Biology 356 - Major Features of Vertebrate Evolution
Dr. Robert Reisz, University of Toronto
Early Gnathostomes
They first appear in the fossil record in the Upper Silurian.
Synapomorphies:
Paired fins with skeletal support (spines in acanthodians, exoskeleton in some
placoderms who had fin endoskeleton as well), mostly endoskeletal.
Jaws and a hyoid arch usually linking jaws to braincase. The jaws and hyoid arch
seem to be serial homologues of the gill arches, but were probably never
unspecialized gill arches.
A horizontal semicircular canal. Agnaths have at most two vertical semicircular
canals.
Myelinized nerve fibers. The nerve fibers (axons) of the nerves of agnaths have
no myelin sheath.
Myoglobin and hemoglobin.
Chondrichthyes (Figures 13 and 14)
Appear in the fossil record in the Upper Silurian.
Active swimmers; dominant marine predators.
Fusiform body and large fins.
Most practice internal fertilization (males have pelvic claspers. They have few
eggs but they have a lot of vitellus.
Mostly marine, but a few groups lived in fresh water.
Modern sharks have a large, oil-filled liver to reduce their weight.
Bone may be present at the base of scales only.
Cleaver-shaped palatoquadrate forms the functional upper jaw.
Teeth form along an infolding of epithelium of the mouth called the dental
lamina. Teeth are continuously replaced (move laterally). This character may be
a synapomorphy uniting them to osteichthyans, but this character could be a
convergence, because acanthodians, which are probably more closely related to
osteichthyans than to chondrichthyans, lack regularly replaced teeth. In addition,
some of the most recently discovered chondrichthyans lack teeth.
Two major periods of adaptive radiation: the first was from the Silurian to the
Devonian; the second was in the Jurassic.
Paleozoic sharks:
Mouth is nearly terminal.
Heterocercal tail.
Triangular pectoral and pelvic fins.
Braincase is undivided, broad and low. We usually divide it into three regions:
ethmoid (anteriorly), orbital, and otico-occipital. The ethmoid region may bear a
rostrum. The orbital region has a wide interorbital septum and a shelf above the
orbits for protection. A postorbital process supports the otic process of the
palatoquadrate. The hyomandibular usually supports the palatoquadrate
(hyostylic and amphistylic suspensions. The palatoquadrate of most early sharks
was supported by the hyomandibular and by the postorbital process of the
braincase. This is called an amphistylic suspension (primitive for
Chondrichthyes). When the hyomandibular alone supports the palatoquadrate,
we have a hyostylic suspension. This is seen in several modern sharks, but it is
not primitive for sharks. This type of jaw suspension allows the upper jaw to be
mobile and to be protruded to capture the prey in modern sharks. The
palatoquadrate of holocephalians and a few other cartilaginous fishes is fused to
the braincase; this is a holostylic or autostylic jaw. This type of jaw suspension
is very sturdy and is mostly used by durophageous animals (who feed on hard
animals such as bivalves).
The gill arches are behind the braincase and each gill has its own external gill
slit (the gills are not covered by an operculum. Most sharks have to swim to
breathe. Water enters through the mouth and goes into the gill
chambers. Contraction of the branchial muscles contracts the pharynx and
expels the water.
Paired scapulocoracoids forms a broad arc; they never fuse.
Pelvic girdle is a small, paired oval or triangular plate.
Fins are supported by several parallel basals and radials. In Paleozoic sharks,
the radials extend to the margins of the fins
The teeth of sharks seem to be derived from scales; they have a pulp cavity
surrounded by a cone of dentine and a thin layer of enamel.
The teeth of modern sharks are vaguely triangular.
Fin spines are located in front of the dorsal fin(s). They also seem to be derived
from scales; they are made of a cone of dentine covered by a thin layer of
enamel.
Cladoselachida:
Known primarily by the genus Cladoselache.
Lived in the Upper Devonian.
No anal fin.
Two dorsal fins preceded by fin spines. The spines are composed entirely of
dentine.
Scales were found only on the margins of the fins and around the eye.
No claspers.
Neoselachii:
Appeared in the Lower Jurassic.
Synapomorphies:
The two halves of the pectoral girdle either articulate with each other or are
fused.
The radials are smaller and ceratotrichia become the major elements supporting
the distal portion of the fins.
The fin spines are smaller and not ornamented.
Hyostylic jaw suspension for more mobile jaws.
Most genera have a rostrum, so the mouth opens ventrally rather than anteriorly.
The nasal capsules are larger than in Paleozoic sharks.
Larger brains than Paleozoic sharks (highest brain weight to body weight ratio of
any group of anamniotic vertebrates, even higher than many birds and
mammals).
Primitively, they have triangular teeth.
Acanthodians (Figure 15):
Appear in the Lower Silurian, become extinct at the end of the Lower
Permian. They are the first jawed fish to appear in the fossil record.
They had large eyes and small nasal capsules. Therefore, they must have been
nektonic.
Heterocercal caudal fin.
Spines support all the fins except the caudal fin. Several genera also had up to
six pairs of intermediate spines between the pectoral and pelvic fins (these
intermediate spines were not associated to fins). The spines may have been for
protection against predators. Most were small and fusiform, but some reached a
length of 2 m.
Primitively, they are completely covered with small, thin, closely-fitting but non
overlapping scales. The scales have no pulp cavity. The scales grew by
concentric addition of layers of bone. They have a bony base and are covered by
dentine on the crown. The head and gill region are covered with large dermal
scales (not homologous to those of osteichthyans).
The fins are covered by parallel rows of scales reminiscent of the lepidotrichia
of osteichthyans. When the endoskeleton of the fins is known (the dorsal fin of
Diplacanthus, the pelvic fin of Cheiracanthus and the pectoral fins of other
genera), it consists in a second, deeper layer of skeletal support resembling
lepidotrichia in size and orientation, but they are not jointed. They are thought to
be basals and radials. They extend only a short way into the fin.
The braincase is similar to an osteichthyan’s braincase. It consists in a large
dorsal ossification separated from a paired lateral occipital ossification by the
lateral occipital fissure (through which passes the cranial nerve X, Vagus). There
is also a ventral occipital ossification grooved dorsally for the notochord and an
anterior basal ossification comparable to the basi-parasphenoid complex of
osteichthyans. The anterior basal ossification has a basipterygoid process for the
basal articulation of the upper jaw.
There is a large, cleaver-shaped palatoquadrate and a long meckelian
cartilage. The palatoquadrate articulates with the hyomandibular, with the
postorbital process of the braincase, and with the basipterygoid process of the
anterior basal ossification. Therefore, the palatoquadrate had little or no
mobility. A trough-shaped dermal bone, the mandibular bone, covers the ventral
surface of Meckel’s cartilage.
Some early forms have no teeth, but when they do, they may have one or two of
the following three types of teeth:
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Isolated elements.
Tooth whorls.
Teeth fused to the surface of the jaw bones.
The fused teeth seem to have been added at the front of the mouth, because the
posterior teeth are more worn than the anterior teeth. The teeth don’t have
enamel and are not regularly replaced. This is strange because chondrichthyans
and osteichthyans share a regular tooth replacement pattern and their teeth are
covered by a layer of enamel. There were no ossified centra and no ossified ribs,
but bony neural and haemal spines supported the notochord.
The shoulder girdle is composed of dermal and endochondral bone, as in
osteichthyans, but the acanthodian girdle was associated with pectoral spines. In
some genera, the spine was mobile, in others it was fixed. The dermal pectoral
girdle covers only the ventral surface of the pectoral area; there are no lateral
dermal bones.
Some interesting trends are visible in the evolutionary history of this group:
Deeper insertion of spines; development of an articulation between the pectoral
spine and girdle; loss of the dermal shoulder girdle; the endochondral pectoral
girdle consists primitively of a single ossification (the scapulocoracoid), but it
becomes subdivided into separate suprascapular, scapular, and procoracoid
ossifications; loss of intermediate prepectoral spines.
Primitively, the fin spines are wide from side to side and have a large posterior
cavity opening on to the fin. In later genera, the spines are laterally flattened and
the cavity opens only in the body, in which the spine is deeply anchored.
We are not absolutely certain of the affinities of acanthodians, but they seem to
be closely related to osteichthyans (bony fishes). Here are the synapomorphies
that seem to justify this:
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Branchiostegal rays.
Large, compact otoliths in the ear.
Spiracular groove on the neurocranium.
Lateral occipital fissure separating occipital and otic regions.
Short occipital region.
Narrow skull with thin interorbital septum.
Small nasal capsules.
Elements of the dermal shoulder girdle of acanthodians may be homologous to
the clavicle and interclavicle of osteichthyans and perhaps the spinal of
placoderms . The problem with most of these characters is that they are only
known in late acanthodians like Acanthodes. We don’t know if early acanthodians
also had these features. Placoderms also share synapomorphies with
osteichthyans, so acanthodians are either the sister-group of osteichthyans or of
the clade including osteichthyans and placoderms.
Placodermi (Figures 9, 10, and 11)
They appeared in the Lower Devonian, and only two genera survived into the
Mississippian. They have no living representatives.
Several paleontologists believe that placoderms are more closely related to
osteichthyans than to chondrichthyans, because they share the following
characters:
A neurocranium protected by overlapping dermal plates with descending
laminae for muscle attachment. Chondrichthyans only have a micromeric
covering of non overlapping scales. The scales of acanthodians do not overlap
much and lack the descending laminae.
A dermal pectoral girdle. Chondrichthyans only have an endochondral shoulder
girdle. Several bones of placoderms may be homologous to those of
osteichthyans. According to Gardiner (1984), we have the following
homologies (compare Denison, 1984, fig. 5a and 6a or fig. 4-15a with Carroll,
1988, fig. 6-13b and c):
Placoderms
Anterior median ventral
Anterior ventrolateral
Anterior lateral
Anterior dorsolateral
Osteichthyans
Interclavicle (only present in
sarcopterygians)
Clavicle
Cleithrum
Anocleithrum
A parasphenoid. This is a dermal bone covering the ventral surface of the
braincase.
A dermohyal fused to the head of the hyomandibula.
Placoderms are characterized by a large number of features:
Their dermal bones had three layers; a basal layer of laminar bone, a middle
layer of trabecular (cancellous) bone, and a superficial layer of semidentine
(cellular dentine). The superficial layer may bear an ornament of tubercles or
ridges and carries the laterosensory lines and grooves. The semidentine is found
only in placoderms.
They have large bony plates (gnathal plates) at the edges of the jaws rather
than teeth. These are not replaced but grow with the animal. There were up to
two in each upper jaw, the anterior and posterior superognathals, and one in
each lower jaw, the inferognathal.
A double cervical joint. There is an articulation between the braincase (by
means of a double occipital condyle) and the synarcual (fused vertebrae; fig. 419) and a joint (sometimes a good articulation, sometimes a sliding joint) in the
dermal shield between the plates covering the skull and the thoracic plates.
The palatoquadrate is lateral to the jaw musculature (the palatoquadrate is
closely integrated with the cheek) rather than medial to it as in all other
gnathostomes.
Several taxa were covered with a thick armor often divided into a cephalic and a
thoracic unit. Each unit is subdivided into smaller plates capable of growing at
their margins. We are not certain if the elements composing the shields are
homologous to the dermal skull elements of bony fishes and tetrapods.
The meckelian cartilage is ossified in two regions, the mentomandibular
anteriorly and the articular posteriorly.
The anterior lateral (cleithrum) and interolateral plates have strongly ornamented
internal laminae which combine to form the hind wall of the gill chamber.
There is no spiracle.
Several have ossified neural and haemal arches. These arches were the only
ossified part of the axial skeleton.
The fins (except for the caudal) are usually supported by elements similar to the
basals and radials found in sharks and osteichthyans.
No anal fin.
Heterocercal tail.
The phylogeny of placoderms has not been fully resolved because we are not
certain of their affinities. Therefore, it is difficult to polarize characters. This may
explain why there is still no consensus on placoderm phylogeny. An especially
controversial question is which placoderms are closest to the primitive condition
for the group: the massively ossified ones or the poorly ossified ones. It seems
that no matter which outgroup we choose among gnathostomes, the poorly
ossified forms are more primitive, because most of the plates of placoderms
(except some in the shoulder girdle) seem to have no homologues among other
groups of gnathostomes.
Most placoderms became extinct at the end of the Devonian. This is one of the
best evidence that there was a mass extinction event toward the end of this
period. This extinction event affected several groups of invertebrates and other
vertebrates and may have been caused by a glaciation. It probably was not
sudden; species probably disappeared over several million years, but the
amplitude of these extinction was greater than the end-Cretaceous event in
which dinosaurs, ammonites and several other animals became extinct.