Download Lab 9 - Fish and Amphibians

PALEO LABORATORY # 9, PAGE 1
PALEO LAB IX - FISH AND AMPHIBIANS
Phylum Chordata
The Chordata differ from the groups previously studied in lab by the presence of a dorsal
nerve cord, gills and notochord at some stage during the life history of the animal. The
Notochord is a flexible, rod-like structure which generally extends from the base of the skull to
the back of the tail and serves as an anti-telescoping device during muscle contraction and for
tissue support. The notochord is found in some lower vertebrates and in the embryonic stages of
the "higher" vertebrates. However, it is lost during the ontogeny of "higher" vertebrate taxa and
is almost never preserved in the fossil record.
EXERCISE # 1 - Observe the specimens of primitive chordates provided [TSU VP 234, VP 235,
VP 600 (preserved specimen)]. What feature(s) are diagnostic of the Phylum Chordata?
Subphylum Vertebrata
The vertebrates are advanced chordates in which there is an increased organization of the
muscular and nervous systems. They are active, mobile creatures and have evolved systems that
reflect this. Vertebrates are bilaterally symmetrical with the long axis of the body usually in a
horizontal position. There is a tendency to concentrate sensory organs on the anterior end, the
direction which vertebrates move. One of the evolutionarily significant morphological features
of the Vertebrata is the vertebral column, which strengthens the body and provides a firm
foundation for the muscles used in undulatory movement. The vertebral column may be
composed of cartilage or bone. Cartilage is soft, translucent material that is confined to deeper
layers of the body and is capable of expansional growth. Bone is much stronger than cartilage, it
is not capable of "expansional" growth (bone "grows" by replacing a cartilaginous precursor) and
bone has many irregular, branching cell spaces not present in cartilage. As cartilage is soft
material, it is typically not preserved in the fossil record except where strengthened by
calcification.
The ancestry of vertebrates has been much debated. Most paleontologists now believe that the
echinoderms are at or near the ancestry of vertebrates due to similarities in embryology,
biochemistry and the similarity of echinoderm larva to those of primitive chordates. R.P.S.
Jefferies, a British paleontologist, believed that the vertebrates came from carpoid
"echinoderms", coined the term Calcichordata for that group, and placed them within the
Chordata. His interpretation is not accepted by the majority of paleontologists but it certainly
shows the similarities between the phyla.
The vertebrates may be divided into two morphological groups, the Pisces (fish-like forms,
constituting five classes) and the Tetrapods ("four-footed", largely land-dwelling vertebrates,
divided into four classes).
Class Agnatha
The oldest, most primitive class of vertebrates are the jawless fish or Agnathans (Upper
Cambrian - Recent). Paired fins are absent in this group or are poorly developed and the ear
region is different from that seen in other vertebrates. Most of these jawless types are long
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extinct, although the lamprey and hagfish are modern representatives which have remained
virtually unchanged for at least 300 million years. Possibly the oldest known agnathans,
represented by isolated bony plates and scales, are found in deposits of Cambrian age in
Wyoming. Other fragmentary specimens of early jawless fish have been found in Ordovicianage rocks in Wyoming, Colorado, as well as in the Arbuckle Mountains of Oklahoma.
There are two major groups of ancient agnathans, the Cephalaspidomorpha and
Pteraspidomorpha. Cephalaspidomorphs have a single, medially-placed nasal (nasohypophyseal)
opening. The early types had a thick, bony shield that extended part way down the body and the
head was dorsoventrally compressed. The osteostracans, or cephalaspids (Upper Silurian - Upper
Devonian; Exs.= Hemicyclaspis, Cephalaspis) had a large shield-like head. Spiny projections on
the edge of the shield probably acted as stabilizers, since paired fins were poorly developed in
this group. The cephalaspid tail is heterocercal, in which the main lobe of the tail fin is bent
dorsally. These animals probably swam close to the sea floor, their gills acting as pumps and
drawing food particles from the mud like a vacuum cleaner through the ventrally-placed, jawless
mouth. The organics were then filtered off and digested and the water pumped out through the
gill slits along the edge of the ventral shield. Characteristic dorsal and lateral fields in the skull
region have variously been interpreted as electric organs or pressure-sensitive sensory structures.
Another group of cephalaspidomorphs is the galeaspids, from the Lower Devonian of China.
This group is similar to osteostracans but usually had widely separated orbits, a large median
dorsal opening (a sensory structure?), and a hypocercal tail (in which the main lobe of the tail fin
is bent ventrally).
Another group often referred to the cephalaspidomorphs are the anaspids (Upper Silurian Upper Devonian). These are small, streamlined (fusiform), laterally-compressed fish with
laterally-placed eyes and a hypocercal tail. The scales of these fish were often elongated
dorsoventrally and the mouth consisted of an oval, vertical slit that was located anteriorly on the
head. Each gill opened to the outside through an independent opening.
The pteraspidomorphs have paired nasal sacs and openings. They include one undisputed
group, the heterostracans (many paleontologists also place the thelodonts within the
pteraspidomorphs). The heterostracans have a solid shield over the head and pharynx. The
mouth is usually anterior and ventral and is often characterized by the presence of long, narrow
oral plates that may have been shoveling devices or used to suction up soft-bodied invertebrates.
This group never developed true fins. The heterostracans are the oldest-known vertebrates, and
range in age from the Upper Cambrian to the Upper Devonian.
The thelodonts (Lower Silurian?, Upper Silurian - Middle Devonian) are small fish that were
flattened dorsoventrally on the anterior end. The head and body were covered with denticles
(primitive scales), the eyes were far apart and lateral, and the mouth was ventral (but nearly
terminal). There were lateral fins (probably not flexible) and the tail was hypocercal. The gill
chambers opened separately to the surface beneath the lateral fins. Unfortunately, we know very
little about the thelodonts as they are primarily represented in the fossil record by isolated
denticles.
EXERCISE # 2 -Observe the casts of Cephalaspis (TSU VP 001, VP 207) and Birkenia (TSU
VP 002, VP 218).
A. Why would these fish be classified as agnathans?
PALEO LABORATORY # 9, PAGE 3
B. What features make Cephalaspis an osteostracan?
C. What features make Birkenia an anaspid?
D. What features of the skeletons reflect the life habits of these fish?
EXERCISE # 3 – Identify the agnathan groups in the power point slides.
Origin of Jaws and Paired Fins
Formerly, it was believed that jaws formed through the structural modification of the anterior
pairs of gills arches (the structures that support the gills); the upper jaws (Palatoquadrate)
correspond to the upper gill arches and lower jaws (Mandible) to the lower gill arches. I learned
this bit of propoganda and taught it to many of my students. However, studies of ancient and
modern fishes suggest that this may not be the case. Many paleontologists now believe that the
cartilaginous supports of the mouth (not the gills) in ancient agnathans were actually the
precursors to jaws. Stay tuned for further developments. The origin of paired fins is even less
obvious, although two theories seem to best explain their derivation. In some agnathans, there is
a long, stabilizing keel along either side of the fish. By shortening and modifying these
structures it is believed that fins could develop, although this Fin-fold Theory has met with some
opposition. A number of paleontologists prefer the Fin-spine Theory in explaining the evolution
of paired fins. In this theory rows of immobile stabilizing or protective spines which lined the
body of the presumed ancestral agnathan could become mobile stabilizing structures (i.e. paired
fins) through the addition of musculature and supportive tissue. Whether the fin-fold or fin-spine
theory is correct is not known. In fact, many paleontologists believe that paired fins were derived
from a combination of folds and spines.
Class Placodermi
Placoderms (Devonian- Lowest Mississippian) showed tremendous advancement over the
agnathans through the development of jaws and paired fins. The placoderms are characterized by
the possession of large skull plates and a joint between the skull and neck, and by the position of
the gills under the head. The most important group of placoderms were the arthrodires. These
were small to very large placoderms in which the dermal bones formed well-developed armor on
the head and trunk. The ball and socket joint between the cranial and thoracic plates was very
well-developed to form a movable articulation. The arthrodires appear to have been better
swimmers than the rest of the placoderms and are characterized by a wide variety of feeding
types including mud-grubbers, plankton-feeders, shell-crushers and fierce carnivores.
EXERCISE # 4
A. Observe the cast of the placoderm Bothriolepis (TSU VP 003). To what group of
placoderms does Bothriolepis belong? What is the basis of your identification?
B. Observe the lower jaw cast of the placoderm Dunkleosteus (TSU VP 606). To what group of
placoderms does Dunkleosteus belong? What is the basis of your identification?
PALEO LABORATORY # 9, PAGE 4
Class Acanthodii
The Acanthodians (Lower Silurian- Lower Permian) include primarily small cartilaginous
fishes with large eyes. Many paleontologists consider them to be primitive bony fish. Typically,
the head of acanthodians was covered with small plates and the body was covered with minute,
closely-fitting scales. The paired and median fins had anterior spines, some of which are used as
index fossils. Silurian and some later acanthodians lived in the seas but by the Early Devonian
many also lived in fresh water.
EXERCISE # 5 - Observe the illustrations of acanthodians, the lab specimens of acanthodian
spines (TSU VP 004), and the skeletons of acanthodians (TSU VP 005, VP 201).
A. Comparing the specimen with the illustrations. The elongate brown structures are finspines.
What are the function of these spines?
B. Observe the specimens of Acanthodes under a microscope. What are the small
rectangular/rhomboidal white structures observed?
Class Chondrichthyes
Sharks and rays (Chondrichthyes) are characterized by the presence of a cartilaginous
skeleton, by the absence of gill covers (the gills open directly to the outside), by the presence of
multiple tooth rows in which the teeth are replaced from the inside of the jaw to the top edge, and
by the presence of isolated cone-shaped Placoid Scales with a shiny enamel-like surface. The
skull of sharks is loosely connected to the upper jaws and most sharks are predators. They are a
tremendously successful group whose geologic record extends from the Middle Devonian
(approximately 380 million years ago) to the present.
Since sharks are cartilaginous, most portions of their skeleton do not usually survive as
fossils. However, the more resistant shark teeth are rather common in the fossil record and
sometimes serve as important index or guide fossils. Many shark genera and species are based
upon isolated teeth and finspines.
EXERCISE # 6 - Observe the modern shark jaw (TSU VP 006).
A. What is the composition of the jaw material?
B. What is the composition of the tooth material?
C. Describe how the teeth are being replaced.
The most primitive sharks are included in the Subterbrachialia, in which the gills are
positioned so far anteriorly that they lie almost beneath the skull. This group includes the
iniopterygians and holocephalians. The iniopterygians (Upper Mississippian - Upper
Pennsylvanian) were bizarre fish in which the pectoral fins were dorsal in position and the
vertebral column extended almost straight into the nearly circular-shaped tail. The
holocephalians (Upper Devonian - Recent) have their upper jaws fused to the braincase. Also,
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the anterior vertebrae are fused and articulate with a dorsal spine. Holocephalians include the
ancient helodonts and "bradyodonts", as well as the modern chimaeras or ratfish. These groups
are often characterized by the presence of broad, crushing toothplates by which they crunch up
their prey.
The elasmobranchs are characterized by posteriorly-placed gills which lie mostly behind the
skull. The gill pouches open separately to the outside and the gill arches are widely spaced.
There are many groups of elasmobranch sharks (see classification above), although only a few
need concern us here. The most important groups paleontologically are the xenacanth and
euselachian sharks.
The xenacanth sharks (Upper Devonian - Upper Triassic) were largely freshwater sharks with
peculiar two- or three-pronged teeth. There was typically a dorsal finspine attached to the
shoulder girdle or to the back of the skull, there were two anal fins, a diphycercal caudal fin (tail
fin with vertebral axis extending straight into the tail) and the pectoral fin had a prominent
central axis.
Another peculiar shark group are the eugeneodonts, which range from the Upper Devonian to
the Triassic. The teeth along the central portion of the jaw (the symphysial teeth) tend to form
wierd circular saw-like cutting devices with peculiar whorl-shaped tooth replacement. The lateral
teeth usually form crushing surfaces. These edestid and helicoprion-type sharks had some of the
most peculiar dental morphologies in the history of vertebrates!
The euselachians (Devonian - Recent) include the modern shark groups. These sharks have
two dorsal fins supported by anteriorly-positioned spines along each of their margins. The
finspines are coated with an enamel-like substance and are supported by a triangular piece of
cartilage. The body tends to be of streamlined (fusiform) shape and there is one anal fin. The
skin is covered with tiny cone-like placoid scales (denticles). The euselachians include the
ctenacanths, hybodonts and neoselachian sharks. The difference between "primitive" and
"derived" sharks primarily involves the improvement of jaw articulation to form a protusible
feeding mechanism. Remember how the space monster in the movie "Alien" moved his jaws
outward to eat all of the people on the spaceship? That reminds me of what neoselachian sharks
do when they are about to consume tourists. Well, as I was saying before, because sharks are
cartilaginous they are often identified by means of isolated teeth. We can typically differentiate
ctenacanths, hybodonts and neoselachians on the basis of their teeth. For example, ctenacanth
sharks often have a "cladodont" dentition, in which several pointed cusps are present on a single
tooth. Hybodonts may also have cladodont-like dentition, although they are often characterized
by very low-crowned crushing teeth. Most neoselachians sharks are active predators, ripping
their victims apart with their sharp dagger-like teeth.
EXERCISE # 7 - Observe the tooth specimen of Stethacanthus altonensis, a symmoriid shark
(TSU VP 008).
A. Describe the tooth shape. This is termed a "cladodont" tooth.
B. What other types of sharks are characterized by this dental morphology?
EXERCISE # 8 - Observe the illustrations of xenacanth sharks and, under a microscope, observe
the xenacanth shark teeth (TSU VP 016, VP 241).
PALEO LABORATORY # 9, PAGE 6
A.What features on the illustrations indicate that this is a xenacanth?
B. What features of the teeth indicate that these are xenacanth sharks?
EXERCISE # 9 - Observe the specimen of Helicoprion bessonowi (TSU VP 570). What feature
does this fossil represent, and what indicates that it represents a eugeneodont shark?
EXERCISE # 10 - Observe the modern ray jaw (TSU VP 024).
A. Is this dentition homodont or heterodont? Explain.
B. Is the morphology of the dentition related to life habits? If so, how?
EXERCISE # 11 – Identify the types of sharks and primitive gnathostomes in the power point
slides.
Class Osteichthyes
The Osteichthyes are characterized by the presence of a bony skeleton and a scale-covered
body. There are two subclasses of bony fishes. The Actinopterygians, or ray-finned fishes, have
fins supported by numerous small rod-like bones. The lobe-finned fishes (Choanichthyes or
Sarcopterygians) are a small group which includes the Crossopterygians (including the
Coelacanths and an extinct group which gave rise to the amphibians, the osteolepids and Dipnoi
(lungfishes).
Actinopterygian Osteichthyes
The most primitive actinopterygians are Palaeoniscoids (Infraclass Chondrostei), primarily
found in rocks ranging from Devonian to Triassic age. These ray-finned fish are characterized by
thick, rhomboidal-shaped scales and by a primitive jaw and fin structure. In these forms, there
are many skull bones and these bones are often tilted forward, forming a strongly-braced
structure. The pelvic fins are placed toward the rear of the fish (in the primitive position) and the
caudal (tail) fin is asymmetrical, with the upper lobe larger than the lower (heterocercal tail).
The palaeoniscoids were probably slow swimmers and their primitive jaw structure did not allow
them to open their mouths widely or quickly.
During the Triassic, the palaeoniscoids gave rise to the fish group which would dominate all
aquatic environments during the latter part of the Mesozoic and the Cenozoic, the Neopterygians.
Neopterygian fishes are actinopterygians that are characterized by their thinner scales (in many
marine and some freshwater teletosts the scales may be entirely absent), an improved jaw
structure (in which the jaw bones are reduced in number, more erect and loosely connected to
form a wide gape for eating larger prey) and fin structure (often the pelvic fins are placed far
forward for improved turning and mobility). The most derived neopterygians are termed
Teleosts. In these fish, the bone at the front of the upper jaw (the premaxilla) was freed so that it
could rotate forward; it became enlarged and developed as the primary tooth-bearing element.
The other major upper jaw element, the maxilla, was freed so that it could be pulled down and
forward by the lower jaw as the mouth opened. The result of these modifications was to greatly
PALEO LABORATORY # 9, PAGE 7
increase the size of the mouth chamber. Therefore, when teleosts open their mouths widely and
quickly, a suction is created which actually draws their victims into the mouth. Another
improvement of the teleosts is in the modification (especially fusion) of the bones at the base of
the tail fin and the formation of a symmetrical caudal fin (homocercal tail). This strengthens the
tail fin, allows for greater propulsive force, and therefore increases the speed of the fish.
"Suction feeding" and modification of the tail fin probably largely account for the tremendous
success the teleosts have experienced throughout the Cenozoic.
EXERCISE # 12. Observe the scale types represented by slides TSU VP 236, VP 237, VP 238
and VP 239.
A. Describe, and be able to identify, each of these scale types.
B. Observe the gar specimen (TSU VP 013). What type of scales are present? Explain.
EXERCISE # 13. Observe the illustrations of primitive and derived actinopterygian fish and the
specimens on display in lab (TSU VP 009, VP010, VP026, VP 202, VP 203, VP208, VP 246, VP
247, VP 609, VP 653 (oversized; on shelf), VP671 (oversized; on shelf).
A. Why are these osteichthyans?
B. Why are these actinopterygians?
C. Observe the paleoniscoid illustrations and cast (TSU VP 010). Why are these
chondrosteans?
D. Observe the teleost illustrations and the display specimens.
1. Based on jaw morphology, why are these teleosts?
2. Based on locomotory mechanisms, why are these teleosts?
3. What type of scale(s) are present on these specimens?
Sarcopterygian Fish (Choanichthyes)
Sarcopterygian fish are differentiated from most actinopterygians by the presence of two
dorsal fins, fleshy lobed fins and the presence of a heterocercal tail. In rhipidistians, coelacanths
and the earliest lungfish the braincase is divided into two parts; the anterior part of the braincase
was able to swing downward to drive the fishes teeth into their prey.
Dipnoans
The dipnoans, or lungfish, are an order of freshwater choanichthyans (some extinct types were
marine) which have adapted to areas with seasonal drought. As the pools of stagnant water dry
up where the lungfish live, the fish come to the surface to breath. Some species, both fossil and
modern, could even withstand a complete drying up of the pools in which they lived by digging
PALEO LABORATORY # 9, PAGE 8
burrows in the mud and Aestivating until rains during the wet season again filled the ponds. All
but the earliest lungfish have their braincase fused into a single unit and their upper jaws are
fused to the braincase. Lungfish have no marginal teeth; in most taxa the dentition consists of
two pairs of upper toothplates (one much smaller than the other) and a single pair of lower.
These broad toothplates were used to crush molluscs or other relatively hard-shelled prey.
EXERCISE # 14. Observe the cast of Scaumenacia curta (TSU VP 043).
A. What features indicate that Scaumenacia is a sarcopterygian?
B. What features indicate that Scaumenacia is a “primitive dipnoan”?
EXERCISE # 15. Observe the toothplate specimens TSU VP 028 and VP 443. What features
indicate that these are dipnoan fossils?
Crossopterygians
The crossopterygians are bony fishes that are characterized by the possession of two dorsal
fins and lobate paired fins. The most important distinguishing feature of the group is the division
of the braincase into anterior and posterior parts with an intracranial joint between them. The
crossopterygians were predominantly predatory fishes. Their small eyes indicate that they may
have mainly relied on their sense of smell to locate and track their prey. There were two types of
crossopterygian fish; the coelacanths and "rhipidistians" (mostly represented by the osteolepidid
fishes).
The coelacanths are a group of relatively large lobe-finned fish which developed a jaw
structure with some morphological features similar to those of actinopterygian fishes. These
were evidently related to improvements in feeding and breathing. Paleozoic coelacanths were
primarily freshwater. Most Mesozoic types appear to have been marine. It was believed that
coelacanths became extinct during the Cretaceous. However, in the 1930's these fish were
discovered to be alive in the deeper waters of the Indian Ocean off the coast of Africa!
The "rhipidistians" (mostly represented by the osteolepiform fish) are important because they
were ancestral to the land vertebrates, or Tetrapods. They differ primarily from coelacanths in the
arrangement and number of dermal bones in the head. In fact, the skull bones of rhipidistians are
largely homologous to those of primitive amphibians. Other features that are similar to those
seen in early amphibians include teeth with infolded enamel (labyrinthodont teeth), marginal
bones of the palate bearing large fang-like teeth, numerous bony plates surrounding the eyes,
formation of bony vertebral centra that are very similar to those of early amphibians in
construction, single proximal elements on the pectoral and pelvic fins (homologous to the
humerus and femur) that articulate with the shoulder and pelvic girdles and pairs of distal limb
elements (homologous to the tibia and fibula and radius and ulna). The rhipidistians evidently
possessed both lungs and gills. The former structure was used for buoyancy control and to
supplement the gills in respiration.
EXERCISE # 16 - Observe specimens TSU VP 204, VP 215 and VP 597.
A. What features indicate that these are sarcopterygians?
PALEO LABORATORY # 9, PAGE 9
B. What features indicate that these are crossopterygians?
C. Which of these specimens are coelacanths and which are osteolepiform “rhipidistians”?
Explain.
EXERCISE # 17 - Identify each osteichthyan fish group observed on the Power Point slides.
Class Amphibia
The amphibians are the oldest group of terrestrial vertebrates and provide the basal stock
which led to all other tetrapods. These vertebrates remain close to fresh water, since their eggs
must be laid in that medium and their larvae typically develop there.
The Amphibia were undoubtedly derived from the rhipidistian crossopterygian fishes. In fact,
the earliest-known amphibian (Ichthyostega) was almost identical to the rhipidistians in skull and
skeletal characteristics and even had a fish-like tail and scales. Rhipidistian fish, ichthyostegids,
and other primitive amphibians are characterized by the infolded enamel of their teeth. This
complex labyrinthine infolding has caused taxonomists to place those amphibian types within the
Subclass Labyrinthodontia (Devonian- Triassic). Besides utilizing tooth structure in
differentiating primitive amphibians, paleontologists also use vertebrae characteristics for
classification. Several bones form each vertebrae in early amphibians, and the size and position
of these bones determines to which group they belong. Lepospondylous, or husk, vertebrae are
found in many small Paleozoic amphibians. In these forms the vertebrae form a single structure,
which is often spool- shaped with a hole in the center for passage of the notochord. "Arch"
vertebrae are found in labyrinthodonts and in all higher vertebrae classes. Each vertebra consists
of two sets of bony arch structures, the intercentra and pleurocentra. Arch vertebrae include
several different types. Rhachitomous vertebrae are the most primitive type with wedge- and
crescent-shaped intercentra and paired pleurocentra; the latter are small structures that lie
between the neural arches (arch-shaped structure above the vertebrae through which the nerve
cord passes) and intercentra. Rhachitomous vertebrae are found in rhipidistians, ichthyostegids
and some temnospondyl labyrinthodonts. Stereospondylous vertebrae have the pleurocentra
reduced or absent. The intercentrum lies below the neural arch and is sometimes ring-shaped.
Stereospondylous vertebrae are found in many Late Permian and Triassic temnospondyls. In one
group, the anthracosaurs, the pleurocentra increase in size, fuse and become a complete ring.
This is homologous to the vertebral centrum of higher vertebrates. In the anthracosaur
amphibian line that led to reptiles, the intercentra are reduced to small wedges between the
pleurocentra. In the second line of anthracosaurs, both the pleurocentrum and intercentrum form
complete ring-shaped structures (the embolomerous condition). Besides vertebral characteristics,
the morphology of the skull is also important in the classification of tetrapods. In general,
tetrapods have a longer snout and shorter skull table than their rhipidistian ancestors. The paired
bones down the midline of the skull (from anterior to posterior) include the elongate nasals and
frontals and the shortened parietals (which houses the pineal opening, a sensory organ) and the
postparietals.
During the Pennsylvanian and Permian Periods amphibians diversified, developing many
different body styles and habits. The diversity of forms presumably inhabited a wide range of
environments. Paleozoic temnospondyls include the "rhachitomes" and "stereospondyls". In
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rhachitomes the intercentra form the major vertebral elements (see above). Some of these, such
as the Pennsylvanian/Permian-age labyrinthodont Eryops, were large flat-headed aquatic
carnivores. Other rhachitomes include the trimerorhachids, which had a flattened skull with a
short face and long skull table and the dissorophids (peculiar terrestrial amphibians which were
often characterized by the presence of body armor!). The dissorophids are important because it is
believed that they gave rise to the modern groups of amphibians. The stereospondyls are a
temnospondyl group (probably polyphyletic; not a natural group) in which the intercentra grew
upward to form complete rings around the notochord and pleurocentra were absent. In these the
skull was typically flattened and there were large openings on the ventral side of the skull. An
example of this group is the large, flat-headed Upper Triassic amphibian Metoposaurus.
The "anthracosaurs" were the most advanced group of labyrinthodonts and adapted well to a
terrestrial mode of life. Many are very reptile-like in their skeletal and skull features and were in
fact the group that gave rise to the Reptilia. One of the best known anthracosaurs is Seymouria, a
lower Permian amphibian from Seymour, Texas. This form is a "missing link", showing many
transitional features in it's skeleton and skull between the amphibians and reptiles. "Amphibian
features" of the seymouriamorphs include an anthracosaur-like skull that was not very well
ossified and gill-bearing larval stages. "Reptile features" of Seymouria and its kin included solid
attachment of the skull and cheek, reduction of the stapes to form a narrow rod (the stapes is one
of the bones that transmits vibrations for "hearing" in higher vertebrates ), formation of reptilelike vertebrae (see discussion above) and expansion of the pelvic girdle to incorporate two
vertebrae for supporting the limbs.
Diadectes was a peculiar heavily-built herbivore that combines amphibian and reptile
characteristics. It is often placed within the “reptiliomorphs”, as it is considered to be closely
related to amniotes. The skull of Diadectes is anthracosaur-like, with a large otic notch which
evidently housed the hearing membrane, or tympanum. However, in other skull specializations
Diadectes is quite advanced. The eight front teeth consist of peg-like and spatulate incisiform
teeth that were utilized to crop vegetation. There are also broad grinding cheek teeth that exhibit
wear patterns indicating precise occlusion of these molariform teeth. Another surprising feature
of Diadectes is the formation of a partial secondary palate; this allowed separation of feeding and
breathing. This feature is not only absent in other amphibians, it is not present in most reptiles
and only becomes a dominant element in mammals.
Another subclass of primitive amphibians are the Lepospondyli (Mississippian- Permian),
characterized by their spool-shaped vertebrae. A Permian representative of this group is the
strange boomerang-headed Diplocaulus, a weak-limbed lepospondyl that lived in lakes and
streams around Seymour, Texas. Diplocaulus and its kin are referred to the Order Nectridia.
Other nectridians are salamander-like in morphology. Another lepospondyl group is the
microsaurs. These amphibians, like the anthracosaurs, were probably terrestrial carnivores and
developed a variety of lizard-like body forms. Other lepospondyl amphibian groups even
developed snake-like bodies! Aistopods reduced or lost their limbs and limb girdles and had up
to 220 vertebrae forming the spinal column. Lysorophids were elongate, short-limbed
lepospondyl amphibians that are often found preserved in nodules. Evidently, members of this
group aestivated, digging burrows on the bottoms of ponds during the dry season and wrapping
their bodies into a tight curl to help prevent dessication as the ponds dried out.
All living amphibians may be placed within the Subclass Lissamphibia. These are rather
advanced, specialized types which can be placed within three orders: the Anura (frogs and toads;
PALEO LABORATORY # 9, PAGE 11
Triassic to Recent), Urodela (Salamanders; Jurassic to Recent) and the Apoda (the caecilians;
curious worm-like amphibians; Jurassic to Recent). These groups are rather stable types which
have not changed much morphologically throughout their known evolutionary history. A feature
common to all of these groups is the presence of pedicillate teeth, in which the base and crown of
each tooth is separated by a zone of weakness. This is probably so that when the animal
protrudes his tongue he doesn't knock all of his teeth out! (or something like that, anyway). All
living amphibians have spool-shaped vertebrae and frogs and salamanders have strange ear
specializations which may indicate close relationships between the two groups.
Frogs and toads have peculiar specializations that are evidently related to jumping habits.
They only have five to nine trunk vertebrae (the posterior-most vertebrae are fused together), ribs
are absent, the U-shaped pelvic girdle braces against the posterior-most vertebral process, the
shoulder girdle is firmly braced, and the tibia and fibula (distal paired bones of the hind limb) are
often fused.
EXERCISE # 18. Identify each of the following amphibian vertebral specimens and indicate why
you believe that this identification is correct.
A. This specimen (TSU VP 032) has a single ossification. Identify the vertebral type.
B. Identify the element represented by the specimens (TSU VP 030); then, identify the vertebral
type.
C. Sketch (and be able to identify) the following structures on specimen TSU VP 031:
intercentrum, pleurocentrum, neural arch, transverse processes. What classification of
vertebrae is this? Why?
D. This specimen (TSU VP 029) has a single ossification. Identify the vertebral type.
E. This vertebral element is the intercentrum (TSU VP 033). What is the vertebral type? Why?
EXERCISE # 19. Observe the amphibian jaw specimen (TSU VP 035); the teeth have been
broken in this specimen. Is this amphibian a lepospondyl or labyrinthodont? Why?
EXERCISE # 20. Compare the pictures of Ichthyostega in your text with those of the
rhipidistians.
A. What similarities are seen between these groups?
B. What differences are observed between these groups?
EXERCISE # 21. Observe the amphibian skulls on display in the lab. Describe the differences
seen between aquatic-adapted amphibian skulls (TSU VP 165 thru VP 167, TSU VP 580, VP
661, VP 674) and skeletons (TSU VP 615, VP 648, VP 659) versus the “reptiliomorph”
Diadectes, a "terrestrial amphibian" (see skull TSU VP 200 and power point slides of
skeleton).
PALEO LABORATORY # 9, PAGE 12
EXERCISE # 22. Observe the cast of the skull (TSU VP 037), skeleton (TSU VP 604) and the
illustration of Seymouria in your textbook.
A. To what group did the seymouriamorph anthracosaurs give rise?
B. What features of Seymouria are “amphibian-like”?
C. What features of Seymouria are “reptile-like”?
EXERCISE # 23. Observe the cast of the skeleton (TSU VP 164) and the skull specimens (TSU
VP 039, an enlarged reconstruction of skull X 5; also TSU VP 038, the anterior portion of
another life-size skull and skull TSU VP 660). To what lepospondyl group do these
specimens belong? Explain.
EXERCISE # 24. Observe specimen TSU VP 040, and compare it with the illustrations in your
textbook.
A. To what lepospondyl group does this specimen belong? Why?
B. What ecological/behavioral adaptation(s) may explain the peculiar skull morphology?
EXERCISE # 25. Referring to the power point illustrations and the lab specimen on display
(TSU VP 160; VP 640; VP 672, on shelf), what osteological (bone) features reflect the habits
of frogs?
EXERCISE # 26 - Identify each amphibian group observed on the Power Point slides.