Ralph E. Molnar

Academic paper

Vertebrate Palaeontology in Australia

Pioneer Design Studio and Monash University, Melbourne

606-702

From a rigorously cladist viewpoint, reptiles do not exist. Reptiles can only be defined arbitrarily as those amniotes which are neither birds nor mammals. In a sense, this is not a real group, for although its members all share derived features not inherited from their ancestors (i.e. share synapomorphies), these synapomorphies are also shared by birds and mammals. Reptiles are defined by subtraction, they are the amniotes that are not birds or mammals. Although this rigorous cladist view is coming to be generally accepted, the more traditional usage will be retained here.
Unlike amphibians, reptiles lay eggs (amniote eggs) with a suite of membranes - the amnion, the allantois, the chorion, and the yolk-sac - which allow them to reproduce on land. They need not lay eggs in water (or very moist environments) as do most amphibians. The features distinguishing birds from reptiles are not generally agreed. One of the most obvious characteristics of birds is their feathers. Although it has been suggested that some dinosaurs may have had feathers, the possession of feathers is coming to be generally accepted as the criterion of being a bird. For mammals the jaw structure is used. In mammals the jaw usually consists of only a single element, the dentary (to be precise, other elements may be present as long as the dentary participates in the jaw articulation). Another feature that distinguishes reptiles (and birds) from mammals is cranial kinesis. This is not used in classification, but is important for understanding reptilian biology. It is common among squamatans and dinosaurs. "Kinesis", of course, refers to movement. Certain elements of the skull (and sometimes of the jaw) can m~e on one another. Usually the mobile units are groups of bones, such as those that make up the snout. In some cases kinesis allows reptiles to swallow larger prey than otherwise could be accommodated (as pythons swallowing pigs). Another suggested function, particularly for some dinosaurs where the amount of motion was small, is to reduce stresses within the skull during feeding. Reptiles that lack kinesis, such as crocodiles, are said to be akinetic. Further information on kinesis may be found in Hildebrand (1974).

English