Amniotes

            The remainder of the tetrapods are amniotes.  They possess a specialized egg (or a derivative of this egg) with a shell and extraembryonic membranes that provide gas exchange, waste storage, and a watery environment for the developing embryo.  The amniotic egg is discussed on p. 693 of Campbell.  Three classes are traditionally recognized in the amniotes, the Reptilia, Aves, and Mammalia.  Refer back to exercise 5 in this lab manual (p. 5-8).  Are the classes of tetrapods (Amphibia, plus the three amniote classes) all monophyletic?  Why, or why not?  Use the cladogram shown on p. 5-8, plus the cladogram on p. 694 of Campbell to help resolve this question.  What would you do to reclassify the tetrapods?

Class Reptilia, with consideration of "Class" Aves

            One lineage of ancient amphibians gave rise to the class Reptilia. With a desiccation-resisting skin, more efficient suspension of the legs, and an amniotic egg with membranes that contain water and a shell for protection, the reptiles became successful on land and underwent a tremendous evolutionary radiation. There are four orders with extant members that have been traditionally included in the Reptilia: turtles (order Testudines) (V & C fig. 7.186), snakes and lizards (order Squamata) (V & C fig. 7.185, 7.1187-7.189), crocodilians (order Crocodilia), and the rare Tuatara of New Zealand (order Rhyncocephalia or Sphenodontia).

            Although we will only briefly touch on it, it is important to consider the groups of extinct reptiles when discussing evolutionary relationships. Information about dinosaurs, for example, can help piece together the fragmentary portrait of relationships of extant reptile groups. An example of this can be seen when considering the birds.

            (1) "Class" Aves?

                         The dinosaurs were, of course, the dominant group of reptiles during the "age of reptiles". These included the large forms we see as skeletons in museums, but also some smaller, fast-running forms. Most biologists now regard birds as having a common ancestry with these kinds of dinosaurs, a relationship that is not concordant with traditional classifications. It may be evolutionarily accurate to include Aves  (V & C fig. 7.190-7.194) within the Class Reptilia as a group equivalent in taxonomic rank to the currently recognized orders of reptiles.

            This doesn’t change what we know about birds - it simply gives a more resolved picture of the phylogenetic relationships among these animals. There are, by the way, many orders of birds, but we won’t have time to delve into them in this lab.