Natural History of Vertebrates
Lecture Notes
Chapter 8 - Origin and Radiation of Tetrapods

These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course. These notes are also linked with the notes from Vertebrate Structure and Development (ZO 515).

List of Terms

Nonamniote Tetrapods

The tetrapods evolved from the sarcopterygian lineage during the Middle Devonian (figure 8-1).

Selective forces on the earliest tetrapods
The thing to keep in mind, is that the characteristics that were advantageous to the earliest tetrapods were also characters that were advantageous to Panderichthys from which the tetrapods came. Panderichthys was a predator that probably laid in wait for victims to come by. It may have used its paired-fins to support the body as it waited or used the paired fins to move through the vegetation that dominated the edges of ponds or estuaries. Phylogenetic comparisons indicate that it would have had lungs and gulped air to breathe.

The most likely explanation for the evolution of the early tetrapods is one in which predation on terrestrial invertebrates allowed the earliest tetrapods to exploit an abundant food resource. Adults of the earliest tetrapods may not have been able to move about on land, but the juvenile stages were probably light enough and mobile enough to move about on land in pursuit of prey. The ability to breath air is also important. Any vertebrate living along the edge of a warm body of water (especially a slow-moving stream or estuary) will find itself in water that is hypoxic. In this case, the ability to breath air is very necessary. Thus, the characteristics that we see as advantageous to panderichthyids are also advantageous to the earliest tetrapods.

There are several tetrapod genera that are known from the Late Devonian. It is clear that by the end of the Devonian, the tetrapods had managed to evolve several forms that were able to exploit different niches of the time.

In the evolution of the tetrapod limb, we see several new innovations that increase the load carrying capacity of the limbs as these animals adopt an increasingly more terrestrial life style. In the pectoral girdle, there is a new bone (the sternum) and the scapulacoracoid attaches to the vertebrae via muscles. In the pelvic girdle, the illium, ischium, and pubis unite and attach firmly to the vertebrae in a bone-to-bone connection. In the forelimb itself, there is one large proximal bone (humerus) that articulates (elbow joint) with two more distal bones (ulna and radius). These articulate (wrist joint) with two smaller bones (ulnare, radiale). The radiale and ulnare then articulate with the carpals and these with the digits. Study figures 8-4 and 8-5 to learn the development of the limb. The hindlimb is very similar.

From the Devonian to the beginning of the Cretaceous, there were many different forms of nonamniote tetrapods. Several terrestrial forms probably gave rise to aquatic forms and thus we see the reappearance of traits that are associated with an aquatic lifestyle. This led to many cases of convergent and parallel evolution that make producing a phylogeny of these organisms very difficult.

The extant amphibians (Lissamphibia) are very different from the many nonamniote tetrapods that were present during the Paleozoic and Mesozoic. The Paleozoic forms are divided into several different groups (though these are probably not monophyletic assemblages), the Bactramorpha, the Reptilomorpha, and the Lepospondyls (figures 8-13 and 8-14). Many of these had labyrinthodont teeth (sharp, pointed teeth with a complex folding of the wall of the pulp cavity) and two parts (bipartite) to each vertebral centrum (figure 8-3). Many were large animals (up to crocodile size) The labyrinthodont grade is represented by two main clades Bactracomorpha (represented by Temnospondylii) generally aquatic, with flat immobile skulls,and reduced numbers of fingers in the hand. The modern Lissamphibia probably came from this group.
Reptilomorpha were more terrestrial, had kinetic skulls, and retained five fingers per hand. The amniota probably came from this group.

Lepospondyls (figure 8-22) tended to be long-bodied and in some forms legless. They were generally smaller than the labyrinthodonts.

Ecologically all of these organisms were predaceous. Some were primarily terrestrial and some were exclusively aquatic. Several forms retained external gills (a larval character) as adults through neoteny. Many were sit-and-wait predators that prowled the edge of bodies of water eating any smaller organism that came close enough to be ambushed.

Last updated on 24 February 2003
Provide comments to Dwight Moore at mooredwi@emporia.edu
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