleroblasts are members of
an interrelated and potentially interconvertible family (Moss, 1964a, 1968c, 1969;
Hall, 1970, 1971). The existence, in both
recent and fossil vertebrates, of tissues intermediate between two well-defined
skeletal tissues (cartilage and bone; bone
and dentine) provides the histological basis
for this concept. The conversion of one
scleroblast type into another after experimental manipulation provides the empirical verification of the concept. Thus, to endeavor to understand why a particular
skeletal tissue develops at a particular site in
the skeleton is to ask the question: How do
different scleroblasts modulate from common stem cells and what causal factors are
involved?
Ectomesenchyme of the neural crest
The derivatives of the neural crest pro-
339
vide an illuminating example of the concept that histologically quite disparate
skeletal tissues may arise from common
stem cells. The neural crest is derived from
the ectoderm at the border between the
epithelial ectoderm and the developing
neural ectoderm and separates off as the
latter transforms into the neural tube.
These cells break free, become mesenchymal in appearance (hence the name ectomesenchyme) and migrate to various sites
within the embryo. In the Amphibia, the
chondroblasts, osteoblasts, and odontoblasts of the cranial skeleton all arise from
the neural crest (de Beer, 1958). Experimental studies on the chick embryo have
provided similar conclusions for the origin
of cranial chondroblasts and several experiments will be discussed.
Johnson (1966) carried out a study in
which excised portions of the neural crest
of 30-hr chick embryos were replaced with
comparable pieces previously labeled with
H3-thymidine. Subsequent examination as
late as 9 days of incubation indicated
labeled chondrocytes (and therefore cartilage) in the cartilages of the head and in the
visceral arches. Although labeled bone was
not observed (and osteogenesis is well advanced in the head of a 9-day embryo),
labeled undifferentiated mesenchymal cells
were seen and were thought to be osteogenic precursors.
Hammond and Yntema (1964) removed
neural crest without replacement from
similarly aged embryos and noted subsequent depletion of cartilage from the
head and lower jaw. No mention of bone
depletion was made. Thus, in the chick, the
origin of head cartilage from neural crest
rests on sound experimental grounds; origin of bone from similar cells is based on
inferential evidence (further evidence is
being sought in my laboratory).
Jollie (1971) has studied the development of the head of the embryonic shark,
Squalus, and has shown that Meckel's cartilage and the teeth (both of which are derived from the cells of the neural crest) arise
from cells in a common blastema situated
under the epidermis. These cells then
stream into the future positions of these
organs, where they differentiate. Thus,
340
BRIAN K. HALL
similar stem cells form different skeletal tis- both fossil and recent vertebrates, and Jolsues according to site-specific factors. Hall lie (1968) has provided an excellent discus(1970, 1971, 1972) has reviewed many of sion of the neural crest and of the consethe factors involved in such transforma- quences of acceptance of the Delamination
tions and an example is discussed later.
Theory (to be discussed below). If the
The sharks, despite their undoubted po- neural crest of the early vertebrates possition as cartilaginous fishes, contain sessed the ability to form a variety of
mesodermal cells capable of producing skeletal tissues, as appears, based on the
bone. Bone has been described in the mod- tissues present in fossil Ordovician verteern sharks and chimeras (Peyer, 1968): at brates, to be the case, then the skeleton of
the base of the teeth in Squalus acanthias, these vertebrates was a highly adaptive orwhere it is acellular (Moss, 1970), and in gan, ideally suited to evolutionary modHeptanchus (Holmgren, 1942). Bone has ification. Mute fossil bones do speak when
also been observed in Ornithoprion hertwigi, asked the right questions.
a specialized Permian edestidaed shark.
This species was partially armored, the Delamination
armor consisting of denticles and scales, the
The previous studies provide experibases of which were imbedded in bone.
These bony elements were fused together mental evidence for the delamination
to form a thick layer of bone around the theory proposed by Holmgren (1940) and
snout and the mandible (Zangerl, 1966). modified by Jarvik (1959) and Moss (1968c,
Whether these bones are of neural crest 1969). In this theory it is postulated that
origin is unknown. Whether they are or are dermal skeletal tissues, formed from the
not, they illustrate the hidden potential re- ectomesenchyme of the neural crest, lose
sident within skeletogenous cells, and indi- their contact with the epidermis and sink in,
cate that views such as, "the process of bone to be replaced at the surface by a succeeddevelopment is, so to speak, much more ing layer of skeletal tissues, not necessarily of
firmly imbedded in the embryological pat- the same type as the first-formed tissues. The
tern of these (Osteichthyes) fishes" (than in first layers to sink in by delamination form
Chondrichthyes) (Romer, 1964, p. 76), are the endoskeleton, the last-formed layers
remain at the surface to become the dermal
too restrictive in concept.
exoskeleton (Tarlo, 1964; Moss, 19686).
Much of the skeleton of the early verte- Hay (1964) has shown that collagen, probrates was probably of neural crest origin, duced in the ectoderm, can migrate down
i.e., the homologous bones in present day into the underlying mesoderm and there
vertebrates are derived from the cells of the contribute to the extracellular matrix proneural crest. Romer (1972) has divided the duced by the scleroblasts—a present day
skeleton into the predominantly cartilagi- visualization of delamination. The exo- and
nous "visceral skeleton" of neural crest ori- endoskeletons and the various scleroblast
gin and the predominately post-cranial types of the neural crest are thus linked by
"somatic skeleton" of mesodermal origin these common developmental processes.
and has argued that the visceral skeleton They may also be linked, as noted earlier,
antedated the somatic skeleton in verte- with inductive interactions with the underbrate evolution. The neural crest must then lying nervous tissue (Schowing, I968a,b,c).
have appeared very early in vertebrate
evolution and its cells possessed the proper- Ectodermal and mesodermal scleroblasts
ties of migration, interaction with ectoderm, and modulation to different
This discussion of the origin of skeletal
scleroblast types equally early. The de- tissues from the neural crest raises the quesvelopment of epithelial-mesodermal in- tion of the origin of skeletal tissues from the
teractions may have been a later develop- mesoderm and/or the ectoderm, for
ment in vertebrate evolution. Maderson skeletal tissues are traditionally regarded as
(1975) has reviewed the evidence for the mesodermal tissues. As Mathews indicates
homology of developmental processes in (1967, p. 500): "Rigid adherence to re-
SKELETAL DIFFERENTIATION AND EVOLUTION
quirements for embryological homology
may lead to unnecessary confinement in the
range of phylogenetic considerations."
Romer (1972) provides a delightful anecdote on this matter—about the Professor
who delayed publication of his student's research on the ectomesenchymal origin of
the mammalian cranial skeleton for 50
years so as not to "commit treason to the
germ layer theory."
Both ecto- and mesoderm may, and do,
produce the macromolecules characteristic
of connective tissues, viz., collagen and acid
mucopolysaccharides. These two groups of
compounds show co-evolution and, at least
for collagen, conservation of amino acid
structure (Mathews, 1971), although Matsumura (1972) has concluded that amino
acid composition is more tissue-related
than phylogeny-related. Some controversy
exists as to whether vertebrate ectodermal
collagen is different from mesodermal collagen (Moss, 1963, 1970; Mossetal., 1964),
or the same (Matsumura, 1972). Various
invertebrates (e.g., squids, tunicates, and
Branchiostoma) form true epidermal cartilage, and some produce composite musculoskeletal tissues as in the intimate association of muscle and cartilage in the odontophoral cartilage of grazing gastropods
(Person and Philpott, I969a,b; Carriker et
al., 1972).
Thus, not only can ectodermal cells produce the macromolecules characteristic of
skeletal tissues, they can also export them to
the mesoderm and so co-participate in
skeletogenesis, or they can elaborate them
locally into skeletal tissues. Moss (I968b,c)
has extended this notion of skeletogenesis
from either germ layer to the development
and evolution of the dermal skeleton of the
ancient vertebrates, especially to the question of the origin of vertebrate skeletal tissues during the evolution of the vertebrates
from an invertebrate stock.
INVERTEBRATE ORIGINS OF THE VERTEBRATE
SKELETON
There is an essential unity in mineralization mechanism throughout the animal
kingdom (and perhaps also in the plant
kingdom) (Travis et al., 1967; Moss,
341
1968a,c) and although we consider
mineralization of a skeleton to be a vertebrate prerogative, two thirds of the living
species which contain mineralized tissues
are invertebrates. The origin of vertebrate
calcined tissues from invertebrate ancestors
probably did not require the evolution of
fundamental new biochemical mechanisms, but the selection and modification of
the appropriate ones from the wide range
of highly developed
calcification
mechanisms possessed by the invertebrates
(Mathews, 1967; Person and Philpott,
I969a,b; Philpott and Person, 1970). Calcification in both vertebrates and invertebrates occurs in a fibrous matrix. The fibers
are often collagen in invertebrates, always
collagen in vertebrates. The mineral is
CaPO4 in vertebrates, but more often
CaCCh in invertebrates. The invertebrates
are able to mineralize tissues of either ectodermal (mollusc shells) or mesodermal
(echinoderm skeletons) origin but have not
evolved mechanisms of interactions between the two germ layers.
Moss (1968c) has enumerated four processes required to produce vertebrate
skeletal tissues which are not possessed by
invertebrates and which therefore must
have evolved very early in the evolution of
vertebrate stock. His four processes may be
simplified to three: (i) formation of a combined (ecto-mesodermal) calcified tissue
with active participation by the ectoderm;
(ii) induction between the mesoderm or ectomesenchyme and the ectoderm; (iii) delamination. The acquisition of the hard
skeleton by the first vertebrates potentially
could have occurred quite rapidly for the
scleroblasts respond very quickly to functional demand.
EPIGENETIC REGULATION OF SCLEROBLAST ACTIVITY
Recent studies on the formation of secondary cartilage on the membrane bones of
the avian skull will serve to illustrate one
environmental factor which can drastically
alter scleroblast behavior and tissue differentiation. The environmental factor involved is the presence or absence, and
amount, of movement acting at articular
342
BRIAN K. HALL
surfaces between membrane bones and
other skeletal elements. In the normal
course of embryonic development, cartilage arises on membrane bones at their articular surfaces late in the 10th or early in
the 11th day of incubation. This coincides
with the time of increased overall movement by the embryo (Hamburger et al.,
1965). If the embryos are paralyzed, or if
the membrane bones are cultured in a
stationary culture, cartilage fails to form
(Hall, 1967, 1968, 1972; Murray and
Smiles, 1965). If mechanical stress is
applied to these bones in culture, cartilage
can be induced, the amount of cartilage
varying in proportion to the duration of the
stimulus (Hall, 1968). The interesting
finding is that the cartilage which forms in
the 1 lth day, and the bone which was forming earlier and which continues to form
after 11 days, arise from a common pool of
mesenchymal cells. Where the cells are protected from the mechanical stimulus osteogenesis continues. Where the cells are
mechanically stressed they become chondrogenic, and they can, if the stress is intermittently applied and removed, alternate between the two scleroblast (Chondroor Osteoblasts) types (Murray and Smiles,
1965). Alterations in mechanical stresses
were extremely important in the evolution
of the jaws and in the evolution of the entire
vertebrate head and, as these recent experiments indicate, mechanical stress is a potent modulator of scleroblast behavior.
possess a narrow range of tissue types the
potential to form other tissue types has not
been lost but is dormant. The tissues
formed in the repair of fractured intramembraneous or endochondral bones
(Hall and Jacobson, 1975; Pritchard and
Ruzicka, 1950; Murray, 1954) are not the
same as the normal skeletal tissues of that
bone or species. Intermediate tissues not
characteristic of the species arise in bone
disease, such as osteosarcoma or chondrosarcoma. Bone forms in "cartilaginous"
fishes. The primitive scleroblast (both ontogenetically and phylogenetically) is an extremely adaptive cell with perhaps more
hidden potential than most other cell types.
A study of the skeletal tissues of the ancient vertebrates which neglects such modifying epigenetic factors is at the very least
incomplete, and at worst inaccurate.
Cellular and acellular bone
The views on the relationships between
cellular and acellular Agnathan bone discussed earlier take on a new dimension
when viewed in the light of the development of the acellular bone of modern
higher teleosts. Such acellular bone is
found in both freshwater and in saltwater
species and so appears not to be correlated
with retention of calcium or of phosphorous (Moss, 1961a, 1965). The functional
Moss (1969) has shown that, within the significance of acellular bone remains unreptilian dermis, a wide range of skeletal known. The acellular bone of the teleosts
tissues may be produced from similar pre- develops in one of three ways from one of
sumptive cells. For example, all stages from three types of cellular tissues: periosteal ostendon, through calcified tendon to bone, teogenesis from the cells (osteoprogenitors)
are found, and he has argued (1964a) that of the periosteum; tendonous osteogenesis
most primitive vertebrates also had the po- from the cells within the tendons; or ostential to produce the whole range of teogenesis by metaplasia of secondary carskeletal tissues during their evolution- tilage cells (Moss, 1964a). Thus, Moss
selection pressure, and modulation dictat- (19646, p. 348) writes: "It is apparent that
i ng which type of tissue would actually form no neccessary relationship exists between
at a given site in a given species at a given the type of bone first formed, and the type
time. The restriction in the range of skeletal which is eventually characteristic of the
tissues as one ascends the vertebrate species." The osteocytes so-formed either
phylogenetic tree indicates that particular are not enclosed in an osseous matrix or, if
skeletal tissues have become preferentially enclosed, are replaced by an osseous
selected for during the course of evolution. matrix, resulting in an acellular bony
However, even in those vertebrates which matrix in the adult. Similar transforma-
SKELETAL DIFFERENTIATION AND EVOLUTION
tions of cytoplasm into supporting tissue
matrix are seen in some invertebrate cartilages and in lignification in plants (Person
and Philpott, 1963).
It is difficult to imagine how the acellular
bone of individual Ordovician Agnatha
could have developed without going
through a cellular phase. Thus, when we
evaluate the evidence for the possible
evolution of acellular bone (aspidin) from
cellular dermal bone we should really be
considering the origin of cellular bone
from cellular bony or other connective tissues. The important evolutionary question
is then really a developmental question:
Why do the osteocytes not persist? (0rvig,
1957,1965; Calder and Hall, unpublished).
Sesamoids and secondary centers of ossification
343
ossification within long bones likewise involved progressive adaptation of preexisting mechanisms of chondro- and osteogenesis, rather than the development of
new mechanisms (discussed by Moss,
1964a). The limitations placed upon the
evolution of the skeletal tissues must then
be assessed on the basis of the differentiative potential of the constitutent cells of the
skeleton.
To summarize the answer to the question
of how the various skeletal tissue types form
at specific sites, it appears that the specificity comes from local environmental factors such as contact with specific epithelia,
local mechanical stresses, and not always
from intrinsic predetermination of the potentially skeletogenic cells. (The cells are
biased toward skeletogenesis, but epigenetic
factors determine which particular scleroblast type will develop at a given site.) These
cells are then potentially and actually interconvertible and the consequences of such
interactions and modulations must be kept
in mind when considering the evolution of
skeletal tissues and especially when basing
phylogeny on tissue and cell structure.
Some structures which are almost universally present in the long bones of recent
mammals, such as secondary centers of ossification within the epiphyses, or sesamoid
bones at joints, appear to have arisen with
the mammals in the Jurassic (Haines,
1969). Do these not indicate the progressive
evolution of skeletal tissue structure? Apparently not, if we look at the development
SIZE AND SHAPE OF THE SKELETON
of such structures. Sesamoid bones are
known to be intratendinous ossifications
We may now turn to the question: What
(Barnet and Lewis, 1958) or intradetermines
the size and shape of the skeleligamentous ossifications (Burton, 1973)
ton?
and it is known that the ornithischian dinosaurs possessed ossified tendons Morphogenesis of the skeleton
(Haines, 1969) and that reptiles tend to
show tendinous rather than periosteal osOnce the cells of the skeleton begin to
sification (Moss, 1969; Enlow, 1969). The differentiate a three-dimensional structure
ligaments and tendons arise from connec- develops. At first the cells within the contive tissue along lines of stress (Schaeffer densed mesoderm are randomly arranged.
and Rosen, 1961) and local tensions appar- As the cells begin to synthesize extracellular
ently induce them to ossify (Haines and matrix and to differentiate, they elongate
Mohuidiin, 1968). Thus, the formation of perpendicular to the Jong axis of the conthe sesamoids need not have involved the densation, and so begin to initiate a pattern
development of radically new processes of and direction to the growth and shape of
ossification, and they are not bones radi- the ruminent. From then on in the decally different in construction from other velopment of the skeletal rudiment factors
bones. Their evolution probably involved
the adaptation of pre-existing processes in other than the direct genetic constitution of
response to new environmental conditions the tissues begin to come into play.
A study of the determination of the mor(the extra stresses imposed on the joints by
phology
of particular bones within the
the adoption of terrestrial life).
skeleton sheds considerable light on the relThe formation of secondary centers of ative contributions of genetic and
344
BRIAN K. HALL
"epigenetic" factors. Thompson (1917), in
a now classic study, considered the maintenance of form in the animal and plant
kingdoms as an adaptation to the environment but avoided the question of the relationship between ontogenetic adaptation
and the inheritance of form and pattern.
Murray (1936) summarized the early literature on the form of bones, most of which
was based on the trajectory theory of Wolff,
viz., that the form of the bone is, in large
measure, molded by the external forces acting upon it. Enlow (1968) has provided a
recent evaluation of Wolffs law. It turns
out that inherent genetic control is more
important than was previously thought to
be the case.
The attainment of the fundamental form
(i.e., the initial three-dimensional morphology, accompanied by considerable
linear growth) of a skeletal element is independent of functional demands, and is
under genetic control (Howell, 1917; Felts,
1961; Chalmers and Ray, 1962; Mawdsley
and Ainsworth Harrison, 1963). For example, if the mesodermal primordium of a
bone, or the early bony anlage after initiation of osteogenesis or chondrogenesis, is
grafted to the chorio-allantoic membrane,
or transplanted subcutaneously or intramuscularly, or cultivated in vitro (even
in the presence of unusually strong
mechanical stresses) this fundamental
form—three-dimensional configuration,
presence and position of condyles,
tuberousities and grooves—develops normally (Murray, 1926, 1928; Monson and
Felts, 1961; Hall, 1967, 1968; Ede, 1971;
Yasuda, 1973). There is expression of the
inherent rates of cell division, cell hypertrophy, and amounts of intercellular
matrix produced per cell.
Once this fundamental form is established, the development of "minor" architectural features of the bone (ridges for
attachment of muscles, ligaments etc.) depends upon functional demand and can be
modified by the environment (Murray,
1936; Chalmers, 1965; Drachman and
Sokoloff, 1966). The appearance of these
"minor" architectural features establishes
the final form of the bone, and the continued action of mechanical factors is nec-
cessary to maintain that form.
The evolutionary consequences of these
studies with growing bones would seem to
be that to change the morphogenetic processes which are responsible for the basic
three-dimensional form of the bone, would
require considerable, integrated alteration
in the genome and so would be a relatively
slow phylogenetic process. Changes in the
minor elements of the bone's form could
occur quite rapidly as ontogenetic modifications within one lifetime.
Functional units and epigenetic factors
Gruneberg (1963) and Moss (1968d)
have reviewed the evidence which indicates
that the final position, shape, size, and
growth of particular skeletal elements are
in large measure secondary responses to
the functional unit (lower jaw, skull, upper
arm, etc.) of which the skeletal element is a
part. That is, organs and tissues adjacent to
the skeleton modify its pattern of growth
and determine (through tissue-tissue interactions) whether the cells destined to
form the tissues of the skeleton will differentiate at all, and further, what type of
tissue they will form. Moss (1968rf) goes so
far as to say that: "It is incorrect to speak of
the evolution of the skeleton as such [for] it
is the functional matrix which evolves, the
bone only responds." I would agree, provided that only the minor features of the
skeleton are included. Some examples
from recent experimental studies are provided below.
If the vitreous humor is drained from the
eye(s) of the embryonic chick late in the 4th
day of incubation, microphthalmia is induced and the growth of the eye slowed
down. If the embryo is examined at 18 days
of incubation, the eye is found to be smaller
than normal. The size, shape, and position
of the orbital bones adjacent to the eye are
also found to deviate from normal. Furthermore, bones further removed from the
orbit, such as the frontal, are also found to
be abnormal (Coulombre and Crelin,
1958). Thus, the growth of the eye exerts a
considerable influence on the morphogenesis, growth, and pattern formation
of the adjacent and subjacent skeletal ele-
SKELETAL DIFFERENTIATION AND EVOLUTION
merits which form parts of the same functional unit.
The muscles of the head also play a role
in controlling the growth of this functional
unit. Bilateral masseterectomy in the newborn rat reduces the size of the facial and
cranial bones and it does so asymmetrically,
one dimension of the bones being more
affected than the others (Moore, 1967).
However, the basic architectural features of
the bones are unaffected by surgical manipulations to the adjacent soft tissues
(Pratt, 1943).
The types of tissues produced at a given
site within the skeleton, the association between adjacent skeletal units, and the plasticity of skeletal tissues for developmental
modulations and responses to environmental stimulii during evolution are amply illustrated by the following studies. Bock
(196CM) and Bock and Morioka (1968)
have carried out a series of studies, illustrating the repeated evolution of elements of
the avian skull. For example, the palatine
process of the premaxilla may be either
fused or unfused and has been lost and
reappeared many times during avian
evolution (presumably in response to newly
appearing stresses and pressures). The
median process of the mandible has
likewise appeared where stress on the
mandible and strength of the quadrate dictated (Bock, 19606), a situation which can
be induced experimentally by paralysis of
avian embryos (Murray and Drachman,
1969). Simonetta (1960) and Bock (1964)
have reviewed the question of the evolution
of the kinetic avian skull and the factors
which modify its skeletal elements of the
skull.
345
viously unconnected skeletal elements as
described above often leads to the development of a new articulation, or to the
modification of existing sutures, joints, or
articulations to accommodate the evolution
of the complex musculo-skeletalconnective tissue functional unit. The basic
(primitive?) suture between two adjacent
bones is what has been termed the flat suture
(Moss, 1957) and consists of two bone surfaces opposed to one another without interdigitation. Such sutures may be modified in response to the functional demand
made on them and develop interdigitations, overlapping surfaces, etc. Such is the
case in the skull of the woodpecker and
provides an answer to the enigmatic question: "Why don't woodpeckers get
headaches?" It turns out that the suture
between the frontal and the nasal bones is
an overlapping one, enabling one bone to
ride over the other and absorb some of the
stresses which would otherwise be directed
onto the articulation. This implies changes
in the soft tissues associated with the articulations related to functional demand, and
although evidence is difficult to obtain from
the fossil record, numerous experimental
examples are available: for example, the
presence or absence of, and the degree of
fibrous development in, the intra-articular
discs of the temporo-mandibular joints of
Marsupials and Monotremes (Sprinz,
1965), or the transformation of fibrous articular tissues to fibrocartilage to allow the
human temporo-mandibular joint to adapt
morphologically to mechanical stresses
(Moffett et al., 1964).
What are the implications of these studies
for establishment of phylogenetic trends in
skeletal tissues?
Single character analyses such as those of
Bock and of Beecher (1950, 1951) and
Cracraft (1968) serve to show the wide vari- Phytogeny of skeletal tissues
ation in morphology which may exist between closely related species and highlight
These studies illustrate the difficulties inthat there are many ways by which skeletal volved in establishing a phylogeny of
elements may respond to environmental skeletal tissues based on the genetic selecconditions. The final form of the character tion of progressively more well-adapted tisdepends upon (at least): (i) selection acting sue types. A partial list of the epigenetic
directly on the character; (ii) influences factors which can influence skeletal hisfrom neighboring structures; (iii) chance togenesis and final structure and which
factors (Cracraft, 1968).
presumably did so early in vertebrate
The establishment of contact of two pre- evolution would include: rate of growth of
346
BRIAN K. HALL
the bone, rate of growth of the rest of the the vertebrate lineage, make such attempts
functional unit, degree of remodeling of difficult, and indeed, there may be no basis
the bone, muscle attachments, degree of in fact for such evolutionary trends. The
vascularization, mechanical factors, size of earliest vertebrates possessed highly
the animal, habitat (terrestrail, aquatic, ae- specialized skeletal tissues and left evirial), length of the development period, age dence, in the form of tissues intermediate
at maturity, seasonal feeding cycles, and between those recognized as discrete endpoints (bone, cartilage, dentine) that their
hormonal milieu.
When we say that no major structural cells were highly adaptive even at the outset
advances were associated with the evolution of vertebrate evolution.
of the skeleton and that a phylogeny of
The readiness with which skeletal tissues
bone or of cartilage, involving progressive respond to epigenetic factors provides conadvancement with time cannot be estab- siderable insight into the mechanisms
lished, we do not wish to imply that no whereby the skeleton and its associated soft
changes occurred with time. Moss (1964a) tissues may have evolved. The skeletal tishas listed four trends in the evolution of the sues are supremely pre-adapted to exploit
skeleton: decrease in the range of skeletal new environmental pressures. Contypes; decrease in the amount of bone per vergence and divergence ought to be exanimal; decrease in the number of bones pected and according to Jollie (1968) ocper animal; and a more restricted location curred in the initial, separation of the
of bone (e.g., decrease in dermal armor). gnathostomes from the agantha. The
Note that none of these involve increased modulation of the scleroblasts to cartilage
specializations of the skeletal tissues; if any- facilitated rapid embryonic growth. Modthing they reflect dimunition of skeletoge- ulation to bone facilitated storage of essennesis with time. Moss also lists three factors tial ions, bearing of increased weight, and
associated with evolution above the fish the transition to land. These are some of
level which he considers as possibly respon- the evolutionary consequences of skeletal
sible for the above changes: the skeleton differentiation.
becomes weight bearing, homeostatic for
calcium and phosphorous, and a source of
NOTE ADDED IN PROOF
hematopoeitic cells.
Recently, le Lievre (1974) has shown that
both bone and cartilage of the avian visceral
CONCLUDING COMMENTS
skeleton are derived from ectomesenchyme
of the neural crest.
The skeletal tissues of both fossil and recent vertebrates form a coherent, interrelated class, according to a number of
criteria: embryological development, proREFERENCES
duction of extracellular matrices consisting
of a fibrous protein and a carbohydrate Alexander, R. McN. 1975. Evolution of integrated
design. Amer. Zool. 15:419-425.
component, ability to calcify the matrix, the
S. P. 1967. A survey of shark hard parts.
potential to modulate to other cell types, Applegate,
Pages 37-67 in P. W. Gilbert, R. F. Mathewson, and
high degree of responsiveness to modificaD. P. Rail, eds., Sharks, skates and rays. Johns Hoption by environmental factors. There is,
kins Press, Baltimore.
therefore, justification in speaking of Barnett, C. H., and O. J. Lewis. 1958. The evolution of
some traction epiphyses in birds and mammals. J.
skeletal tissues as closely related to one
Anat. 92:593-601.
another and this has led to attempts to es- Beecher,
W. J. 1950. American Orioles. Wilson Bull.
tablish phylogenies of skeletal tissues. The
62:51-86.
data presented in this paper indicate that Beecher, W. J. 1951. Adaptation to food-getting in the
American blackbirds. Auk 68:411-440.
the very high degree of plasticity shown by
these cells, their readiness to modulate in Beer, G. de. 1958. Embryos and ancestors. 3rd ed.
Oxford Univ. Press, London.
response to epigenetic factors, and their Berrill,
N.J. 1955. The origin of the vertebrates. Oxearly appearance as specialized tissues in
ford Univ. Press, London.
SKELETAL DIFFERENTIATION AND EVOLUTION
347
54:803-822.
Biltz, R. M., and E. D. Pellagrino. 1969. The chemical
anatomy of bone. 1. A comparative study of bone Enlow, D. H. 1969. The bone of reptiles. Pages 45-80
in C. Cans, ed., Biology of the Reptilia. Vol. 1.
composition in sixteen vertebrates. J. Bone Joint
Academic Press, New York.
Surg. 51A:456-466
Bock, W. J. 1960a. The palatine process of the pre- Enlow, D. H., and S. O. Brown. 1956. A comparative
histological study of fossil and recent bone tissues.
maxilla in the Passeres. A study of the variation,
Part I. Texas J. Sci. 8:405-443.
function, evolution and taxonomic value of a single
character throughout an avian order. Bull. Mus. Enlow, D. H., and S. O. Brown. 1957. A comparative
histological study of fossil and recent bone tissues.
Comp. Zool. 122:361-488.
Part II. Texas J. Sci. 9:186-214.
Bock, W. J. 19606. Secondary articulation of the avian
mandible. Auk 77:19-55.
Enlow, D. H. and S. O. Brown. 1958. A comparative
histological study of fossil and recent bone tissues.
Bock, W. J. 1964. Kinetics of the avian skull. J. MorPart III. Texas J. Sci. 10:187-230.
phol. 114:1-42.
Bock, W. J., and H Morioka. 1968. The ecthemoid- Felts, W. J. L. 1961. In vivo implantation as a technique
in skeletal biology. Int. Rev. Cytol. 12:243-302.
mandibular articulation in some Meliphagidae
(Aves). Amer. Zool. 8:808. (Abstr.)
Felts, W. J. L., and F. A. Spurrell. 1966. Some structural and developmental characteristics of cetacean
Bock, W. J., and G. von Wahlert. 1965. Adaptation
(Odontocete) radii. A study of adaptive osand the form-function complex. Evolution 19:269teogenesis. Amer. J. Anat. 118:103-134.
299.
Brul, E. L. du. 1964. Evolution of the temporoman- Friant, M. 1959. Sur l'ossification enchondrale du cardibular joint. Pages 3-27 in B. G. Sarnat, ed., The
tilage de Meckel chez les Rongeurs. Bull. GR. Int.
temporomandibular joint. C. C. Thomas, SpringRes. Sci. Stomatol. 4:1-11.
field, 111.
Friant, M. 1964. Sur l'ossification du cartilage de MecBurton, P. J. K. 1973. Structure of the depressor mankel d'une chauvre-souris, le grand Murin (Chiropdibulae muscle in the Kokako Callaeas cinerea. Ibis tera), Myotis myotis (Borkh). Acta Anat. 57:66-71.
115:138-139.
Friant, M. 1966. Vue d'ensemble sur revolution du
Carriker, M. R., P. Person, R. Libbin, and D. van
"cartilage de Meckel" de quelques groupes de
Zandt. 1972. Regeneration of the proboscis of
Mammiferes. Acta Zool. 47:67-80.
muricid gastropods after amputation, with em- Friant, M. 1968. L'evolution de cartilage de Meckel du
phasis on the radula and cartilages. Biol. Bull.
pore (Sus scrofa dom., Gray). Ann. Fac. Med. Vet.
143:317-331.
Pisa 21:1-17.
Chalmers, J. 1965. A study of some of the factors Friant, M. 1969. L'evolution du cartilage de Meckel du
controlling growth of transplanted skeletal tissue.
cheval (Equus caballus L.). Ann Fac. Med. Vet. Pisa
Pages 177-184 in L. J. Richelle and M. J. Dal22:253-275.
lemagne, eds., Proc. 2nd Eur. Symp. Calcified Tis- Frommer.J., and M. R. Margolies. 1971. Contribution
sues. Coll. des Colloq. de l'Univ. de Liege.
of Meckel's cartilage to ossification of the mandible
in mice. J. Dent. Res. 50:1260-1267.
Chalmers, J., and R. D. Ray. 1962. Transplantation
immunity in bone homografting. J. Bonejoint Surg. Goedbloed, J. F. 1964. The early development of the
44B:149-164.
middle ear and the mouth cavity. A study of the
Coulombre, A. J., and E. S. Crelin. 1958. The role of
interaction of processes in the eqithelium and the
the developing eye in the morphogenesis of the
mesenchyme. Arch. Biol. 75:207-244.
avian skull. Amer. J. Phys. Anthropol. 16:25-37.
Griineberg, H. 1963. The pathology of development.
A study of inherited skeletal disorders in animals.
Cracraft, J. 1968. The lacrimal-ectethmoid bone comBlackwell Scientific Publications, Oxford.
plex in birds: a single character analysis. Amer. Midland Natur. 80:316-359.
Haines, R. W. 1969. Epiphyses and sesamoids. Pages
Crelin, E. S., and W. E. Koch. 1967. An autoradiogra81-116 in C. Cans, ed., Biology of the Reptilia.
phic study of chondrocyte transformation into
Academic Press, New York.
chondroclasts and osteocytes during bone forma- Haines, R. W., and A. Mohuiddin. 1968. Metaplastic
tion in vitro. Anat. Rec. 158:473-484.
bone. J. Anat. 103:527-538.
Denison, R. H. 1963. The early history of the verte- Hall, B. K. 1967. The formation of adventitious cartibrate calcified skeleton. Clin. Orthop. Related Res.
lage by membrane bones under the influence of
31:141-152.
mechanical stimulation applied in vitro. Life Sci.
6:663-667.
Drachman, D. B., and L. Sokoloff. 1966. The role of
movement in embryonic joint development. De- Hall, B. K. 1968. In vitro studies on the mechanical
velop. Biol. 4:401-420.
evocation of adventitious cartilage in the chick. J.
Ede, D. A. 1971. Control of form and pattern in the
Exp. Zool. 168:283-306.
vertebrate limb. Symp. Soc. Exp. Biol. 25:235-254. Hall, B. K. 1970. Cellular differentiation in skeletal
tissues. Biol. Rev. Camb. Philos. Soc. 45:455-484.
Enlow, D. H. 1966. An evaluation of the use of bone
histology in forensic medicine and anthropology. Hall, B. K. 1971. Histogenesis and morphogenesis of
Pages 93-112 in F. G. Evans, ed., Studies on the
bone. Clin. Orthop. Related Res. 74:249-268.
anatomy and function of bone and joints. Hall, B. K. 1972. Immobilization and cartilage transSpringer-Verlag, Heidelberg.
formation into bone in the embryonic chick. Anat.
Enlow, D. H. 1968. Wolffs law and the factor of arRec. 174:391-403.
chitectonic circumstance. Amer. J. Orthodont. Hall, B. K., and H. N. Jacobson. 1975. The repair of
348
BRIAN K. HALL
fractured membrane bones in the newly hatched
chick. Anat. Rec. 181:55-70.
Halstead, L. B. 1969a. Calcified tissues in the earliest
vertebrates. Calcified Tissue Res. 3:107-124.
Halstead, L. B. 196%. The pattern of vertebrate
evolution. Oliver and Boyd, Edinburgh.
Hamburger, V., M. Balaban, R. Oppenheim, and E.
Wenger. 1965. Periodic motility of normal and spinal chick embryos between 8 and 17 days of incubation. J. Exp. Zool. 159:1-14.
Hammond, W. S.,andC. L. Yntema. 1964. Depletions
of pharyngeal arch cartilages following extirpation
of cranial neural crest in chick embryos. Acta Anat.
56:21-34.
Hancox, N. M. 1972. Biology of bone. Cambridge
Univ. Press, Cambridge.
Hay, E. D. 1964. Secretion of a connective tissue protein by developing epidermis, Pages 97-116 in W.
Montagna and W. Lobitz, Jr., eds., The epidermis.
Academic Press, New York.
Ho, T. Y. 1967. The amino acids of bone and dentine
collagens in Pleistocene mammals. Biochim.
Biophys. Acta 133:568-573.
Holtfreter, J. 1968. Mesenchyme and epithelia in inductive and morphogenetic processes. Pages l-30m
R. Fleischmajer, ed., Epithelia-mesenchymal interactions. Williams and Wilkins, Baltimore.
Holtrop, M. E. 1966. The origin of bone cells in endochondral ossification. Calcined Tissues, Proc.
Eur. Symp. 3:32-36.
Holmgren, N. 1940. Studies on the head infishes.Acta
Zool. 21:51-267.
Holmgren, N. 1942. Studies on the head in fishes. Part
III. The phylogeny of elasmobranch fishes. Acta
Zool. 23:129-261.
Horstadius, S. 1950. The neural crest. Oxford Univ.
Press, London.
Howell, J. A. 1917. An experimental study of the effects of stress and strain on bone development.
Anat. Rec. 13:233-252.
Jacobson, W., and H. B. Fell. 1941. The developmental mechanics and potencies of the undifferentiated
mesenchyme of the mandible. Quart. J. Microscop.
Sci. 82:563-586.
Jarvik, E. 1959. Dermal fin-rays and Holmgren's principle of delamination. Kgl. Svenska Vetenskapsakad
Handl. 6:3-49.
Johnson, M. C. 1966. A radioautographic study of the
migration and fate of cranial neural crest cells in the
chick embryo. Anat. Rec. 156:143-156.
Johnson, M. C., and M. A. Listgaren. 1972. Observations on the migration, interaction, and early differentiation of orofacial tissues. Pages 55-80 in H. C.
Slavkin and L. A. Bavetta, eds., Developmental aspects of oral biology, Academic Press, New York.
Jollie, M.C. 1968. Some implications of the acceptance
of a delamination principle. Pages 89-107 in T. 0rvig, ed., Current problems of lower vertebrate
phylogeny. Almquist and Wiksell, Stockholm.
Jollie, M.C. 1971. Some developmental aspects of the
head skeleton of the 35-37mm Squalus acanthias
foetus. J. Morphol. 133:17-40.
Kobayashi, S. 1971. Acid mucopolysaccharides in calcified tissues. Int. Rev. Cytol. 30:257-371.
Koch, W. E. 1972. Tissue interaction during in vitro
odontogenesis. Pages 151-164 in H. C. Slavkin and
L. A. Bavetta, eds., Developmental aspects of oral
biology. Academic Press, New York.
Kolliker, A. 1859. On the different types in the microstructure of the skeleton of the osseous fishes. Proc.
Roy. Soc. London 9:656-668.
Lash, J. W. 1968. Chondrogenesis: genotypic and
phenotypic expression. J. Cell Physiol. 72 (Suppl.
l):35-46.
le Lievre, C. 1974. Role des cellules mesectodermiques
issues des cretes neurales cephaliques dans la formation des arcs branchiaux et du squelette visceral.
J. Embryol. Exp. Morphol. 31:453-477.
McLean, F. C, and M. R. Urist. 1968. Bone. Fundamentals of the physiology of skeletal tissues. 3rd ed.
Univ. Chicago Press, Chicago.
MacConaill, M. A. 1973. Calcopheritic calcification of
cartilage. J. Anat. 115:23-28.
Maderson, P. F. A. 1975. Embryonic tissue interactions as the basis for morphological change in evolution. Araer. Zool. 15:315-327.
Mereel, M. 1967. Recherches sur la relation inductrice
entre chondrocytes et perioste dans le tibia erabryonnaire du poulet. Arch. Biol. 78:145-166.
Mathews, M. B. 1966. The molecular evolution of
cartilage. Clin. Orthop. Related Res. 48:267-283.
Mathews, M. B. 1967. Macromolecular evolution of
connective tissues. Biol. Rev. Camb. Philos. Soc.
42:499-551.
Mathews, M. B. 1971. Comparative biochemistry of
chondroitin sulphate-proteins of cartilage and
notochord. Biochem. J. 125:37-46.
Matsumura, T. 1972. Relationship between amino
acid composition and differentiation of collagen.
Int. J. Biochem. 3:265-274.
Mawdsley, R.,andG. A. Harrison. 1963. Environmental factors determining the growth and development of whole bone transplants. J. Embryol. Exp.
Morphol. 11:537-547.
Moffett, B. C, Jr., L. C.Johnson, J. B. Macabe, and H.
C. Askew. 1964. Articular remodelling in the adult
human temporomandibular joint. Amer. J. Anat.
115:119-142.
Monson, J. W., and W. J. L. Felts. 1961. Transplantation studies of factors in skeletal organogenesis. II
The response of the immature mouse humerus to
longitudinal compressive forces. Amer. J. Phys. Anthropol. 19:63-67.
Moore, W. J. 1967. Muscular function and skull
growth in the laboratory rat (Rattus norvegicus). J.
Zool. (London) 152:287-296.
Moss, M. L. 1957. Experimental alteration of sutural
area morphology. Anat. Rec. 127:569-590.
Moss, M. L. 1961a. Studies on the acellular bone of
teleost fish. 1. Morphological and systematic variation. Acta Anat. 46:343-362.
Moss, M. L. 19616. The initial phylogenetic appearance of bone: an experimental hypothesis. Trans. N.
Y. Acad. Sci. 23:495-500.
Moss, M. L. 1963. The biology of acellular teleost
bone. Ann. N. Y. Acad. Sci. 109:337-350.
Moss, M. L. 1964a. The phylogeny of mineralised
tissues. Int. Rev. Gen. Exp. Zool. 1:297-331.
Moss, M. L. 19646. Development of cellular dentin and
lepidosteal tubules in the bowfin, Amia calva. Acta
SKELETAL DIFFERENTIATION AND EVOLUTION
Anat. 58:333-354.
Moss, M. L. 1965. Studies of the acellular bone of
teleost fish. V. Histology and mineral homeostasisof
fresh-water species. Acta Anat. 60:262-276.
Moss, M. L. 1968a. Bone, dentin and enamel and the
evolution of vertebrates. Pages 37-65 in P. Person,
ed., The biology of the mouth. Amer. Ass. Advan.
Sci., Washington, D.C.
Moss, M. L. 1968A. Comparative anatomy of vertebrate dermal bone and teeth. I. The epidermal coparticipation hypothesis. Acta Anat. 71:178-208.
Moss, M. L. 1968c. The origin of vertebrate calcified
tissues. Pages 359-371 in T. 0rvig, ed., Current
problems of lower vertebrate phylogeny. Almquist
and Wiksell, Stockholm.
Moss, M. L. 1968d. Functional cranial analysis of
mammalian mandibular ramal morphology. Acta
Anat. 71:423-447.
Moss, M. L. 1969. Comparative histology of dermal
sclerifications in reptiles. Acta Anat. 73:510-533.
Moss, M. L. 1970. Enamel and bone in shark teeth:
with a note on fibrous enamel in fishes. Acta Anat.
77:161-187.
Moss, M. L. 1972a. The vertebrate dermis and the
integumental skeleton. Amer. Zool. 12:27-34.
Moss, M. L. 19726. The regulation of skeletal growth.
Pages 127-141 in R.J. Goss.ed., Regulation of organ
and tissue growth. Academic Press, New York.
Moss, M. L., S.Jones, and K. A. Piez. 1964. Calcified
ectodermal collagens of shark tooth enamel and
teleost scale. Science 145:940-942.
Murray, P. D. F. 1926. An experimental study of the
development of the limbs of the chick. Proc. Linnean Soc. N.S.W. 51:187-263.
Murray, P. D. F. 1928. Chorio-allantoic grafts of fragments of the two-day chick, with special reference to
the development of the limbs, intestine and skin.
Aust. J. Exp. Biol. Med. Sci. 5:237-256.
Murray, P. D. F. 1936. Bones. Cambridge Univ. Press,
Cambridge.
Murray, P. D. F. 1954. The fusion of parallel long
bones and the formation of secondary cartilage.
Aust. J. Zool. 2:364-380.
Murray, P. D. F.,and D. B. Drachman. 1969. The role
of movement in the development of joints and related structures: the head and neck in the chick
embryo. J. Embryol. Exp. Morphol. 22:349-371.
Murray, P. D. F., and M. Smiles. 1965. Factors in the
evocation of adventitious (secondary) cartilage in
the chick embryo. Aust. J. Zool. 13:351-381.
Noble, H. W. 1973. Comparative functional anatomy
of temporomandibular joint. Oral Sci. Rev. 2:3-28.
0rvig, T. 1951. Histological studies of placoderms and
fossil elasmobranchs. I. The endoskeleton with remarks on the hard tissues of lower vertebrates in
general. Arkiv. Zool. 2:321-456.
0rvig, T. 1957. Palaeohistological notes. I. On the
structure of the bone tissue in the scales of certain
Palaeonisciformes. Ark. Zool. 10:481-490.
0rvig, T. 1965. Palaeohistological notes. 2. Certain
comments on the phyletic significance of acellular
bone tissue in early vertebrates. Ark. Zool. 16:551556.
0rvig, T. 1967. Phylogeny of tooth tissues: evolution
of some calcified tissues in early vertebrates. Pages
349
45-110 m A. E. W. Miles, ed., Structural and chemical organization of teeth. Academic Press, New
York.
0rvig, T. 1968. The dermal skeleton: general considerations. Pages 373-397 in T. 0rvig, ed., Current
problems of lower vertebrate phylogeny. Almquist
and Wiksell, Stockholm.
Person, P., and D. E. Philpott. 1963. Invertebrate cartilages. Ann. N. Y. Acad. Sci. 109:113-126.
Person, P., and D. E. Philpott. 1969a. The nature and
significance of invertebrate cartilages. Biol. Rev.
Camb. Philos. Soc. 44:1-16.
Person, P., and D. E. Philpott. 19696. The biology of
cartilage. I. Invertebrate cartilages: Limulus gill cartilage. J. Morphol. 128:67-94.
Peyer, B. 1968. Comparative odontology. Univ.
Chicago Press, Chicago.
Philpott, D. E., and P. Person. 1970. The biology of
cartilage. II. Invertebrate cartilages: squid head cartilage. J. Morphol. 131:417-430.
Pratt, L. W. 1943. Experimental masseterectomy in
the laboratory rat. J. Mammalogy 24:204-211.
Pritchard, J. J., and A. J. Ruzicka. 1950. Comparison
of fracture repair in the frog, lizard and rat. J. Anat.
84:236-261.
Ricqles, A. de. 1968. Recherches paleohistologiques
sur les os longs des tetrapodes. I. Origine du tissu
osseux plexiforme des dinosauriens sauropodes.
Ann. Paleontol. 54:133-145.
Ricqles, A. de. 1969. Recherches paleohistologiques
sur les os longs des tetrapodes. II. Quelques observations sur la structure des os longs des teriodontes.
Ann. Paleontol. 55:3-52.
Ricqles, A. de. 1972. Recherches paleohistologiques
sur les os longs des tetrapods. III. Titanosuchiens,
dinocephales et dicynodontes. Ann. Paleontol.
58:17-60.
Romer, A. S. 1942. Cartilage an embryonic adaptation. Amer. Nautr. 76:394-404.
Romer, A. S. 1963. The ancient history of bone. Ann.
N. Y. Acad. Sci. 109:168-176.
Romer, A. S. 1964. Bone in early vertebrates. Pages
13-37 in H. M. Frost, ed., Bone biodynamics. Little,
Brown and Co., Boston.
Romer, A. S. 1972. The vertebrate as a dual animalsomatic and visceral. Evol. Biol. 6:121-156.
Schaeffer, B. 1961. Differential ossification in the
fishes. Trans. N. Y. Acad. Sci. 23:501-505.
Schaeffer, B., and D. E. Rosen. 1961. Major adaptive
levels in the evolution of the actinopterygian feeding mechanism. Amer. Zool. 1:187-204.
Schowing, J. 1968a. Influence inductrice de I'encephale embryonnaire sur le developpement du
crane chez le poulet. I. Influence de l'excision des
territoires nerveux anterieurs sur le developpement
cranien.J. Embryol. Exp. Morphol. 19:9-22.
Schowing, J. 19686. Influence inductrice de 1'encephale embryonnaire sur le developpement du
crane chez le poulet. II. Influence de l'excision de la
chorde et des territories encephaliques moyen et
posterieur sur le developpement cranien. J. Embryol. Exp. Morphol. 19:23-32.
Schowing, J. 1968c. Influence inductrice de l'encephale embryonnaire sur le developpement du
crane chez le poulet. III. Mise en evidence du role
350
BRIAN K. HALL
inducteur de l'encephale dans l'osteogenese du Urist, M. R. 1962. The bone-body fluid continuum:
calcium and phosphorous in the skeleton and blood
crane embryonnaire du poulet. J. Embryol. Exp.
of extinct and living vertebrates. Perspect. Biol.
Morphol. 19:83-94.
Med. 6:75-115.
Sicher, H. 1966. Orban's oral histology and embryology. 6th ed. C. V. Mosby Co., St. Louis.
Urist, M. R. 1963. The regulation of calcium and other
ions in the serums of hagfish and lampreys. Ann. N.
Simonetta, A. M. 1960. On the mechanical implicaY. Acad. Sci. 109:294-311.
tions of the avian skull and their bearing on the
evolution and classification of birds. Quart. Rev. Urist, M. R. 1964. Further observations bearing on the
Biol. 35:206-220.
bone-body fluid continuum: composition of the
skeleton and serums of cyclostomes, elasmobranchs,
Smith, H. M. 1947. Classification of bone. Turtox
and bony vertebrates. Pages 151-180 in H. M. Frost,
News 25:234-236.
ed., Bone biodynamics. Little, Brown and Co., BosSmith, H. W. 1961. From fish to philosopher. Doubleton.
day, N.Y.
Sprinz, R. 1965. A note on the mandibular intra- Urist, M. R. 1970. Induction and differentiation of
articular disc in the joints of Marsupialia and
cartilage and bone cells. Pages 504-528 in O. A.
Monotremata. Proc. Zool. Soc. (London) 144:327Schjeide and J. de Vellis, eds., Cell differentiation.
338.
Van Nostrand Publishing Co., Amsterdam.
Tarlo, L. B. H. 1964. The origin of the bone. Pages Wake.D. B.,andR. Lawson. 1973. Developmental and
3-17 in H. J. J. Blackwood, ed., Bone and tooth.
adult morphology of the vertebral column in the
Pergamon Press, Oxford.
Plethodontid salamander Eurycea bislineata, with
comments on vertebral evolution in the amphibia. J.
Thompson, D'A. W. 1917. On growth and form. CamMorphol. 139:251-300.
bridge Univ. Press, Cambridge.
Travis, D. F., C. J. Francois, L. C. Bonar, and M.J. Yasuda, Y. 1973. Differentiation of human limb buds
in vitro. Anat. Rec. 175:561-578.
Glimcher. 1967. Comparative studies on the organic
matrices of invertebrate mineralized tissues. J. Ul- Zangerl, R. 1966. A new shark of the family Edestidae,
trastruct. Res. 18:519-550.
Ornkhoprion hertwigi. Fieldiana Geol. 16:1-43.