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By Phil Myers
Reptilia, presented as a Class in our classification, includes turtles (Testudines), snakes and lizards (Lepidosauria), crocodiles and their relatives (Crocodilia), and birds (Aves), as well as a number of extinct groups. Reptiles (including birds!) are amniotes; that is, their eggs are protected from dessication and other environmental problems by an extra membrane, the amnion, not found in the first terrestrial vertebrates (amphibians). Mammals (Mammalia) are also amniotes, but they differ from reptiles in the structure of their skulls (especially the regions associated with chewing and hearing). Mammals also have hair and feed their young with milk produced by modified skin glands (mammary glands).
In addition to being amniotes, all reptiles have (or did have, in their evolutionary history) horny epidermal scales made of a particular kind of protein, paired limbs with 5 toes, skulls with a single occipital condyle, lungs instead of gills for respiration, and a 3 or 4 chambered heart. Their eggs are covered with a leathery or calcium-based shell (partially or completely lost in some species that give birth to live young), and fertilization occurs inside the female, rather than outside, as it does in most amphibians. Members of Reptilia generally share many additional traits, for example in their nervous and excretory systems, locomotion, and reproduction.
Why are birds included within Reptilia, and how are they and other members of this group related to each other? Both the fossil record and comparative analyses of living species (especially those based on molecular evidence) convincingly establish that, among living reptiles, birds and crocodiles are more closely related to each other than they are to lepidosaurs (snakes and lizards). The position of turtles is more controversial; in the past they were thought to represent an early branch of Reptilia. Recent evidence suggests they may have a special relationship with crocodiles and birds. Because birds clearly arise from within the groups we traditionally consider to be reptiles, not separately from them, most systematists now formally consider birds (Aves) to be a subgroup within Reptilia.
References:
- Hickman, C. P. Jr., L. S. Roberts, and A. Larson. 2003. Animal Diversity, 3rd edition. McGraw Hill, Boston.
- Laurin, M. and J. A. Gauthier. 2001. Amniota.
http://tolweb.org/tree?group=Amniota&contgroup=Terrestrial_Vertebrates
Related Sites:
Last updated 24 February 2004.
Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
