Avetheropoda (avis - bird; a reference to birdlike features of many members of this group) is that group of theropods more closely related to birds than are the preceding genera and their near relatives. Avetheropods share many derived features, and consist of two clades, Carnosauria and Coelurosauria (Figure 9.28).
Coelurosaurs adapted the wrist with the development of the semi-lunate carpal, a wrist bone modification that increased manual dexterity and the ability to sever flesh from bone (Figure 9.29). This group of theropods includes many large forms, most famously the tyrannosauroids. It also includes some small forms (Figure 9.30), as well as Maniraptora, such as ornithomimosaurs and oviraptorosaurs (which we've met), therizinosauroids (which we have not), and the non-avian theropods closest to birds, Eumaniraptora.
Therizinosauroids represent a strange theropod venture into herbivory. Large and ponderous, these highly evolved and distinctive maniraptoran theropods were apparently
Figure 9.27. Zygapophyses in tetanurans. Note how these processes extend across the adjacent vertebrae both anteriorly and posteriorly, hindering flexibility.
Figure 9.28. Cladogram of Avetheropoda. Derived characters include: at 1, enlarged external nares, participation of lateral surface of nasal in antorbital cavity, presence of nasal recesses, prefrontal excluded from rostral rim of orbit, supraorbital notch between postorbital and prefrontal, paroccipital processes directed strongly ventrolaterally from occiput to below level of foramen magnum, very short basipterygoid processes, mid-cervical centrum length less than twice the diameter of forward articular surface, front margin of spinous processes of proximal mid-caudal vertebrae with distinct kink, spur along front margin of spinous processes of mid caudals; at 2, presence of a pterygopalatine fenestra, short mandibular process on pterygoid, presence of sternal ribs (three pairs), reduction of coracoid process, semi-lunate carpal, U-shaped ischial obturator notch, loss of transverse groove on astragalar condyle; at 3, crenulated ventral margin of premaxilla, parietal at least as long as frontal, U-shaped mandibular symphysis.
herbivorous and have been compared to giant sloths (Figure 9.31). Their affinities have been a matter of considerable confusion, but the consensus is that they were an aberrant group of theropods.
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