Late Cretaceous Early Cretaceous Late Cretaceous Late Cretaceous Late Jurassic
Alberta, Canada Niger
Mongolia; China Mongolia, Kazakhstan Western USA
Western USA; Alberta, Canada Western USA; Alberta, Canada Western USA Mongolia; China China
Late Cretaceous Late Cretaceous Early Cretaceous Late Cretaceous Late Jurassic
Recognition of such increasingly exclusive differences between taxa may represent minutiae to a non-paleontologist, and they are in some cases minor distinctions. Nonetheless, paying attention to such small details is important in defining these clades. By now, a student of cladistics could compile all of the characters of a dinosaur, saurischian, theropod, tetanuran, and avetheropod. This listing would give a progressively more detailed hypothesis of an avetheropod's evolutionary history, beginning with archosaur ancestors in the Middle Triassic and leading up to the present. With Linnaean classification, a hierarchy of categories existed without so much of an evolutionary context. Cladistics has thus provided a hypothetical framework for unraveling relationships between dinosaurs, particularly for theropods.
Avetheropoda includes the stem-based clade Carnosauria, which has within it the allosaurids (such as Allosaurus, Carcharodontosaurus, and Giganotosaurus) and sinraptorids (such as Sinraptor and Yangchuanosaurus, Fig. 9.6B), as well as the stem-based Coelurosauria. Considerable diversification of theropods is represented within Coelurosauria. Coelurosaurs have several characters in their clade, but one of the most distinctive is a semilunate carpal, a carpal with a half-moon shape. Two coelurosaur outgroups consisting of one species each, Compsognathus and Dryptosaurus, have been proposed as more primitive coelurosaurs than Maniraptoriformes, a node-based clade within Coelurosauria. Maniraptoriformes includes the stem-based clade Arctometatarsalia, which is named for a specific novelty in metatarsal development, the middle metatarsal "pinched" between the metatarsals on either side of it (Fig. 9.7). Within Arctometatarsalia are ornithomi-mosaurs and tyrannosaurids, and possibly troodontids. Troodontids are somewhat problematic in their placement, and some paleontologists place them outside of Arctometatarsalia. Only about 20 years ago, such a grouping would have been considered an unlikely phylogenetic association on the basis of overall appearance. Ornithomimosaurs are the so-called "ostrich dinosaurs", because of their close resemblance to modern ostriches in overall morphology and size. Tyrannosaurids are mostly huge carnivores, and troodontids are small- to medium-sized theropods with relatively large braincases (mentioned earlier). Representatives of all three groups lived at the same time during the Late Cretaceous (including Troodon, Ornithomimus, and Tyrannosaurus). Nevertheless, an objective observer during that time may have had little reason to see a recent common ancestry for such apparently divergent forms, based on their superficial appearances (Fig. 9.8).
Other groups within Maniraptoriformes include what most paleontologists agree are the most unusual theropods, therizinosaurs (such as Therizinosaurus and Segnosaurus), which evidently shared ancestry with oviraptorisaurians (Oviraptor and Ingenia, for example). Oviraptorisaurians were also unusual, as they had behavior
FIGURE 9.7 "Pinched" Arctometatarsalia.
metatarsal in pes of Tyrannosaurus, a character of the clade
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