Used by permission of John W. Kimball
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
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:
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.)
Vertebrate features:
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:
Class Agnatha
2 - Anaspida
3 - Thelodonti
4 - Galeaspida
5 - Pituriaspida
6 - Petromyzontia (lampreys)
7 - Myxinoidea (hagfishes)
Hagfishes - primarily scavengers
2 - probably related to modern bony fishes
3 - small (less than 20 cm long) with large eyes
4 - Acanthodians most likely died out because of the rapidly increasing number of ray-finned fishes and sharks during the Permian
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')
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
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
2 - skeleton of fin lobe corresponds closely to proximal skeletal elements of early tetrapod limbs
4 - had swim bladders that may have been used as lungs
Class Amphibia
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 Archaeornithes
2 - weakly developed keel &, probably, weakly developed flight muscles
Subclass Prototheria
- egg-laying mammals
Summary - The Vertebrate 'Family Tree':
Useful links:
In Search of Vertebrate Origins
Pushing back the origins of animals
Back to BIO 342 Syllabus
