Archaeopteryx as a dinosaur

Higher relationships of Archaeopteryx. Archaeopteryx has an antorbital opening; therefore Archaeopteryx (and thus modern birds) is an archosaur. In the hind foot of Archaeopteryx and living birds, three toes point forward (digits II, III, and IV), and the fourth (digit I) is reduced; the toes are symmetrical around digit III, and all toes are clawed (Figure 10.5d). This condition is diagnostic of ornithodirans (see Figures 4.11 and 10.7). All living birds (as well as Archaeopteryx) have a fully erect stance, in which the shaft of the femur is 90° to the head, and the ankle of Archaeopteryx (and all birds) is a modified mesotarsal joint; these characters diagnose Dinosauria.

Archaeopteryx, because it bears characters diagnostic of Dinosauria, is a dinosaur. That being the case, living birds must be a subset of Dinosauria, and both of them should be part of an expanded Reptilia.

Archaeopteryx as a theropod. Archaeopteryx (and all modern birds, for that matter) have hollow bones: a character diagnostic of Theropoda. Moreover, Archaeopteryx bears an enlarged three-fingered hand with the deep pits at the end of the metacarpals so diagnostic of Theropoda (Figure 10.8, and see Chapter 9). Birds must therefore be theropods.

Archaeopteryx as a tetanuran. Archaeopteryx has a furcula, a character diagnostic of teta-nurans. Archaeopteryx also has elongate zygophophyses (leading to a stiffened tail), a shortened tooth row, and an astragalar groove. With a high ascending process on its astragalus, Archaeopteryx is clearly not at the base of Tetanurae.

Archaeopteryx as a coelurosaur. Archaeopteryx possesses the distinctive semi-lunate carpal, a shortened ischium (far shorter than the pubis) and large, circular orbits. Coelurosaurs have a furcula, as does Archaeopteryx. Archaeopteryx is a coelurosaur (Figure 10.8).

Archaeopteryx as a maniraptoran. As befits their name, all maniraptoran coelurosaurs have a grasping, three-fingered hand which is a modification of the ancestral theropod condition. Another maniraptoran character on Archaeopteryx is a shortened opisthopubic pubis; that is, one pointing directly down, in which the anterior face of the footplate is missing. The distinctive maniraptoran addition to that hand is an elongation of the middle digit (II). Archaeopteryx has this feature. Other maniraptoran features found in Archaeopteryx include a highly flexed neck, elongate forelimbs, and a distinctive bowed ulna. Archaeopteryx is a member of Maniraptora.

Archaeopteryx as a eumaniraptoran. Archaeopteryx has a highly reduced fibula, a eumaniraptoran character, as well as forelimbs that are equal to or greater than the length of the hindlimbs (Figure 10.8).

Figure 10.6. Cladogram ofAvialae (all living birds + Archaeopteryx). Derived characters include: at 1, very long arms, narrowing of the face and reduction ofthe size and number of the teeth, enlargement of the braincase, reduction of the fibula toward the ankle, and, significantly, the presence of feathers. As we shall see, although this last character had been valid since the discovery ofArchaeopteryx, it has become clear in the last seven years that it diagnoses a group significantly more inclusive than Avialae.

Figure 10.6. Cladogram ofAvialae (all living birds + Archaeopteryx). Derived characters include: at 1, very long arms, narrowing of the face and reduction ofthe size and number of the teeth, enlargement of the braincase, reduction of the fibula toward the ankle, and, significantly, the presence of feathers. As we shall see, although this last character had been valid since the discovery ofArchaeopteryx, it has become clear in the last seven years that it diagnoses a group significantly more inclusive than Avialae.

Saurischia

Dinosauria

Ornithodira

0 0

Post a comment