Tetrapod: Encyclopedia II - Tetrapod - Anatomical features of early tetrapods

Tetrapod - Anatomical features of early tetrapods

The amphibian's ancestral fish must have possessed similar traits to those inherited by the early amphibians, including internal nostrils (to separate the breathing and feeding passages) and a large fleshy fin built on bones that could give rise to the tetrapod limb. The rhipidistian crossopterygians fulfill every requirement for this ancestry. Their palatal and jaw structures were identical to those of amphibians, and their dentition was identical too, with labyrinthine teeth fitting in a pit-and-tooth arrangement on the palate. The crossopterygian paired fins were smaller than tetrapod limbs, but the skeletal structure was very similar in that the crossopterygian had a single proximal bone (analogous to the humerus or femur), two bones in the next segment (forearm or lower leg), and an irregular subdivision of the fin, roughly comparable to the structure of the carpus / tarsus and phalanges of a hand.

The major difference between crossopterygians and amphibians was in relative development of front and back skull portions; the snout is much less developed than in most amphibians and the post-orbital skull is exceptionally longer than an amphibian's.

A great many kinds of early amphibians lived during the Carboniferous period. Therefore, their ancestor would have lived earlier, during the Devonian period. Devonian Ichthyostegids were the earliest of amphibians, with a skeleton that is directly comparable to that of rhipidistian ancestors. Early Labyrinthodonts (Late Devonian to Early Mississippian) still had some ichthyostegid features such as similar skull bone patterns, labyrinthine tooth structure, the fish skull-hinge, pieces of gill structure between the cheek and shoulder, and the vertebral column. They had, however, lost several other fish features such as the fin rays in the tail.

In order to propagate in the terrestrial environment, certain challenges had to be overcome. The animal's body needed additional support, because buoyancy was no longer a factor. A new method of respiration was required in order to extract atmospheric oxygen, instead of oxygen dissolved in water. A means of locomotion would need to be developed to traverse distances between waterholes. Water retention was now important since it was no longer the living matrix, and it could be lost easily to the environment. Finally, new sensory input systems were required if the animal was to have any ability to function reasonably while on land.

Tetrapod - Classification

Labyrinthodontia Diagnostic features unique to the Labyrinthodontia are hard to find at first glance; the complex dentine infolding tooth structure was shared with crossopterygian fish. The labyrinthodonts are divided into the Temnospondyli and the Anthracosauria, the main difference between the two groups being their respective vertebral structures. The Anthracosauria had small pleurocentra, which grew and fused, becoming the true centrum in later vertebrates. In contrast, the Temnospondyli had a conservative vertebral column in which the pleurocentra remained small in primitive forms, vanishing entirely in the more advanced ones. The intercentra are large and form a complete ring.

Temnospondyls A diagnostic feature of the Temnospondyli was that the tabular bone in the skull roof is relatively small and had no contact with the parietal bone, whereas contact between the two bones was present in all anthracosaurs.

Although the temnospondyls flourished in many forms in the Late Palaeozoic and Triassic, they were an entirely self-contained group and did not give rise to any later tetrapod groups. It was the sister group Anthracosauria that gave rise to the reptiles.

Within the Temnospondyli are the two suborders Rachitomi and Stereospondyli, also distinguished by their vertebrae. There were three distinct successive stages within the Rachitomi, the first occurring in the Carboniferous. The second happened mostly in the Pennsylvanian, continuing into Permian, of which Erydops is characteristic. The third and final stage was in the Late Carboniferous and Early Permian, from which Eryops of the Texas Permian red beds is best known. Just as there were numerous side branches throughout the evolution of the temnospondyls, so too were there many of the rachitomes.

Of special interest in regard to the Rachitomi, is Branchiosaurus. This relatively tiny amphibian lived from the Late Pennsylvanian to the Early Permian and was very similar to the Rachitomi, differing only in its small size. However, it had a much less ossified skeleton, a short skull and other distinguishing features. Clear traces of gills are present in many fossilized samples, hence the name. Thought to have vertebra differing from rachitomous vertebrae, it was placed in a separate order named Phyllospondyli. Only later was it realized, by studying growth stages and seeing increasing ossification in larger specimens, that it was in fact the larval stage of a much larger rachitome like Eryops.

Adapted from the Wikipedia article "Anatomical features of early tetrapods", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki