Figure 10.7. Cladogram depicting the position of Archaeopteryx within Archosauria. Derived characters include: at 1, antorbital opening (Archosauria); at 2, four-toed, clawed foot, with symmetry around digit III, digit I reduced, lying closely appressed to and along side digit II (Ornithodira); at 3, semi-perforate acetabulum (Dinosauria); at 4, ascending process on the astragalus (Saurischia). The cladogram shows that Archaeopteryx, and therefore birds, are dinosaurs. Within Dinosauria, the character at 4 among others (see introduction to Part III: Saurischia) indicates that Archaeopteryx, while a bird, is also a saurischian dinosaur.
224 Theropoda II
Figure 10.8. Cladogram depicting the position of Archaeopteryx within Dino-sauria. Some of the characters defining each node are as follows: 1 is the same as 3 in Figure 10.7; 2 is the same as 4 in Figure 10.7. At 3, one obvious diagnostic character (of many) is the hollow bones possessed by all theropods. At 4 is shared elongation of the hemal and neural arches, and possession of a furcula; at 5, opisthopubic pubis.
Archaeopteryx as an avialan. In Archaeopteryx, as in all avialans, the tail vertebrae all show an extensive elongation of the hemal and neural arches. Likewise, the teeth of avialans lose their serrations - as in Archaeopteryx.
What can we conclude from all this? That Archaeopteryx is an avialan theropod. Because Archaeopteryx is also a bird, we conclude that birds are avialan theropods as well (Box 10.1).
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