A new North American therizinosaurid and the role of herbivory in 'predatory' dinosaur evolution

Abstract

Historically, ecomorphological inferences regarding theropod (i.e. 'predatory') dinosaurs were guided by an assumption that they were singularly hypercarnivorous. A recent plethora of maniraptoran discoveries has produced evidence challenging this notion. Here, we report on a new species of maniraptoran theropod, Nothronychus graffami sp. nov. Relative completeness of this specimen permits a phylogenetic reassessment of Therizinosauria-the theropod clade exhibiting the most substantial anatomical evidence of herbivory. In the most comprehensive phylogenetic study of the clade conducted to date, we recover Therizinosauria as the basalmost maniraptoran lineage. Using concentrated changes tests, we present evidence for correlated character evolution among herbivorous and hypercarnivorous taxa and propose ecomorphological indicators for future interpretations of diet among maniraptoran clades. Maximum parsimony optimizations of character evolution within our study indicate an ancestral origin for dietary plasticity and facultative herbivory (omnivory) within the clade. These findings suggest that hypercarnivory in paravian dinosaurs is a secondarily derived dietary specialization and provide a potential mechanism for the invasion of novel morpho- and ecospace early in coelurosaurian evolution-the loss of obligate carnivory and origin of dietary opportunism.

Figure 1.

Skeletal reconstruction (preserved shown in white) and representative elements of N. graffami (UMNH VP 16420): (a) cranial dorsal vertebra, cranial view; (b) mid-dorsal vertebra, craniolateral view; (c) left ischium, lateral view; (d) left pubis, lateral view; (e) dorsoventrally crushed iliosacrum, dorsal view; (f) proximal caudal vertebra, left lateral view; (g) right ischium/pubis, lateral view; (h) iliosacrum, ventral view; (i) furcula, cranial view; (j) right scapulacoracoid, lateral view; (k) proximal caudal vertebra, caudal view; (l) right iliosacrum, oblique ventrolateral view, showing crushed peduncles, acetabulum, brevis fossa and cubic tuberosity of postacetabular process; (m) four distalmost caudal vertebrae, lateral view; (n) left and right humeri, cranial view; (o) chevrons, cranial view; (p) left femur, cranial view; left tibia (q) proximal and (t) caudal views; left fibula (r) proximal and (u) lateral views; (s) left metacarpus, dorsal view; (v) antebrachium; (w) manual unguals, lateral view; (x) proximal (left) and dorsal (right) views of pes (missing PI–II, PIII–II, PIII–IV and PIV–V); and (y) left astragalus, cranial view. For abbreviations, see electronic supplementary material. Scale bars: 10 cm (a–e, g–y); 5 cm (f). Skeletal drawing modified from Victor O. Leshyk, copyright 2007.

Figure 2.

Time-calibrated phylogeny showing the temporal range of the 76 coelurosaurian dinosaurs included in this study only, with diet interpreted for nine major coelurosaurian subclades based on ecomorphological traits supported by CCTs and MP optimizations. Clades interpreted as predominant herbivory or hypercarnivory possess species-level ecomorphology correlated with direct evidence of diet using CCTs. Interpretation of at least facultative herbivory in basal ornithomosaurs, therizinosaurs and oviraptorosaurs based on the presence of at least two CCT-supported traits per basal species (CCT characters 263 and 349 in Pelecanimimus, 263 and 355 in Falcarius, 349 and 355 in Incisivosaurus). Ecomorphological characters potentially related to herbivory listed at select nodes. Traits supported by CCTs are listed at the right of the tree and denoted by the ‘dagger’ symbol (distribution illustrated in flow chart). Optimization of ecomorphological dental characteristics based on MP is listed at the left of the tree and denoted by an asterisk. MP optimization of direct fossil evidence of herbivory listed at node Paraves. The origin of facultative herbivory (i.e. omnivory) and the point of dietary diversification for the clade is posited at the base of Maniraptorformes (grey box). Coelurus fragilis is excluded from the tree owing to widely conflicting interpretations regarding its taxonomic and phylogenetic relationships (e.g. Senter 2007; Turner et al. ,; this study).