BIO 342
Comparative Vertebrate Anatomy
Lecture Notes 1 - Chordate Origins & Phylogeny

Comparative vertebrate anatomy - the study of structure, of the function of structure, & of the range of variation in structure & function among vertebrates:

Vertebrate characteristics:

Used by permission of John W. Kimball

Notochord = rod of living cells ventral to central nervous system & dorsal to alimentary canal

Fate of notochord during development:


Pharynx - region of alimentary canal exhibiting pharyngeal pouches in embryo; pouches may open to the exterior as slits:

Dorsal, hollow central nervous system - consists of brain & spinal cord & contains a central cavity (called the neurocoel)

Vertebrate beginnings

Among the oldest & best known = ostracoderms

Before ostracoderms?


Before Vertebrates?


Non-vertebrate chordates still alive today include tunicates (or sea squirts; urochordates) & amphioxus (or branchiostoma). (cephalochordates)

Phylum Chordata - established in 1874 & included organisms with:


A 530 million-year-old (although perhaps as old as 560 million years) creature, Cheungkongella ancestralis, probably a tunicate, found in the Chengjiang fauna in China's southwest Yunnan Province, might be the earliest known fossil evidence of primitive chordates (Shu, D.-G., L. Chen, J. Han, X.-L. Zhang. 2001. An early Cambrian tunicate from China. Nature 411:472 - 473.)

Hemichordates = acorn worms

Bateson added acorn worms to the phylum Chordata in 1884 because they have:

Present consensus = the stomochord is not homologous with the notochord and Hemichordates are placed in a separate phylum

Possible invertebrate ancestors:

Agnathans vs. Gnathostomes:

Class Agnatha

Ostracoderms (Osteostraci, Anaspida, Heterostraci, & Coelolepid): Cyclostomes (Petromyzontia & Myxinoidea):


The relationships of acanthodians to other vertebrates has been the subject of considerable debate. Early researchers considered them to be most closely related to the ray-finned fishes, but most scientists during the mid-20th Century considered acanthodians to have a closer affinity to the sharks. Opinion has now generally swung back in favor of a closer relationship with ray-fins, but this is far from universally accepted.

Class Placodermii:
Placoderms were armored jawed fishes that first appeared about 420 million years ago (MYA) during the Silurian Period. They had diversified dramatically by the beginning of the Devonian and came to dominate most marine and freshwater ecosystems before becoming extinct at the end of that period (355 MYA). About 200 genera of placoderms have been discovered, with most of these occurring during the Devonian radiations. The rapid evolution and diversity of placoderms have made them useful in dating Devonian rocks. Placoderms (= plated skin) were named for their heavy armor of dermal bone, which formed large shields on the head and thorax. The rest of their bodies was covered with small bony scales or was without dental armor. The head and trunk shields of most placoderms were articulated by bony joints. This joint apparently allowed the forward part of the skull to tilt up, increasing the gape. Placoderms lacked teeth, but biting or grinding structures are often be found in the dermal bones lining their mouths. Placoderms evolved into a variety of body forms in a relatively short time. Many were torpedo-shaped, but there were notable expections, including the flatten Phyllolepida and the bottom feeding Antiarchi. Most placoderms were less than 30 cm (2 feet) in length, but some members of the dinichthyids (= terrible fish) reached or exceeded 6 m (20 ft), making them the first giants of the vertebrate lineage.

Class Chondrichthyes - cartilaginous fishes

Class Osteichthyes - bony fishes

1 = operculum, 2 = dorsal fin, 3 = caudal peduncle (The narrow section of a fish's body directly
anterior to the insertion of the tail but before the mid-body.), 4 = caudal fin or "tail", 5 = anal fins,
6 = pelvic fins, & 7 = pectoral fins

Used with permission of John Kimball

Class Amphibia

Labyrinthodonts are distinguished by deeply folded structure of enamel and dentine layers in the teeth, that look like an intricate labyrinth in the cross section, hence the name of this group. Labyrinthodonts were probably similar to fishes in their mode of living. Labyrinthodonts, like fishes and most modern amphibians, laid eggs in the water, where their larvae developed into mature animals. All labyrinthodonts had special sense organs in the skin, that formed a system for perception of water fluctuations. Moreover, some of them possessed well developed gills. In contrast, many labyrinthodonts seemingly had primitive lungs. They could breath atmospheric air, that was a great advantage for residents of warm shoals with low oxygen levels in the water. The air was inflated into the lungs by contractions of a special throat sac. Primitive members of all labyrinthodont groups were probably true water predators, and only advanced forms that arose independently in different groups and times, gained an amphibious, semi-aquatic mode of living. Mature individuals of advanced labyrinthodonts could live on land, feeding mainly on insects and other small invertebrates. Well ossified robust skeletons in some Late Carboniferous and Early Permian labyrinthodonts prove their adaptation to the terrestrial mode of life. It suggests that amphibians had successfully 'organized' actual terrestrial assemblages prior to the wide expansion of reptiles.

The most diverse group of the labyrinthodonts was the batrachomorphs ('similar to a frog'). Though these animals looked more like crocodiles, they most probably gave rise to the order Anura, the amphibians without tails, which include, in particular, the modern frogs. Batrachomorphs appeared in the Late Devonian, but they had worldwide distribution in the continental shallow basins of the Permian (Platyoposaurus, Melosaurus) and Triassic Periods (Thoosuchus, Benthosuchus, Eryosuchus). Some batrachomorphs existed until the end of the Cretaceous.

Modern amphibian characteristics:

Class Reptilia - the first amniotes:

Stem reptiles = Cotylosaurs (about 300 mybp)

Representative ichthyosaurs

Saurischia (sawr-RIS-kee-ah) & Ornithischia are the two orders of dinosaurs, with the division based on the shape of the pelvic bone. The saurischian pubis (left) juts forward, and its ischium points backward. The ornithischian pubis and ischium (right) both point backward. The ornithischians were all herbivorous, and included some of the most interesting-looking dinosaurs. Ornithischian dinosaurs include three suborders: Ornithopoda, Marginocephalia and Thyreophora. The famous carnivorous dinosaurs were from the saurischian order, as were the largest herbivorous dinosaurs. The saurischian dinosaurs include two suborders: Theropoda and Sauropodomorpha.

The first vertebrates to evolve true flight were the pterosaurs, flying archosaurian reptiles. After the discovery of pterosaur fossils in the 18th century, it was thought that pterosaurs were a failed experiment in flight; a humorous mishap; or that they were simply gliders, too weak to fly. More recent studies have revealed that pterosaurs were definitely proficient flyers, and were no evolutionary failure; as a group they lasted about 140 million years (about as long as birds have)! Pterosaurs are thought to be derived from a bipedal, cursorial (running) archosaur in the late Triassic period (about 225 million years ago). No other phylogenetic hypothesis has withstood examination; however, the early history of pterosaurs is not yet fully understood because of their poor fossil record in the Triassic period. We can infer that the origin of flight in pterosaurs fits the "ground up" evolutionary scenario, supported by the fact that pterosaurs had no evident arboreal adaptations.

The pterosaur wing was supported by an elongated fourth digit (imagine having a "pinky finger" several feet long, and using that to fly!). Pterosaurs had other morphological adaptations for flight as a keeled sternum for the attachment of flight muscles, a short and stout humerus (the first arm bone), and hollow but strong limb and skull bones. Pterosaurs also had modified scales that were wing-supporting fibers, and that possibly formed hairlike structures to provide insulation -- bird feathers are analogous to the wing fibers of pterosaurs, and both are thought to possibly have been evolved originally for the primary purpose of thermoregulation (which implies, but does not prove, that both pterosaurs and the earliest birds were endothermic).

Early pterosaurs (such as Dimorphodon) had long tails that assisted balance, but later pterosaurs had no tails, and may have been more adept flyers. The most derived pterosaurs, such as Pteranodon and Quetzalcoatlus, were so large that soaring was the only feasible option; these were the largest flyers ever to cast a shadow on the Earth's surface.

Reptile subclasses - classified in part according to presence or absence of temporal openings

Temporal fenestration has long been used to classify amniotes. Taxa such as Anapsida, Diapsida, Euryapsida, and Synapsida were named after their type of temporal fenestration. Temporal fenestra are large holes in the side of the skull. The function of these holes has long been debated. Many believe that they allow muscles to expand and to lengthen. The resulting greater bulk of muscles results in a stronger jaw musculature, and the longer muscle fibers allow an increase in the gape.

Class Aves - birds
Class Mammalia

Summary - The Vertebrate 'Family Tree':

Useful links:

Discovering Dinosaurs

In Search of Vertebrate Origins

Pterosaurs rule the air

Pushing back the origins of animals

The Shape of Life

Walking with dinosaurs

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