Vertebrates
Part 1
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Not as diverse as some of the invertebrates
Generally larger in size with the ability to adapt gives them a leading role
We are vertebrates
What is a vertebrate?
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Vertebrates have:
 Internal skeletons with a multisegmented backbone
 Red blood cells
 A thyroid gland
Only the bony skeleton is preserved in the fossil record except in very rare
cases
What is a chordate?
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Chordates, including vertebrates, have a notochord running along the dorsal
side for support.
In vertebrates that chord runs inside the spinal column
Orientation - terminology
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Chordates and vertebrates can be discussed by using specific terminology
 Dorsal – refers to the back – or the surface below which the notochord runs
 Ventral – the surface opposite the back (the belly)
 Anterior – the end which generally holds the mouth and the sensory organs
(the head)
 Posterior – the end opposite the head
Vertebrate ancestors
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Hemichordates – soft bodied organisms with a dorsal nerve and tubular
structure, pharynx with multiple openings
Graptolites – very similar to living hemichordates and important geologically
as index fossils in the Ord. and Silurian
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Urochordates – “sea squirts” – adults have a pharynx with openings, but the
larvae have notochords, dorsal nerves and muscular tails that are all lost in the
adult form.
Cephalochortates – well defined notochord, dorsal nerve, ventral blood
vessel, pharynx with multiple openings. Found in the Mid Cambrian Burgess
shale
While all of these primitive forms have an anterior end with a “mouth” none of
them has a distinct head.
In the Craniates we see the development of a distinct head and a true internal
skeleton made of cartilage. The lamprey and the hagfish are living examples
of this group. They are the only living jawless vertebrates
Early vertebrates
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The earliest known vertebrates are the jawless fishes. The “Agnatha”.
First found in the Cambrian, they are known through the Devonian in marine
deposits worldwide.
All had a cartilaginous skeleton, but had an exterior covering of boney plates
on the head and part of the body
Four groups of jawless fishes:
 Pteraspids – laterally compressed bodies with plates covering the head
and thorax. From a few cm to 2m in length, the tail was asymmetrical with
the major lobe pointed down (hypocercal); no pectoral or pelvic fins
 Thelodonts – not well known, “bag of scales” with a reversed heterocercal
(hypocercal) tail.
 Cephalaspids (Osteostracans) – best known group; large flattened head
shield with upward directed eye openings; fine scales covered the rest of
the body and the tail pointed upward (heterocercal)
 Anaspids – many rows of fine bony scales, paired eyes and a reversed
heterocercal tail. Thought to be most closely related to the lampreys
Conodonts - microfossils
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Conodonts were long studied as microfossils and are significant index fossils
for most of the Paleozoic. In 1983 impressions were found surrounding
“conodonts” that appear to belong to an eel-like animal. But it still remains a
question mark in many respects.
Early vertebrates
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The evolution of jaws was a marked advancement in vertebrate evolution
It is generally believed that jaws evolved from the early gill arches of the
anathids
The first gill arch separates into the upper and lower jaw and the second gill
arch forms the support for the hinge.
All vertebrates except the anathids are gnathostomes – i.e. jawed
Jawed fish
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The first jawed fish appeared in the Devonian. Of the four groups that
appeared then, only two survived to the recent.
Placoderms – from the greek plakos meaning plate and derma meaning skin.
Dominant Devonian form; thick armored plates in their skin with a bony shield
covering their head and thorax that developed separately from the skeleton; no
armor on the rest of the body; cartilaginous skeleton; well developed pectoral
and pelvic fins and a long dorsal fin; heterocercal tail; no true teeth
Placoderms – Several groups:
 Arthrodires – major predator of the Devonian; head shield set on a balland-socket joint that allowed the head to move up and the jaw to drop – or
open; up to 10m in length; presence of a sclerotic ring in many forms
indicated protection for the large eyes.
 Antiarchs – freshwater forms; heavily armored, even on the fins, with long
thoracic shields and a flattened head; eyes pointing upward; early to mid
Devonian
 Ptycodonts with less armor, a long whip-like tail and thick tooth plates for
crushing molluscs;
 Rhenanids with flat body and broad pectoral fins similar to living rays; and
Petalichthyids with long bony spines projecting in front of their pectoral
fins are representative of other groups of Placoderms.
Devonian extinction
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In the mid- late Devonian Placoderms ruled the seas. But the extinction event
inn Late Devonian meant the end of 35 families. All the remaining jawless fish
died out, all the placoderms, and 10 families of lobe-finned fish.
By the latest Devonian and early Mississippian more modern fish had become
dominant
The rise of modern fish
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The sharks and other chondrichthyans evolved in the Devonian alongside the
placoderms.
Sharks never had bony scales so they are not well preserved in the fossil
record. Teeth and fin spines are most often found, as well as denticles, sharp
objects in the skin.
The chondrichthyans (greek chondros, for cartilage, and ichthys for fish)
have several distinct features:
 Labial cartilage around the mouth
 Distinctive fin structure not seen in other groups
 Bony teeth and denticles
 Pelvic claspers in males permitting internal fertilization and, in some cases,
birth of live young
The two main groups of chondrichthyans found in the fossil record are still
alive today:
 The elasmobranchs including the sharks and rays first appeared in the
Late Devonian. They underwent several radiations and first appear with
calcified vertebrae in the Late Triassic.
 The holocephalans, known as chimeras or ratfish live today at depths over
80m. They are known as fossils as early as the Carboniferous but are quite
rare.
The third group of paleozoic fish are known from a few isolated spines in the
Late Ordovician.
The acanthodians are the earliest known jawed fish and they survived until
the Early Permian. Most notable feature were the spines that supported all
their fins. They also had permanent teeth in advanced jaws and large eyes.
The osteichthyans are comprised of two groups of fish with bony skeletons
(osteon meaning bone and ichthys meaning fish.)
Some of the skeletal bone is derived from the skin (dermal bone) and is found
in the shoulder girdle, palate, the jaw margin, bones covering the gill flap and
in the hyoid region (throat)
The rest of the bones form from skeletal cartilage as embryos.
These fish also have a swim bladder formed from the gut and muscles of the
lower jaw which attach medially rather than on the outside.
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The first group of osteichthyans are the actinopterygians (aktinos meaning
ray and pterygos meaning wing) the ray finned fish.
 Their fins are supported by many parallel rods of bone that articulate directly
with the limb girdles. They also have a single dorsal fin. Early forms had
ganoid scales forming a type of body armor
 The earliest actinopterygians (paleoniscoids) are found in the Late Sil and are
rare in the Dev. But are the most common fish after the Late Dev. Extinction.
They disappeared in the Cretaceous.
 The second radiation of actinopterygians occurred in the Permian. The
neopterygians were dominant in the mesozoic.
 They had a symmetrical homocercal tail and advanced jaw structures which
allowed them to open wide. The maxiliary bone detached from the skull at the
posterior end and is hinged on the anterior. This permitted the development of
much larger and stronger jaw muscles.
 The final radiation of actinopterygians began in Mid Triassic and culminated in
the Cretaceous. The teleosts make up 99% of living fish. The teleost jaw is
more advanced – two supra maxillary bones found in no other group. The
skull is lighter with fewer bones, fins are repositions to allow fine steering –
pectoral to the middle of the body, pelvic fins moved forward.
 The teleost tail has been modified into adistinctive series of radiating
uroneurals that originate from the spinal column.
 Teleosts were highly diverse including both extinct forms and those that have
survived to modern day.
 The second group of bony fishes are the sarcopterygians (sarcos meaning
flesh and pterygos meaning wing) – these are the “lobed fin fishes” including
the Coelancath and modern lung fish.
 They have robust muscles and bone that support the fins and are ancestral to
the tetrapods.
 The sarcopterygians were as diverse as the actinopterygians in the Dev.
 By Late Dev they had given rise to land based tetrapods
 Today only 4 species of sarcopterygians are non-tetrapods – three lungfish
and the coelacanths.
 The sarcopterygians characteristic that links them to the tetrapods in addition
to the structure of the bony elements in the lob fin is their labyrinthodont teeth.
A porous layer of bone called cosmine covers the teeth and gives them a
wrinkled appearance.