Comparative vertebrate anatomy - the study of structure, of the function of structure, & of the range of variation in structure & function among vertebrates:
2 - pharynx with pouches or slits in wall (at least in the embryo)
3 - dorsal, hollow nervous system
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:
Among the oldest & best known = ostracoderms
Non-vertebrate chordates still alive today include tunicates (or sea squirts; urochordates) & amphioxus (or branchiostoma). (cephalochordates)
Phylum Chordata - established in 1874 & included organisms with:
2 - pharyngeal pouches or slits
3 - dorsal, hollow nervous system
4 - cells that produce the hormone thyroxine
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:
2 - gill slits
3 - a short diverticulum of the gut called the stomochord
Possible invertebrate ancestors:
& longitudinal nerve cord
Agnathans vs. Gnathostomes:
Hagfishes - primarily scavengers
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.
2 - probably off the main line of vertebrate evolution
3 - many had bony dermal shields
4 - some were probably predators (with large, sharp 'tooth plates')
Class Chondrichthyes - cartilaginous fishes
2 - pelvic fins of males are modified as claspers
3 - placoid scales
4 - numerous today but more abundant in the past
O. Selachii - 'modern' sharks
O. Batoidea - rays & skates
2 - naked gill slits (no operculum)
3 - mouth located ventrally
Class Osteichthyes - bony fishes
2 - gill slits are covered by a bony operculum
3 - skin has scales with, typically, little bone
4 - most have a swim bladder
4 - had swim bladders that may have been used as lungs
2- fin-rays in the tail (for swimming)
3- a skull similar to that of some Crossopterygians
4- a sensory canal system (like the lateral line system) that indicates a primarily aquatic existence
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:
2 - middle ear cavity with ear ossicle (columella)
3 - no bony scales (except apodans)
Class Reptilia - the first amniotes:
2 - clawed
3 - large, yolk-laden, shell-covered eggs laid on land
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.
2 - lost several dinosaur characteristics (e.g., long tail & teeth) but retained others (e.g., claws, scales, diapsid skull, single occipital condyle &, perhaps, feathers) (see AMNH website & ABC News website)
Subclass Prototheria - egg-laying mammals
Summary - The Vertebrate 'Family Tree':
In Search of Vertebrate Origins
Pterosaurs rule the air
Pushing back the origins of animals
The Shape of Life
Walking with dinosaurs
Back to BIO 342 Syllabus