Lecture Outline:
Origin and Early History of the Vertebrates
(History of Life, Mark A. Wilson, College of Wooster, Fall 2008)
 
The Phylum Chordata is separated from other animals by four major features:
1. Dorsal nerve tube
2. Notochord
3. Gill slits
4. Post-anal tail

All chordates (including humans) possess these features at some stage in their ontogeny (growth and development).

The fossil record does not give us much information about the origin of the chordates.

Hypothesis for the origin of vertebrates --

Through comparative embryology, scientists now believe that the chordates arose from a primitive sessile-benthic arm-feeding echinoderm.
 
Tunicates (Subphylum Urochordata) are small marine invertebrate chordates quite common today; they may hold the key to vertebrate origins.
 
Tunicates have a sessile adult stage in which they are filter-feeders with gill slits but no notochord or dorsal nerve tube.

Tunicate larvae, though, are free-swimming filter-feeders; they are characterized by a notochord and dorsal nerve tube lost later when they metamorphose into sessile adults.
 
It is possible that a tunicate-like organism in the Cambrian evolved by way of neoteny into a free-swimming adult chordate.
 
Neoteny is the evolutionary retention of juvenile features into maturity. In this case, the juvenile features of tunicate-like organisms (notochord, dorsal nerve tube, and swimming ability) may have been evolutionarily retained in the succeeding chordates.
 
The living lancelet (Amphioxus, in the Subphylum Cephalochordata) is a chordate with a free-swimming adult stage. It is also a filter-feeding bottom-dweller.
 
We may predict, then, that the first vertebrates (Subphylum Vertebrata) were free-swimming, bottom-dwelling filter-feeders ... and they are!

Fossil record of the earliest vertebrates --

The earliest confirmed chordate fossil is Cathaymyrus from the Lower Cambrian Chengjiang Fauna in China (a soft-bodied fossil assemblage much like that of the somewhat younger Burgess Shale). The form of Cathaymyrus suggests that it was a cephalochordate, but not a vertebrate. Pikaia of the Burgess Shale is very similar.
 
The first vertebrate fossils are tiny bits of bone (calcium phosphate) found in the Late Cambrian Deadwood Formation of Wyoming; this is a marine sandstone. They belong to an ostracoderm of the Class Agnatha.
 
The Class Agnatha is characterized by:
1. No jaws.
2. No paired fins.
3. Simple vertebrae.
4. Primitive brain.
(Lamprey eels are modern representatives of the Class Agnatha.)
 
Ostracoderms are the most ancient agnathan fish. They are characterized by:
1. An armored head shield.
2. No joint between the head and the body.
3. A mouth consisting of a series of small plates.
4. A body that is usually flattened dorso-ventrally.
5. Eyes usually on the dorsal part of the head.
6. A cartilaginous internal skeleton.

Ostracoderms appear to have been bottom-dwelling organisms that sucked up organic material from the sediment like vacuum cleaners. Later ostracoderms became more streamlined and "fishlike". Ostracoderms went extinct in the Devonian, probably because they could not compete with the jawed fish.
The placoderms (Class Placodermi, Devonian-Mississippian) originated from the agnathan fish; they included both freshwater and marine forms. Placoderms show advances over the agnathans by:
1. Jaws.
2. A mobile head (in advanced forms).
3. Presence of paired fins.
4. Partial attainment of a bony internal skeleton.

The most primitive placoderm (Bothriolepis) looks very much like an ostracoderm, except that it has weak jaws.
 
Jaws apparently were derived from the third of the gill arches (cartilaginous and bony supports for the fleshy gills) in agnathans.
 
Many placoderms (like Dunkleosteus) were effective predators.
Sharks (Class Chondrichthyes, Silurian-Recent) differ from other fish in the following ways:
1. An entirely cartilaginous skeleton.
2. Internal fertilization.
3. No air bladders or lungs.

One of the first and best known early sharks (Cladoselache) is found in the Ohio Black Shale (Devonian) near Cleveland.
 
Sharks may have evolved from placoderms, but those placoderm ancestors in the Silurian have not yet been found.
 
Acanthodians or "spiny fishes" (Class Acanthodii, Silurian-Permian) were small freshwater fish characterized by:
1. Jaws.
2. The first true scales.
3. Spines along the leading edges of their fins.
4. Paired fins for mobility.

The evolutionary origins of acanthodians are obscure; they probably descended from the agnathans.
 
The bony fish (Class Osteichthyes, Silurian-Recent) were apparently derived from the acanthodians. They have the following characteristics:
1. Completely ossified (bony) skeleton, both internal and external.
2. Streamlined body (in most).
3. Complex vertebrae for constrained lateral movement.
4. Air bladder in advanced forms (from 6th gill pouch).
The two major divisions of the bony fish are as follows:
Subclass Actinopterygii ("Ray-Fins")
-- 95% of all fish today.
-- Silurian to Recent.
-- Fins consist mostly of cartilaginous rays.

Subclass Sarcopterygii ("Lobe-Fins")
-- Rather rare fish today.
-- Devonian to Recent.
-- Fins contain a sequence of bones homologous to our own limbs.
The first lobe-finned fish looked very much like the first ray-finned fish, except for the structure of the fins and the occasional presence of lungs (suggested by internal nostrils).
 
The lungfish (Order Dipnoi, Devonian-Recent) are lobe-fins once thought to have been ancestral to the amphibians, but they do not possess:
1. Radius and ulna (and tibia and fibula) of the amphibians; only a single bone in those positions.
2. Sharp, pointed teeth; they have flat tooth plates.
3. A division between the front and back halves of their skulls, as do the early amphibians.

The crossopterygiians (Order Crossopterygii, Devonian-Recent) are lobe-finned fish with limb bone sequences, teeth and skull characteristics very similar to those of early amphibians.
 
The Suborder Coelacanthiformes includes the "living fossil" Latimeria.
-- This suborder is not ancestral to the amphibians.

 

The Suborder Rhipidistia includes fish which closely resemble early amphibians; the best studied is Eusthenopteron, which possessed:
1. Labyrinthodont teeth (characterized by enfolded enamel).
2. A strong bony skeleton.
3. Lungs (deduced from the presence of internal nostrils).
From the evidence presented, it appears that the Class Amphibia (Devonian-Recent) evolved in the Devonian from a fish very similar to Tiktaalik, a remarkable fossil recently found in the Canadian Arctic.
 
Two terms in evolutionary biology are important here:
Homology is the relationship of features of organisms which share the same evolutionary origin but not necessarily the same function.
For example, the limb bone structure in bats and humans is homologous since the same structure was derived from a common mammalian ancestor in the past; these structures are homologous regardless of function.

Analogy in evolutionary biology is when two unrelated structures evolve similar functions.
For example, the wings of bees and birds serve the same function, they are thus analogous, but they are not both derived from a common ancestor with wings; each developed its wings independently (through convergent evolution).

Partial Classification of the Phylum Chordata:

 

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