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19. Theropods
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347 Theropods Thomas R. Holtz Jr. The Theropoda (“beast-footed ones”) are a clade of saurischian dinosaurs commonly referred to as the carnivorous dinosaurs. It is certainly true that the majority (perhaps all) of the dinosaurs with a taste for flesh were theropods , and indeed, from the beginning of the Jurassic Period (199.6 Ma) until the end of the Cretaceous (65.5 Ma), these were the dominant terrestrial predators on every continent (Figs. 19.1, 19.2). However, this group produced a series of omnivorous and herbivorous forms, just as the placental group Carnivora has produced omnivorous raccoons and herbivorous pandas. Theropods famously include the largest terrestrial carnivores in Earth’s history: taxa such as Spinosaurus, Giganotosaurus, and Tyrannosaurus dwarf both their rauisuchian predecessors and their mammalian successors. Yet many theropods were quite small: Epidendrosaurus, Microraptor, Mei, and Hesperonychus are among the smallest dinosaurs of the Mesozoic. Theropods represent the most successful group of dinosaurs; they alone survived the catastrophe at the end of the Cretaceous in the form of that most specialized group of feathered theropods: Aves (the birds), about which see Naish (this volume). 19 19.2. Phylogeny of the coelurosaurian theropods , based primarily on Holtz et al. (2004), Li et al. (2009), Longrich and Currie (2009a, 2009b), Mayr et al. (2005), Senter (2007), Zanno et al. (2009), and Zhang et al. (2008). Timescale from Gradstein et al. (2004). Solid lines = known range from oldest to youngest specimens; dashed lines = inferred branching relationships between groups. Question marks show uncertainty of phylogenetic position of Archaeopteryx (Mayr et al. 2005). Date for Eshanosaurus is uncertain; may be as old as Early Jurassic or as late as Early Cretaceous (Barrett 2009). Thomas R. Holtz Jr. 348 Theropods are the only major clade of dinosaur to retain the ancestral dinosaurian habit of obligate bipedality throughout their entire history. In other clades, larger and more specialized forms often reverted to partial (iguanodontian ornithopods) or obligate (derived sauropodomorphs, derived thyreophorans, derived marginocephalians) quadrupedality in response to increasing body size, development of heavy armor, enlarged frilled skulls, and so forth. Even the largest theropods–rivaling modern African elephants in mass–were strict bipeds. Most theropods retained the ziphodont (bladelike) teeth typical of other archosauriforms (such as Euparkeria, ornithosuchids, rauisuchians, and erythrosuchians), and indeed of many other carnivorous vertebrates (Farlow et al. 1991). Additionally, typical theropods retained the long grasping hands shared by primitive members of the other dinosaur groups (heterodontosaurids and Eocursor among the ornithischians; early sauropodomorphs). In particular, the manual claws of theropods were recurved, tapering to sharp points; this presumably allowed them to more effectively clutch onto prey items. The skull bones of theropods show numerous chambers and openings associated with a complex of air sacs (pneumatic diverticula), especially in the maxillae, nasals, orbital region, palate, and (especially in derived forms) throughout the braincase (Witmer 1997). Postcranially, theropods had hollow limb bones and vertebrae (in the cervical region in primitive theropods, and more extensively in derived forms). The pattern of postcranial pneumaticity strongly suggests the presence of a birdlike flow-through lung, which was developed relatively early in theropod history (O’Connor and Claessens 2005), or perhaps even earlier in dinosaurian history (Wedel 2006). Theropoda has been defined (as a branch-based taxon) as Passer domesticus (the European house sparrow) and all taxa sharing a more recent common ancestor with it than with Cetiosaurus oxoniensis (Holtz and Osmólska 2004). However, a revision (D. Naish et al. personal communication) has suggested replacing Passer domesticus with Allosaurus fragilis. Consequently , the origin of theropods and sauropodomorphs occurred as a single divergence. Because there is debate about whether the earliest carnivorous dinosaurs were truly theropods (see below), the oldest unquestionably theropod fossils are present from the Norian Age (middle Late Triassic) onward (Nesbitt et al. 2007). The existence of the Carnian Age (early Late Triassic ) sauropodomorphs Panphagia and Saturnalia indicates that the split between Theropoda and its sister group Sauropodomorpha had occurred by the early Late Triassic, and at least some studies have suggested that the Carnian carnivorous dinosaurs are true theropods. Three-toed footprints from Middle Triassic of Argentina (Marsicano et al. 2007) might represent extremely early theropods, but they might alternatively be from a nondinosaur instead. There are definite Carnian Age carnivorous dinosaurs in the form of Eoraptor of Argentina and Herrerasauria (including Herrerasaurus of Argentina and Staurikosaurus of Brazil) (Fig. 19.3A). These are considered basal members of Theropoda by some workers (Rauhut 2003; Sereno 2007; Nesbitt et al. 2009). However, other...