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zoological Journal o f t h e Linnean Society (1990), 100 73-100. With 16 figures
The syncranial osteology of the southern eelcod family Muraenolepididae, with comments
on its phylogenetic relationships and on the
biogeography of sub-Antarctic gadoid fishes
G. J. HOWES
Department of <oology, British Museum (Natural History), Cromzerell Road, London
SW75BD
Received July 1989, accepted for publication Januav 1990
The monogeneric gadoid family Muraenolepididae has been neglected both taxonomically and
anatomically. Without good evidence it has been the opinion of most authors that the family
possesses mainly primitive features. A comparison of the syncranial osteology with that of other taxa
shows that this can be refuted and that Muraenolepis is a relatively derived ‘higher’ gadoid, having its
relationships close to the Phycidae and Gadidae. The circum-Antarctic distribution of Muraenolepk is
unique within gadoids and poses the question of its origin. This is discussed with reference to bipolar
distributions exhibited by some other gadoid families. I t is concluded that gadoids have evolved with
the Atlantic continental shelves and their distribution (and bipolarity) is a consequence of the
geological processes which have formed the Atlantic Ocean.
K E Y WORDS:-Gadoidei
~
Muraenolepis - osteology
-
sub-antarctic biogeography
-
bipolarity.
CONTENTS
Introduction . . . . . . . . . . . . . . .
Material and methods . . . . . . . . . . . .
The syncranium of Muraenolepis and comparisons with other gadoids.
Cranium.
. . . . . . . . . . . . .
Derived cranial features . . . . . . . . . .
Suspensorium and palato-pterygoid bones . . . . . .
Derived suspensorial features . . . . . . . . .
Jaws.
. . . . . . . . . . . . . .
Discussion,
. . . . . . . . . . . . . . .
. . . .
The phylogenetic position of Muraenolepididae
Biogeography . . . . . . . . . . . . .
Acknowledgements
. . . . . . . . . . . .
. . . . . . . . . . . . . . .
References
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INrRODUCTION
The genus of southern eel-cods, Muraenolepis was described by Gunther (1880)
from two specimens of M . marmoratus Gunther collected by HMS Challenger from
Kerguelen Island. At first placed by Gunther in the Gadidae, Regan (1903) later
recognized certain unique features which caused him to refer the genus to its
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73
0 1990 The Linnean Society ofLondon
G . J . HOWES
74
TABLE
1. Numbers of vertebrae, dorsal and anal rays, and gillrakers (on inner side of 1st arch) in
Muraenolepis species
Species
.W rnarnioratus it\pe,
.M microp3 * 1 n7 1
A1 rnirrocephalus (type;
orangtensis I n 1 1
1:ertrbrae
Dorsal rays
Anal rays
Gillrakers
70
66-70
82
75
140
129-137
I78
171
I04
96-107
I35
I50
I1
9-10
9
9
*Fahay & Markle (3984) give for M .microps 67-69 vertebrae, 127-141 dorsal and 98-1 12 anal fin rays.
Pectoral fin rays number 37-38 in all species and are the highest numbt-r of any gadoid.
own family. Svetovidov ( 1939) considered those characters sufficiently distinctive
to recognize a suborder for the Muraenolepididae, a category which has been
acknowledged by subsequent authors (e.g. Cohen, 1984).
In Svetovidov’s ( 1 939) opinion, the Muraenolepididae represent the “. . . most
generalized family of all Gadiformes”, a view shared by most subsequent
authors. For example, Cohen (1984) remarks that “Muraenolepis is not obviously
related to any other gadiform and appears to represent an ancient lineage”.
In attempting to provide a cladistic framework for gadoid relationships,
Howes (1989, 1990) treated the Muraenolepididae as a n advanced member of
the Gadoidei. This assumption was made principally on an analysis of
myological and arthrological characters (based in part on Howes, 1988).
The few published anatomical data for Muraenolepis comprise: Regan’s (1903)
brief diagnosis; Svetovidov’s (1939) short description of the skull and pectoral
girdle; Fahay & Markle’s (1984) figure of the caudal fin skeleton; and Howes’
(1987, 1988) respective descriptions of the palatine bone and the cranial muscles.
During previous studies on gadoid anatomy (Howes, 1987, 1988) certain
osteological features of the muraenolepidoid skull were recognized as unique or
unusual and its is one purpose of this paper to present a detailed description of
them together with a comparative analysis of those features amongst other
gadoids.
Muraenolepis currently contains four species; M . marmoratus Gunther 1880
(Fig. l ) , the type species of the genus from Kerguelen Island and M . microps
Lonnberg, 1905 from the Falklands, South Shetlands, South Georgia and
Balleny Islands, and the Ross Sea shelc M . orangiensis Vaillant, 1888 from
Patagonia and Kerguelen Is., M . microcephalus Norman, 1937, known only from
the type taken at 63” 51’S, 54” 16’E. The distinctions between the species have
been given by Norman (1937) and depend on differences in body depth, head
length and length of the first dorsal fin ray. There are, however, differences in
vertebral and dorsal and anal fin ray numbers which distinguish the species
(Table 1). The snout length (Fig. l ) , mouth shape and dentition of M . marmoratus
also differ from those of the other three species (Fig. 12G). Virtually nothing is
known of the species’ biology. Only incidental observations have been made on
habitat and food (Burchett et al., 1983); eggs and larvae have been described by
Efremenko ( 1983), Fahay & Markle ( 1984) and Gon ( 1988). Muraenolepis species
are apparently inshore fishes occurring over the Continental Shelf in depths
recorded between 10-400 m; the type of M . microcephalus was caught in midwater open-ocean nets (Norman, 1937).
SYNCRANIAL OSTEOLOGY
%
-
75
-
Figure 1 . A, Muraenolepis microps, drawing o f a specimen 275 mm TL (BMNH 1986.7.10 : 2; South
Georgia). B, C, Comparisons between the heads of Muraenolepis microps (B) of 160mm T L and
M . marmoratus (C) of 1543 mm TL (scale pattern shown above for M . microps). Note differences in
eye-size, snout length and shape, development of sensory papillae at tip of lower jaw and extent of
gill-openings.
MATERIAL AND METHODS
The following descriptions are based almost entirely on dry skeletal
preparations of Muraenolepis microps, prepared by dissection. Attempts at
preparing cleared and stained material were largely unsuccessful owing to the
intensively fatty and tenacious nature of the flesh and connective tissue. For
comparison, crania of the following gadoid genera were examined: Bathygadus,
Brosme, Ciliata, Rhinonemus, Euclichthys, Gadomus, Gaidropsarus, Gadus, Lepidion, Lota,
Macruronus, Merlangius, Merluccius, Molva, Phycis, Physiculus, Pollachius, Raniceps,
Trisopterus, Urophycis (all dry preparations). In addition, the material listed in
Howes (1988) was utilized.
THE SYNCRANIUM OF MURAENOLEPZS AND COMPARISONS WITH OTHER GADOIDS
Cranium
(Figs 2-5)
In its overall dorsal outline, the cranium is roughly an equilateral triangle
and, in this respect, is typical of some members of the Gadidae (cf. Eleginus,
Pollachius, Trisopterus; see Svetovidov, 1948). There are no prominent crests and
the laminae which roof the frontal and pterotic sensory canals lie flush with the
cranial surface. Neither is there a deep or elaborate anterior frontal depression
(mucosal cavity of Svetovidov, 1948) which is the hall-mark of the Gadidae.
76
G. J. HO\I'ES
~~
ex0
\
-a"a
Figure 2. .Ifurtittiolepi.! m r t r o p ~ ,cranium in dorsal view: left, ethrnoid region, right, occipital region.
.Abhre\ iations: m a - 'arrcssory' neural arch. bo- basioccipital, epo- rpioccipital, exo- rxorripital,
fr- frontal. ic- intercalar. le- lateral ethmoid. pa- parietal, ptr- pterotir, ptt- posttrmporal,
rc- rostrodermosupr;rcthni~~id.
so- supraoccipital. sp- autosphenotic, vo- vomcr. Scale bar in mm.
r.
1here is no prominent supraoccipital crest; any existing crest is confined to the
posterior slope of that bone.
The cranium is deep, being 30°, of its length (measured through its orbital
depth). Since the cranial roof remains virtually horizontal for its entire length,
the depth of the ethmoid region is nearly the same as that of the otic. The
ethmoid bloc is strongly ossified and has a posterior extension of the mesethmoid
which contacts the parasphenoid. The anterior face of the ethmoid is narrow and
vertical; a thin layer of ethmoid cartilage extends posteriorly to fill an anterior
fissure of the parasphenoid. A prominent process extends from the lateral face of
the mesethmoid bloc, to which attaches the palatine ligament. The lateral
ethmoids have thin, cartilaginous \rentromedial lamina which are separated from
one other in the midline by the ethmoid cartilage. T h e lateral wing of each bone
is in the form of a stout strut whose outer process is thick, truncate and indented.
The vomer has a broadly flared and exceptionally thick head; i t is edentulous.
The jrontals are fused together and form a convex surface. Anteriorly each
frontal narrows and is indented where it meets the nasal. Posteromedially, the
bones rise to form a slight crest where they join the supraoccipital. The nasal
bone is an elongate, open channel, sloped anteroventrad. Its ventral margin lies
in the same horizontal plane as the base of the lateral ethmoid (Fig. 3 ) .
Posteriorly, the nasal is attached to the mesethmoid and anteriorly to the 1st
infraorbital (lachrymal) by strong connective tissue. I n preserved specimens the
nasals are filled with a fat-like substance.
SYNCRANIAL OSTEOLOGY
77
fr
---
I
I
I
mec
vo
I
PS
.n..r
I
vo
I
io
Figur? 3. Muraenolepis microps, ethmovomerine region; above, lateral view of'skeletal elements, below,
showing superficial elements and position of nasal organ. Abbreviations: io- infraorbital (lachrymal),
me- mesethmoid, mec- mesethmoid cartilage, na- nasal bone, nr- nasal rosette, ps- parasphenoid;
others as in previous figure.
The pterosphenoid is a small, oblong, lamellate bone curving somewhat mesad
where it meets the frontal (Figs 4, 5). The autosphenotic is elongate and dorsally
occupies only a small area between the frontal and pterotic (Figs2, 4, 5); its
posterolateral portion contains much of the anterior part of the hyomandibular
fossa. Both the pterotic and parietal are large and together form the majority of the
occipital cranial roof (Figs ZB, 4). The ventrolateral face of the pterotic bears th.posterior part of the hyomandibular fossa. The epioccipital has a rounded
posterior margin and slopes outward to meet the intercalar; medially it contacts
the exoccipital and supraoccipital (Figs 4,5).
The parasphenoid is narrow for most of its orbital length; anterodorsally there
extends a thick, medial lamina which sutures with the posterior extension of the
mesethmoid (Fig. 3). The ascending process is shallow, with a long, low-angled
slope; the posterior part of the parasphenoid is deep and covers the lateral face of
the basioccipital (Fig. 4).
The prootic is small with a deeply indented anterior margin forming the border
of the optic-trigeminal foramen (see below for further discussion); anterodorsally
78
Pro
I
I
,
PS
ic
bo
I
I
7"
Figure 4. Mura~nolepi~mzcrops, occipital and otic cranial regions in lateral (above) and ventral
(below) views. Abbreviations: fg- glossophayngeal foramen, pro- prootic, pts- pterosphenoid; others
as in Fig. 2 .
fr
PfS
w
P'e
k
IC
W
,
'
A
am
6
bo
Figure 5 Muroenolepzs mtrrops, cranium in A, anterior and B, posterior (B) views Abbrevmtions.
psn- parasphenoid ascending process; others as in previous figures
SYNCRANIAL OSTEOLOGY
79
it bears the lower part of the hyomandibular fossa (Fig. 4). The intercalar is
extensive and is bordered by the basioccipital, prootic, pterotic, parietal,
epioccipital and exoccipital. The glossopharyngeal foramen lies in its posterior
aspect, below the attachment of the posttemporal (Figs 4, 5B).
The lateral face of the basioccipital is covered for much of its length by the
parasphenoid and thus only the posterior part of the bone is visible in lateral
aspect; the central articulatory facet is almost circular. The two lateral facets of
the exoccipitals are small and ventromedially directed. A prominent flange rises
behind the facet and forms the posteroventral extension of a triangular element
overlapping the posterior surface of the exoccipital and which appears to be an
‘accessory’ neural arch (ana, Figs 2, 4, 5).
Derived cranial features
Frontals fused and absence of ventral laminae
Other than in Muraenolepis, fused frontals are encountered only amongst
Phycidae ( Urophycis, Phycis, Rhinonemus) and Gadidae (e.g. Gadus, Trisopterus,
Pollachius) although not occurring in all species of those genera (Svetovidov,
1948; Mujib, 1967).
Unlike those in the majority of gadoids the frontals of Muraenolepis lack any
ventral laminae, a condition shared with the Melanonidae. Ventral frontal
laminae are variously developed, being shallow and widely separated in
Bathygadidae, Steindachneriidae and Euclichthyidae; moderately developed
and posteriorly convergent in Macruronidae and Phycidae (Phycis, Urophycis);
parallel in Merlucciidae and Ranicipitidae, and in Lotidae (Lota) meet the
ascending laminae of the parasphenoid to form paired septa. In the lotid Molua,
the frontal laminae, together with the pterosphenoids form parallel arches
(Fig. 6C), the posterior section of the arch meeting the pterosphenoid and
parasphenoid to form a narrow, walled passage leading to the optic foramen. In
the Gadidae, front ventral laminae curve mesad to meet almost in the midline,
forming an enclosed channel reminiscent of that in some Moridae (cJ Fig. 6A,B).
In most gadoid taxa the olfactory tract runs medial to the lateral ethmoid,
passes ventrally, attaching (via connective tissue) to the ventral margin of the
frontal lamina. The course of the tract then exhibits two conditions: (1) lying
along the outer edge of the lamina before turning inward to the optic foramen,
where the lamina becomes reduced and meets the parasphenoid; (2) lying
medial to the lamina for the remainder of its course and enclosed by a membrane
stretching between the laminae. The second condition is present in the Gadidae.
The condition in the Moridae is often cited as being one whereby the olfactory
tracts are entirely enclosed within an osseous canal formed by medial front
laminae (Svetovidov, 1948; Paulin, 1983; Howes, 1990). However, this is not
entirely correct since the condition within the Moridae appears to be as variable
as that throughout the Gadoidei. For example, in Lepidion eques there is no
ossified canal, each olfactory tract passing into the orbital cavity via a foramen
between the frontal and lateral ethmoid, the tracts lying freely beneath the
frontals; in Halargyreus johnsonii the tracts are partially enclosed by a
membranous septum. In Physiculus the tracts each pass through a canal enclosed
within the respective frontal and meet beneath the centre of the orbital cavity
within a narrow, ventrally closed canal through which they continue into the
G. J . HOM'ES
80
:.-
.
.~
-
-.
Figure 6 . Degrer of drvelopmrnt of the frontal laminar. A. Trtsopleru.\ luscuj (Gadidar). B, Lepidion
fl- fi.onta1
c q w s [Moridaei, both ventral views. C , .%fo/wmohn (Lotidae),lateral view. Abbreviations:
ventral lamina. psk- parasphenoid kerl; others as in pre\-ious figures.
cranial cavity. I n the two species of Physiculus examined, P. brevisculus and
P. argyropastus, there are marked differences in the degree of ossification of the
canal, its structure, and the extent of the lamina extending from beneath the
canal to form the medial septum.
The widespread occurrence of frontal ventral laminae and their association
with the olfactory tract appears to be a characteristic (synapomorphy) of the
Gadoidei (in Macrouroidei there is a medial septum of, apparently, ossified
membrane separating the tracts). Their absence in both Muraenolepididae and
Melanonidae is taken to be a derived, rather than a plesiomorphic situation. In
both taxa, but particularly the Melanonidae (which also has markedly short
frontals), the brain protrudes forward into the orbital cavity and so obviates the
need for long olfactory tract supporting structures.
Reduced sphenotic
Although never a large bone, the sphenotic of gadoids nearly always has a
laterally produced dorsal surface. In the phycids Ciliata and Gaidropsarus,
however, it is longitudinally extended (see TablesXXII and XXIII in
Svetovidov, 1948). In the Moridae and Ranicipitidae there is usually a gap or
81
SYSCRANIAL OSI'EOLOGY
I
re
I
vo
-.-
'\J
pc
I
P O
Figure 7. Rhinonnus cimbriur, antericir part of the cranium in latcral vicw. Abbreviations as in
prcvious figures.
indentation between the frontal and sphenotic and the sphenotic projects beyond
the lateral border of the pterotic as it does in other phycids (Phycis and
Urophycis). In Merlucciidae and Gadidae, the sphenotic is relatively small,
largely overlapped by the frontal, and has its lateral margin confluent with that
of the pterotic.
In Muraenolepis the dorsal surface of the sphenotic is reduced to such an extent
that only a small area is exposed and its lateral margin is confluent with that of
the frontal (Fig. 2B). In its overall morphology, disposition and size it resembles
the sphenotic of the Gadidae more than that of any other family.
The parasphenoid
In Muraenoleflzs a thick median lamina rises from the orbital part of the
parasphenoid to meet the posterior extension of the mesethmoid (Fig. 3A). Many
gadoid taxa have a median ridge along the parasphenoid but it is never
produced to the extent it is in Muramolepis, although in some specimens of
Trisopterus luscur there is a prominent eminence of the medial ridge. Rhinonemus is
exceptional in having a well-developed medial process (Fig. 7) which
antcrodorsally meets the posteriorly extended medial walls of the lateral
ethmoids and in this respect is unlike the condition in Muraenolepis. The median
septum in Rhinonemus consists mostly of lateral ethmoid with the parasphenoid
contributing to only a third of its length. Furthermore, the parasphenoid part of
the septum is broad and appears to consist of two lateral laminae which have
become joined in the midline. Paired, parallel and outwardly directed
parasphenoid laminae which contact the bases of the lateral ethmoids are
common to most gadoids. I n Bathygadidae and Steindachneriidae (and
macrouroids) the lateral laminae are elevated anteriorly, a feature which
Okamura (1989) had incorrectly interpreted as the same condition as in
Muramolepis (see above). In the majority of taxa the parasphenoid is broad and
flat ventrally. Exceptions are Manuronus (Macruronidae) and P h y i s (Phycidae)
in which the keel of the parasphenoid is narrow and near-circular in crosssection, the parallel laminae having curved toward the midline to form a tubelike structure.
82
G. J . HOiYES
In Muraenolepis the medial walls of the lateral ethmoids are separated from one
another across the midline by the ethmoid cartilage, and contact with the
parasphenoid is with that bone’s anterior margin. I n the majority of gadoids, the
lateral ethmoids are also separated across the midline by ethmoid cartilage but
contact with the parasphenoid is between their medial walls and the
parasphenoid lateral laminae. T h e extent of this contact is variable, being
narrow in Melanonidae, Steindachneriidae, Moridae, Macruronidae and more
extensive in Phycidae, Lotidae, Ranicipitidae and Gadidae. I n the latter, there is
often considerable posterior extension of the medial wall. I n the phycids Phycis
and Urophycis the lateral ethmoids contact one another across the midline. I n
Rhinonemus, as described above, although the lateral ethmoids extend
posteriorly along the parasphenoid laminae and closely approach one another,
they are at no point in contact.
Supraoccipital crest
According to Svetovidov ( 1939) Muraenolepis has a prominent supraoccipital
crest. This is not so; there is only a slight median ridge which posteriorly slopes
below the level of the cranial roof (Fig. 4A). The majority of gadoids possess a
well-developed supraoccipital crest but in Ranicipitidae and Lotidae (Lota,
Molva, Brosme) the crest is reduced to a posterior eminence, level with the cranial
roof. I n the physids Rhinonemus and Gaidropsarus the crest is represented by a
slight ridge, while the Bregmacerotidae lack any form of crest.
Optic fenestra
Gadoid fishes, in common with macrouroids, possess a single common opening
for the passage of rectus muscles, cranial nerves and vessels which pierce the
membranous covering, T h e width of the opening is variable, as is the disposition
of the bones surrounding it.
Muraenolepis has an extensive optic foramen which deeply indents the border of
the prootic, occupying half its length (Fig. 4A). I n the Moridae, Euclichthyidae,
Melanonidae, Bathygadidae and Macruronidae the anterior part of the prootic
is directed medially so that the border of the foramen is orientated transversely
(Fig. 8A-D) . Morids also have a transversely directed parasphenoid ascending
process which, together with a descending pterosphenoid lamina provides two
openings, the outer allowing the passage of the trigeminal trunk, the medial for
the optic nerve and rectus muscles (Fig. 8A & Howes, 1989). Among Phycidae
(Phycis, Urophycis), and in Ranicipitidae, the notch lies entirely in the sagittal
plane, although the prootic wall is thickened posterior to the notch (Fig. 8E-F).
Other ‘higher’ gadoids (including all other phycids) have a thin-walled prootic
with the trigeminal notch more deeply incised into its anterior border, as in
Muraenolepis (Fig. 8G-I). I n some of these taxa there occurs a further derived
condition in which a ventral lamina extends from the pterosphenoid to contact
the ascending process of the parasphenoid so forming, as in the Moridae (see
above), two separate foramina for the optic and trigeminal nerves. The taxa
having this condition are Merlucciidae, Lotidae (Molva, Fig. 6C, and Brosme; in
Lota the pterosphenoid lamina remains separated from the parasphenoid) and
Gadidae ( Trisopterus, Fig. 8, and Micromesistius),
a3
SYNCRANIAL OSTEOLOGY
IB
F
D
E
F
G
H
I
A
Figure 8. The size and shape of the optic-trigerninal opening and the dispositions of the surrounding
bones. A, Moridae (Lepidion). B, Euclichthyidae. C, Bathygadidae (Bathygadur). D, Macruronidae.
E, Phycidae (Phycis). F, Ranicipitidae. G , Gadidae (IriSopterus). H, Gadidae (Gadus).
I, Muraenolepididae. Each divided diagram shows lateral (left) and anterior (right) views.
Pterosphenoid- shaded; autosphenotic- hatched; optic-trigeminal opening- black; mt- membranous
tissue; other abbreviations as in previous figures.
Suspensorium and palato-pteryp.oid bones
(Fig. 9)
The hyomandibular is longer than broad and is orientated vertically; its
anterodorsal portion is formed into a large cranial articulatory condyle, its
posteroventral margin contacts the preoperculum and anteroventrally the
metapterygoid and symplectic. The lateral face of the hyomandibular stem is
partially covered by the large, triangular ‘symplectic process’ of the
preoperculum (spp, Fig. 9C).
The fan-shaped body of the quadrate is widely separated from its
ventroposterior spine which lies along the lower limb of the preoperculum. The
quadrate condyle is broad and anteroposteriorly expanded. The symplectic is a
large, broadly triangular bone with a concave posterior margin, widely
separated from the hyomandibular stem and preopercular limb. Dorsally, the
symplectic bears a medial process which lies inside the anteroventral lamina of
the hyomandibular; the posterior margin of the symplectic contacts the vertical
limb of the preoperculum.
The metapterygoid is small and oblong and in the specimen illustrated is
G . J. HOWES
84
hY0
Figurc 9. .Lfum~nolepismil-rops. suspensorial and palaroptcrygoid bones. In A, medial, B, anterior and
C. lateral views. Abbrcviations: ecp- rctopterygoid, rnp- entopterygoid, hyo- hyomandibular,
met- metapterygoid, pal- palatine, po- preopercular, qu- quadrate, spp- symplectic process of
prcopercular. sy- synplectic.
fragmented into two parts (Fig. 9A). The anterior border of the bone is convex
and widely separated from the entoplevgoid; ventrally, contact is with both the
quadrate and symplectic. The entopterygoid is shallow and is in the form of a
thin lamina along the posterodorsal border of the ectoptevgoid which is a wellossified plank-like bone. The palatine is a hammer-head shaped bone, its medial
head being ligamentously attached to the medial part of the lateral ethmoid; the
area between the two heads is tied by connective tissue to the lateral ethmoid
wing, and the lateral head o\,erlies the maxilla (Howes, 1987: fig. 4). The
palatine sits in a right-angled terminal recess of the ectopterygoid (Fig. 9B).
Dericed suspen rorial features
The suspensorium and palatopterygoid series of Muraenolepis are distinctly
modified in many respects when compared with those of other gadoids, viz:
palatine double-headed; entopterygoid reduced; symplectic enlarged; quadrate
with large space between its body and posterior spine; large interosseous space
between symplectic and quadrate-preopercular.
The uniqueness of the muraenolepidid palatine morphology has been
mentioned elsewhere I Howes, 1987). According to Dunn ( 1989), a rounded
lower process of the gadoid hyomandibular is the derived condition, compared
Mith the plesiomorphic pointed process. .+furaenolepis has an almost intermediate
condition in that the process is elongate and truncate. Another feature of the
h) omandibular, which Dunn regarded as derived, is a horizontal preopercular
process, a f'eature lacking in LCluraenolejis. Such a process is present in the
SYNCRANIAL OSTEOLOGY
85
Figure 10. Suspensorial and palatopterygoid bones. A, Lotidae (Lotu), medial view. B, Ranicipitidae
(Ranzceps), lateral view. C, Phycidae (Rhinonamus), medial view. D, Gadidae (Gudur), medial view.
Abbreviations: hpo- hyomandibular preopercular process; others as in previous figures.
Entopterygoid is shaded, metapterygoid vertically hatched.
Bregmacerotidae, Phycidae and Lotidae, but in the Ranicipitidae, Gadidae and
Merlucciidae it is ventrally directed.
A small entopterygoid is not unique amongst gadoids and occurs in the lotids
Lota and Brosme (Fig. 10A); in the phycids Rhinonemus, where it is a shallow
lamina running the length of the ectopterygoid (Fig. lOC), and Gaidropsarus,
where it is a small oblong lamina lying above and separated from the central
part of the ectopterygoid; in Merlucciidae and most Gadidae it is shallow and
confined above the anterior half of the ectopterygoid (Figs IOD, 11D). In the
Bregmacerotidae there is only a single pterygoid element, which from its
disposition between the quadrate border and palatine, identifies it as an
ectopterygoid (Fig. 11B); the entopterygoid thus appears to be lost.
The gadoid symplectic is variable in shape. In Bathygadidae, Melanonidae,
Steindachneriidae,
Moridae,
Euclichthyidae,
Bregmacerotidae
and
Macruronidae, it has a thin, narrow stem and a broad triangular head. In other
taxa the bone varies from triangular to oblong and, apart from Muraenolepis, is
deepest in the phycid Gaidropsarus (where there is no space between it and the
preopercular limb). A large symplectic-preopercular space occurs in the
Macruronidae, Lotidae, Merlucciidae and Gadidae; such is not the case in other
families.
The ectopterygoid of most gadoids is of similar shape to that in Muraenolepis
although in most taxa the posteroventral tip of the bone extends down the
anterior border of the quadrate, a feature absent in Muraenolepis. The largest
ecto- and entopterygoids occur in Bathygadidae, Melanonidae, Moridae,
Steindachneriidae, Euclichthyidae (Fig. 1 1A) and Macruronidae, taxa which
also possess the largest metapterygoids. Although in comparison to these taxa,
the metapterygoid of Muraenolepis is small it is equally proportioned to that found
G . ,J. HOM’ES
86
Figure 1 I . Suspcnsorial and palatopterygoid bones. A, Euclichthyidae (Euclichthys) medial view.
B, Bregmauerotidae (Rregmarerm) medial view. C. Phycidae (Phyris) lateral view. D, Merlucciidae
Merluccius~ lateral view.
in the Phycidae, Lotidae, Ranicipitidae, Gadidae and Merlucciidae. Only in the
Bregmacerotidae is it so reduced that i t fails to contact either the hyomandibular
or symplectic (Fig. 1 1B ) , Dunn ( 1989) considers the sharply pointed, triangular
metapterygoid of Gadus (Fig. 1OD) to represent the plesiomorphic gadoid
condition. My out-group comparisons suggest the contrary, that this
morphotype is derived and can be considered as synapomorphic for the Gadidae.
The gadoid quadrate usually bears a posteroventral spine but it is not
separated, or only slightly, from the body of the bone; Muraenolepis, with the
Lotidae and Gadidae, exhibits the widest separation (cf. Figs 9, 10A,D). Dunn
(1989) considered the length of the posterior process of the quadrate but not its
degree of separation from the body of the bone. His analysis suggested that the
more elongate spine is the more derived. In this respect Muraenolepis has the
plesiomorphic state. T h e anteroposterior expansion of the quadrate condyle in
Muraenolepis is doubtless correlated with the medially extended articular surface
on the anguloarticular.
Contact between the hyomandibular and preopercular is variable amongst
gadoids. The central part of the preopercular extends anterodorsally and
somewhat medially and is referred to as the ‘symplectic process’ since it normally
articulates with the head of the symplectic or with the symplectic cartilage. I n
the Bathygadidae, Steindachneriidae, Melanonidae and most of the Moridae,
the symplectic process contacts the symplectic head and the ventral border of the
hyomandibular stem; the upright limb of the preopercular lies close to or
contacts the posterior edge of the hyomandibular. This condition is also present
SYNCRANIAL OSTEOLOGY
a7
in the Lotidae and Ranicipitidae except that the upright preopercular limb is
widely separated from the posterior margin of the hyomandibular and contacts a
posteroventral process of that bone (hyomandibular-preopercular process,
Fig. lOA,B). I n both the Macruronidae and Merlucciidae the symplectic process
contacts the symplectic cartilage but in the former the preopercular margin
overlaps the posterior border of the hyomandibular, whereas in the latter the
bones are widely separated (Fig. 11D & Howes, 1989). I n the Bregmacerotidae
the preapercular lacks a symplectic process and contacts the entire posterior
margin of the hyomandibular (Fig. 1 1B).
Surface-to-surface contact between the preopercular symplectic process and
the lateral face of the hyomandibular occurs in some of the Moridae, Phycidae
and all of the Gadidae. As in the Muraenolepididae the symplectic process
covers the posteroventral face of the hyomandibular (Figs 10D, 11C). The
dissociation of the preopercular symplectic process with the symplectic cartilage
and its repositioning against the lateral face of the hyomandibular mark this as a
derived feature. Dunn (1989) considered the shape and length of the symplectic
process; rounded and low he identified as plesiomorphic, and pointed and high
as derived; Muraenolepis has the latter.
Gill (1890) drew attention to the modifications of the suspensorium of Raniceps
(one of the set of features which caused him to establish a separate family for the
genus). H e noted particularly the forward inclination of the hyomandibular,
that its articulation was on a line in front of the quadrate, and that the
". . . relation of the hyomandibular, quadrate, symplectic and metapterygoid to
each other and the neighbouring bones" differed from that of other gadoids
known to him (Fig. 10B). In fact the relationships of these bones, one to the
other, is no different from that in any other gadoid (as noted above). It is,
however, unique for the hyomandibular to have a forward inclination, which as
Gill surmised, is probably a correlate of the short cranium of Raniceps.
Whereas in other gadoids the contact zone between the quadrate and
metapterygoid is angled posteroventrally, that in Muraenolepis is horizontal;
furthermore, the synchondrosis is so weak that it is in the nature of a hinge-joint
allowing the pterygoid bones to be directed medially (Fig. 9C). Such a possibility
is not available to other gadoids where the posteroventral stem of the
ectopterygoid abuts against the quadrate margin and offers resistance to any
medial flexure. It is apparent that the wide, posterior bifurcation of the quadrate
has been an essential attribute in bringing about the level metapterygoidquadrate junction.
Jaws
(Fig. 12)
The premaxilla has high ascending and articular processes, the latter being
somewhat shorter than the former (Fig. 12A). The postmaxillary process is
shallow and the distal tip of the maxilla is blunt and outwardly curved. There
are three rows of conical teeth anteriorly, those in the outer row being the larger;
posteriorly, the outer row teeth diminish in size and the row terminates prior to
the tip of the jaw so that the inner rows take on the function of the outer row
(Fig. 12B). The maxilla has a slight concavity just posterior to its head; this is the
88
G. J. HOM’ES
Figure 12. Jaw bones of Muraenolepis microps. A-F, Premaxilla in A, lateral and B, ventral views.
C, Maxilla in lateral view. Lower jaw in D, dorsal, E, medial and F, ventral views. G, dentigerous
area of dentary of M . marmorafus (drawn from alcohol preserved specimen, BMNH 1986.7.10: I . ) .
.4bbreviations: aa- anguloarticular, cm- coronomeckelian bone, de- dentary, pma- premaxillary
articular process, pmp- postmaxillary process of the premaxilla, pms- ascending process of the
premaxilla, ra- retroarticular.
insertion point of adductor muscle A1 b. Posteriorly, the bone is deep, with a
slightly convex dorsal margin and straight posterior margin (Fig. 12C).
The dentary is deep with a blunt anterior tip, and has a narrow dentigerous
surface; ventrally it has 7-8 large openings to the sensory canal (Fig. 12D).
There is an inner row of small, thin, conical teeth, interspersed with a few larger
teeth. An outer row of small, straight unicuspid teeth ends halfway along the
dentigerous surface; anteriorly, near the symphysis, the two rows become
irregular and broaden into a patch of moderately-sized, recurved teeth
(Fig. 12D). Muraenolepis marmoratus has a similar arrangement of dentary teeth to
that of M . microps except that there is a regular arrangement of large teeth in the
inner row and the toothed area is broader (Fig. 12G).
The anguloarticular is long with a blunt posterior process and a relatively short
articular surface (for the quadrate) which extends onto its medial face
(Fig. 12D,E). The retroarticular is an elongate element confined to the ventral
surface of the anguloarticular; the coronomeckelian bone is large and boot-shaped
(Fig. 12E).
In comparison with other gadoids the upper jaw is rather short and the
maxilla has a truncate rather than an indented or produced margin. The
premaxilla lacks a posterior indentation, that is ‘gadoid notch’ of Rosen &
Patterson (1969: 401). Howes (1988) drew attention to the various types of
upper jaw morphology and noted the correlation between the development of
the labial ligament and the stepped form of the premaxilla. According to Dunn
SYNCRANIAL OSTEOLOGY
89
( 1989) a rounded postmaxillary process is the plesiomorphic gadoid condition
whereas the triangular one (of Gadidae) represents the derived condition.
Likewise, Dunn considers a truncate to bifurcate posterior maxillary border as
plesiomorphic and recognizes as secondarily derived the truncate condition
exemplified by Gadiculus. The shortened form of the muraenolepidid maxilla
appears to be secondarily derived. The posterior convex dorsal elevation of the
bone appears to correspond with the anterior or central elevation in the Gadidae
(Fig. 12C).
DISCUSSION
The phylogenetic position of Muraenolepididae
The ‘higher gadoids’ is a group recognized by Howes (1988, 1989, 1990) as
comprising the Macruronidae, Bregmacerotidae, Phycidae, Lotidae,
Ranicipitidae, Gadidae, Merlucciidae and Muraenolepididae, and is diagnosed
primarily on the basis of fused upper hypurals; complex division and medial shift
of the adductor mandibulae musculature; well-developed facet on the
interoperculum for the articulation of the interhyal, and a firm articulation
between the 1st infraorbital (lachrymal) and the posterior or ventral surface of
the lateral ethmoid wing (in Muraenolepis articulation is within the ventral
surface). The Muraenolepididae clearly belongs to this category from its
possession of these characters, but its sister-group relationship is ambiguous and
it was placed with the Ranicipitidae, Phycidae, Gadidae and Merlucciidae as
part of an unresolved polychotomy (Fig. 13; Howes, 1990).
Muraenolepis has several derived syncranial osteological features, four of which
are autapomorphic, namely, absence of frontal ventral laminae; median
parasphenoid septum contacting a posterior extension of the mesethmoid;
cranial part of the parasphenoid dorsally extended, and a double-headed
palatine. Two unique non-osteological features are the anguillid-like scales
(Fig. 2B) and high number of pectoral fin rays (viz 36-38).
Four derived cranial characters are shared with a broad spectrum of taxa,
namely reduced supraoccipital crest, with the Bregmacerotidae, Ranicipitidae,
Lotidae and Phycidae (Rhinonemus and Gaidropsarus) ; reduced sphenotic, with the
Gadidae and Merlucciidae; thin-walled prootic with deeply indented optictrigeminal notch, with the Phycidae (Rhinonemus, Ciliata, Gadiropsarus, ? Motella),
Merlucciidae and Gadidae; and fused frontals with the Phycidae (Phycis,
Urophycis, Rhinonemus) and some of the Gadidae.
A general reduction in size of pterygoid bones is a feature shared by all
‘higher’ gadoids apart from the Macruronidae and a further reduction in
entopterygoid size occurs in the Lotidae, Phycidae, Gadidae and Merlucciidae.
Muraenolepis, Raniceps, Phycis and Urophycis share a direct attachment to the
urohyal of the rectus communis muscle, a feature interpreted as derived by Howes
(1988: 47). I had reported earlier (Howes, 1988) the shared specialization
between Muraenolepis, Phycis, Urophycis and Lota of an opercular insertion of part
of the epaxialis musculature. My interpretation of the epaxial insertion in
Muraenolepis is, however, somewhat flawed since examination of subsequent
specimens has shown that the so-called epaxialis could well be a levator operculi
with a shifted origin. Of two specimens, one has the levator operculi as a single
90
Figurr 13. (:ladogram olgadoid phylogenctir relationships (after Howcs, 1990). In addition
to
those
c.haracters diagnosing 'higher' gadoids, \.iz: fused upper h!-purals; medial shift of adductor mondihulap
musc ulature; interopcrcula~facet for iiiwrhyal; 1st infiaorhital articulation with posteroventral area
of'lateral rthmoid wins. should be added reduction in sizc of pterygoid bones and, long preopercular
procc'\s of' the hyomandibular
text -1'Iir reinterpretation oC the epaxial muscle-operculum
<liars( tcr p. 89: and the presence of an extensive optic-trigrminai notrh in the prootic repositions
t l i c lluraeriolcpididat.~~~~pididae
b e t w r n the 'Phycidae' r Rhtnonrtnus. Goidrop.rarn.t. Cilioto, Moidlnr-Lotidae,
t.
and l l r t l u c & l a e+ Gadidaz.
element whereas in the other, it is derived, the outer part stemming partly from
the posttemporal. Concerning Phycis, Uropfvcis and Lota, however, I believe my
earlier interpretation of opercular insertion of epaxialis to be correct.
Rosen & Patterson i1969: 428) noted that muraenolepidids have a "normal
SYNCRANIAL OSTEOLOGY
91
gadoid skull roof and occipital region and that the caudal skeleton, although
reduced, has the characteristic gadoid dorsoventral asymmetry”. According to
these authors, the jaw muscles “closely resemble those of ophidioids and some
macrouroids”. Although the cranial morphology is characteristic of ‘higher’
gadoids as is that of the caudal skeleton, I would disagree that there is any
resemblance between the jaw muscles of Muraenolepis and ophidioids and
macrouroids. Indeed, the musculature of Muraenolepis is typical of a ‘higher’
gadoid (Howes, 1988).
The caudal fin skeleton ofMuraenolepis is simple in that hypurals 3-5 are fused
with the terminal centrum; hypurals 1 and 2 are also fused together and there is
a single large (possibly two fused) epural(s). Fahay & Markle (1984) identified
the muraenolepidid caudal skeleton as the ‘primitive state’, but it more likely
represents a derived condition in the continuity of its dorsal and anal rays with
the caudal rays. Indeed, the caudal skeleton does not differ significantly from
that of Macruronus, or for that matter any other ‘higher’ gadoid (see Howes, 1990
for discussion).
The muraenolepidids resemble the phycids in having a small dorsal fin. In
Muraenolepis the fin comprises a single ray supported by a feeble radial but with a
full complement of arrector and depressor muscles and innervated by a spinal
nerve branch. I n contrast, the first dorsal fin of the phycids Mutella, Ciliata,
Rhinonemus and Gaidropsarus is a specialized ‘vibratile’ structure lying in a sensory
groove and innervated by the facial nerve (see Whitear & Kotrschal, 1988 for
description and references). Markle ( 1989) relates the Muraenolepididae to the
Bregmacerotidae on the basis of loss of ligamentous connection between the 1st
and 2nd epibranchials and in turn, this ‘clade’ to the Phycidae through the
possession of an elongate first dorsal fin. From these descriptions, however, it is
clear that there is a marked distinction in dorsal fin structure between
muraenolepidids and phycids.
The Muraenolepididae possess a simple swimbladder lacking the anterior
diverticula of phycids and gadids. There is no intimate connection between the
swimbladder wall and the skull as in Phycis and Urophycis where the swimbladder
lies adnate to an intercalar-exoccipital fenestra (similar to the condition in the
Moridae).
Dunn (1989) has presented a series of osteological characters in the Gadidae;
his polarity assignments are derived through out-group comparisons with the
Ranicipitidae, Phycidae (Rhinonemus), Lotidae and Merlucciidae. Of the 28
characters presented, Muraenolepis scores 16 as plesiomorphic (state l ) , and 9 as
derived (8, state2; 1, state3). Two characters (14 & 15, concerning the
preopercular process of the hyomandibular) are not applicable. The remaining
character (No. 17, shape of the metapterygoid), is scored in Muraenulepis as
derived. However, I would question Dunn’s interpretation of this character
polarity since I believe the rounded or truncated metapterygoid is plesiomorphic
and the narrow, triangular element, confined to the Gadidae, is synapomorphic
for that family (see discussion above, p. 89). Using Dunn’s character analysis,
Muraenolepis scores higher than Merluccius (i.e. is more derived).
The irregular distribution of the above detailed apomorphies amongst ‘higher’
gadoids does not provide a succinct dichotomous pattern of taxa. Which of these
features are homoplastic has yet to be determined through further analysis of
other character suites. Nonetheless, it is clear that the Muraenolepididae shares a
92
G . J . HOWES
higher number of apomorphies with the Gadidae, Merlucciidae and some of the
Phycidae than with other taxa, and may stand a5 the sister group to this
assemblage. Howes ( 1990) sought to demonstrate the sister-group relationship of
the Gadidae with the Merlucciidae, which were aligned in a polychotomy with
the Lotidae and Phycidae. One of the features used to unite this assemblage as
the marked attrition of the prootic border, but this also occurs in
Muraenolepididae (see above). The Phycidae were recognized there as
paraphyletic, a view upheld by this study. T o determine more precisely the
phylogenetic relationships of the Muraenolepididae it will first be necessary to
identify those monophyletic lineages within the 'Phycidae' and determine their
relationships within this complex of gadoids.
Biogeography
Those localities sampled indicate that the distribution of the
Muraenolepididae is circum-Antarctic, a possibility considered by De Witt
(1971). Captures of Muraenolepis are from the Falklands Plateau, South Georgia
and South Sandwich Islands (i.e. between the subtropical and Antarctic
convergences) ; Kerguelen Island and the Antarctic shelf area of Balleny Islands,
Ross Sea and off Enderby Land, and also from Indian Ocean sub-Antarctic
islands and seamounts; recorded by Duhamel & Hureau (1982)) Tomo &
Hureau (1985), Hulley et al. (1989) and Gon & Klages (1988) (Fig. 14).
Among gadoids, the Muraenolepididae is unique in that it is the only family
endemic to the Antarctic zone, i.e. south of the Antarctic convergence. Only one
other is endemic to the Southern Ocean (the New Zealand area, or Campbell
Plateau), namely, the Euclichthyidae. Other families of gadoids represented in
the southern temperate regions are the Moridae, Macruronidae, Phycidae,
Gadidae and Merlucciidae, all of which have a bipolar or bitropical distribution.
The Moridae contain eight bipolar genera (Mora, Halargyreus, Lotella,
Physiculus, Laemonema, Letidion, Anlimora and Gadella), a pattern also exhibited at
species level by Antimora rostrata, Mora moro and Halargyreus johnsonii. The
Phycidae has one (bipolar) genus, Urophycis, with two or three species on the
Brazilian shelf, between 35"-40" S. The Gadidae has two (bipolar) genera: (1)
Gaidropsarus, with three species, G. nouaezelandiae (Campbell Plateau), G . capensis
(South African Cape) and G. insularurn (S.W. African coast, Tristan d a Cunha,
Gough and St Paul Islands; see Penrith, 1967 for details of species of their
distribution); and (2) Micromesistius with a single species, M . australis, recognized
by Inada & Nakumura (1975) as comprising two subspecies respectively on the
Campbell Plateau and Patagonian shelf (further specimens reported by these
authors were collected off Chile). The single merlucciid genus Merluccius has four
species which occur throughout the tropics (Inada, 1981). The Macruronidae
has three species of Macruronus which occur off Patagonia, South African Cape
and the Campbell Plateau (Howes, 1990). Before considering in more detail the
distribution of these Southern Ocean families, a brief synopsis of gadoid
distribution follows.
The Bathygadidae, like the Bregmacerotidae, have a tropical-subtropical
distribution; the former between 50"N and 50"s (in the Atlantic), the latter
within 40"N and 40"s.T h e Moridae has a cosmopolitan distribution; of its
eighteen genera, seven are confined to the Southern Ocean. The Melanonidae
SYNCRANIAL OSTEOLOGY
93
Figurc 14. Distribution of Muracnolcpididac; bascd on data from Dc Wit1 (1971), Duhamel &
Hurcau (1982), Tom0 & Hurcau (1985) and Hullcy rr of. (1989).
are recorded as cosmopolitan, occurring antiequatorially in the Atlantic with
one species (Melanonu gmcilzs) being circum-Antarctic. Species also occur in the
western Pacific (recorded by Parin, 1978, from off the Solomon Islands) and
eastern Pacific (off California and British Columbia; Berry & Perkins, 1966;
Peden, 1974). Restricted distributions are exhibited by the Steindachneriidae
(tropical western Atlantic) and Ranicipitidae (European coasts). The Lotidae
are restricted to the temperate Northern Hemisphere and have an amphiAtlantic distribution.
Svetovidov (1948) attempted to reconcile the distribution of the Gadidae (his
concept of which included the Phycidae, Lotidae, Merlucciidae and
Ranicipitidae, thus corresponding to my ‘higher’ gadoids) with the
zoogeographical subregions as proposed by Ortman (1896) and later modified
by Eckman (1953). Like Eckman (1953: 144), he was unable to recognize precise
boundaries of such regions, only ‘transitional areas’. For example, Svetovidov
could not recognize a Mediterranean gadoid group since most species (all but
two) occur (and spawn) in the Atlantic. He noted, however, that unlike most
Atlantic-Mediterranean fishes, gadoids show no relationship with the Indo-west
Pacific.
Svetovidov (1953) noted “The peculiar distribution of the Gadidae is contrary
to the distribution of other (cold-water) fishes.. .”. The pecularity of this
distribution is that the majority of gadoid families are more speciose in the north
94
L J HOI\ tS
Atlantic than in the north Pacific. T h e absence of a n Atlantic-Indo-west Pacific
link in ‘higher’ gadoids appeared to Svetovidov (1953) to be correlated with
climatic factors, but it seems also, and perhaps primarily, to be linked to their
association with the continental shelf and upper part of the continental slope.
There are no continental shelf gadoids in the Indian or western Pacific oceans
i the ‘lower’ gadoids in these oceans, ZYC bathygadids and morids are associated
with the upper and lower continental slopes). Inada (1981) explains the absence
of merlucciids (the onlv shelf gadoids crossing the tropics) in the Indian Ocean
as being due to ecological factors, i.e. high temperature of the Mozambique
current and he proposes an elaborate dispersal pattern to account for their
present-da) distribution). Inada’s concept of dispersal appears to follow the
classical ideas of Darlington (1957) whereby the assumed advanced species lie at
the centre of origin, and furthermore, he has viewed this dispersal against the
background of present-day ecological factors.
Hocutt i 1987) has argued that vicariant events responsible for Indian Ocean
endemism bere the drift of India and its eventual amalgamation with the Asian
marqin. Likewise, corrclated e\ents such as the closure of the Tethys sea in the
Palaeocenr and the opening of the Atlantic might correlate with the phylogeny
of the shelf-dwelling (‘higher’) gadoids in that region. Fedotov & Bannikov
i 1989) point to the origin of distinctive gadoid genera in late Oligocene/early
Miocene, re5ulting from “hydrological changes in the seas of the later Tethys”.
1 hc complcx geomorphologies resulting from the opening of the ‘Atlantic
corridor’ (various papers in Soug) & Rodgers, 1988) undoubtedly produced
continental slope and shelf areas offering a wide variety of new habitats
(I’alentine 8r Moores, 1970, 1972).
Antarctica became the remaining segment of Gondwanaland after the others
had nioked northward ‘misted by rapid sea-floor spreading of the Indian Ocean
‘Norton & Sclater, 19791. AuTtralia broke awa) from Antarctica in the Eocene,
followed by South America during the Oligocene (30 Myr B P ) . Any groups of
fishes associated with Antarctica during this period and which have survived to
the present would be expected to reflect in their distribution the disposition of
these continental associations. ‘Those gadoids with a sub-Antarctic distribution
certainl) do so. The GaidropJarzrJ species occur around the eastern rim of
Ytarctica, outside the con\ ergence; the .Merluccius, Mzcromesislius and Macruronus
species indicate a former range linhing the Campbell Plateau and Patagonia.
There $\as certainl) a continuous link between South America and the Antarctic
Peninsula in the late Slesozoic (which incorporated the Burdwood Bank, South
Georgia and the South Shetlands, but not the Falkland Islands) and which was
disrupted during the early Tertiar) b) the eastward bending of the Scotia Arc,
displacing South Georgia to its present position (Dalziel & Elliot, 1973). T h e
link or track from Patagonia to the Campbell Plateau is one evidenced by other
groups of organisms (Fig. 15; see also Humphries et al., 1986).
The presence of the same Gaidroprarus species on southern Atlantic and Indian
Ocean islands and the South Afi-ican we5t coast needs confirmation, nevertheless,
the presence of the same genus suggests a former]) close geographic association of
these areas. Tristan and Cough islands lie at the end of a chain with Tristan
occupying the location of a hot-spot (Heezen el al., 1972; Morgan, 1981).
Likewise, Prince Edward and St Pauls Islands occupy a hot-spot track.
,4lthough some might explain the distribution of Gaidropsarus in these Southern
r .
‘1
SYNCRANIAL OSTEOLOGY
95
Figure 15. Distribution of ‘higher’ gadoid taxa in the Southerh Ocean. Hatched area off Atlantic
South America- Urop/ycis (Phycidae); dots- G ~ i d r o p s ~ r u s(Gadidae); squares- Micromesittius
(Gadidae); shaded areas- Macrumnus (Macruronidae); dashed lines- extent of Mel-luccius
(Merlucciidae). The connecting lines indicate the number of shared genera. Map drawn on
Hammer transverse elliptical equal-area projection.
~
Ocean localities as the result of dispersal, it apptars that there may be a more
general explanation involving island tracking.
Rather than viewing distributions of gadoids on a map of the present-day
world it seems more promising to view them c)n maps showing the assumed
dispositions of the continents during the Palaeoclene-Neogene periods when the
Atlantic Ocean was forming. O n this basis the Merlucciidae, for example, are
seen to have dispersed through vicariant processed of continental rifting (Fig. 16),
rather than via the elaborate routes postulated qy Inada (1981).
The present-day circum-Antarctic distributioln of the Muraenolepididae is
most likely a reflection of a former Gondwqnic coastal distribution. The
Muraenolepididae is an example of one of tHose groups (others being the
Nototheniiformes and some Ophidiiformes) which have accommodated
relatively rapidly to changing conditions of 1 the progressive isolation of
Antarctica. As pointed out by Grande & Eqstman (1986), the increasing
distance, from South America, Africa and Austrblia, depth and coldness of the
Southern Ocean and development of the circumtAntarctic circulation resulting
in the Antarctic Convergence made coloniza$on from the north virtually
impossible. The only migration route would have been the Scotia Ridge
G . J. HOWES
96
I
Figure 16. Hypothesized phylogeny of the Merlucciidae in the Southern Ocean (hatching) with
respect to the separation of oceanic margins. A, Arrangement of the southern continents in early
Palaeogene. B, In late Palaeogene. C, In mid-Neogene. Abbreviations: Af- Africa, Ant- Antarctic,
Aus- Australia, Ker- Kerguelen Plateau, Mel- Melanesia, NZ- New Zealand. Jagged lines between
the continents indicate rift-transform systems. Base map modified from Heezen et al. (1972).
connecting Patagonia with the western Antarctic Peninsula. De Witt (1971), on
the other hand, has proposed that the Scotia Ridge has provided an eastward
dispersal route and notes that Muraenolepis possibly had a n Antarctic origin and
extended its range to South America. However, in the same paper, he
contradicts Andriashev’s ( 1965) opinion that Muraenolepis is an element of the
original Antarctic fish fauna and proposes that it is a “more recent invader”
since it occurs primarily on the continental slope and not on the inner regions of
97
SYNCRANIAL OSTEOLOGY
the continental shelf. The occurrence of Muraenolepis around su b-Antarctic
islands does not in itself lend support to the idea of invasion since these islands,
for the most part, appear to have had past continental associations with
Antarctica (Morgan, 1981).
Miller’s (1987) view of Antarctic fish distribution combines some of the factors
discussed above, together with others, namely, passive dispersal by tectonic plate
movements; ocean currents and modification to habitats by glaciation and sealevel changes. My views concerning the ‘origin’ of Antarctic fishes are more in
accord with those of Regan (1914) and Andriashev (1965) both of whom
recognized ‘original’ Antarctic faunal elements. However, ‘Antarctica’ must now
be viewed as part of Gondwanaland. Contrary to Regan’s (1914) opinion that
the distinctiveness of the Antarctic ichthyofauna provides no evidence for a
former continuity of continents, as noted by Miller (1987) it is that very
distinctiveness that does so since the evolution of that fauna has been ‘advanced’
by the Continent’s rapid isolation.
At present no more an explicit sister-group relationship for the
Muraenolepidid can be established than the ‘Phycidae’-Gadidae Merlucciidae,
groups which are predominantly northern in their distribution. The hierarchical
pattern of this relationship (i.e. family level) suggests an earlier bipolar
dichotomy than those dichotomies exhibited only by genera among the other
families. On the other hand, the bipolarity might have been contemporaneous in
all the groups and thus the morphological divergence exhibited between
Muraenolepididae and other taxa could be a reflection of its extreme isolation.
The bipolarity exhibited by the gadoid taxa detailed above suggests a
formerly continuous latitudinal distribution (the Merlucciidae of course still
retain such a range). Eckman’s (1953) now classic discussions of bipolarity posit
that the majority of marine bipolar distributions occurred during the greater
part of the Tertiary period. Eckman (1953: 257-8) assumes the cause of
bipolarity to have been climatic change but, at least for higher taxonomic
categories, concedes to “geographical changes” which “influenced the course of
the cold ocean currents or there may have been other causes”. T o Eckman,
bipolarity was a modification of Theel’s (1885) and Ortman’s (1896) dispersal
theories which declared that from centres of origin at high latitudes, taxa crossed
the equatorial region wherein they later became extinct, leaving bipolar relicts.
In the more recent literature, Briggs (1987) supports Theel’s theory whilst
arguing against a vicariance approach to bipolarity (advocated by Nelson, 1985
and White, 1986). Briggs, however, maintains that the youngest genera are
found in the tropics, whilst the “higher the latitude, the more ancient they
become”. Nelson (1985) considers that bipolarity, at least as manifest in the
Pacific, could be the result of earth expansion or fragmentation of a former
southern continent and dispersal of those fragments to the now Pacific margins.
Concerning Atlantic bipolarity (of engraulids), Nelson has no firm explanation
other than those “possibilities for dispersal of Pacific fishes into the Atlantic by
way of a northern route”.
Few gadoid taxa occur in the tropics but those that do such as Melanonidae,
Steindachneriidae, Bregmacerotidae and Merlucciidae contradict Briggs’s
(1987) notion that high latitude equals primitive taxa since the former two
families are considered relatively plesiomorphic (Howes, 1989, 1990). Further
contradiction comes from macrouroid distribution, which is latitudinally wide
+
G J HOiVES
98
for several genera. T h e absence of gadids and phycids in the tropical Atlantic is
probably due more to the lack of suitable broad continental shelves which
presumably once existed in that region during the time of continental apposition
but have now vanished owing the geomorphological processes which have
shaped the Ocean. T h e present eastern Pacific coastal distribution of gadids and
merlucciids appears to me to be part of a once continuous distribution around
the respective southern and northern coasts of the North and South American
continents.
In summary, the circum-Antarctic distribution of the Muraenolepididae is
thought not to be the result of dispersal from South America but the retention of
a distribution along the continental shelves of the formerly closely apposed South
America and Antarctica. T h e bipolarity exhibited by some gadoid taxa is seen as
the result of equatorial disruption of formerly latitudinally continuous
distributions. T h e extant Southern Ocean distributions of Macruronidae,
Merlucciidae, some Phycidae and Gadidae reflect former continuous continental
margins linking the Campbell Plateau with Patagonia.
.~C:RSO\VLEL)GE:Xl~s
IS
I am most grateful t o my colleagues, P. H. Greenwood, N. R. Merrett, C.
Patterson and A, C. IYheeler, and an anonymous referee for their critical
comments and helpful suggestions for improving the manuscript. My thanks also
to P. Campbell, 0.A. Crimmen arid hl. Holloway for their assistance in
preparing and radiographing specimens.
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