Halszkaraptor escuilliei and the evolution of the paravian bauplan

Abstract

The evolution of birds from dinosaurs is a subject that has received great attention among vertebrate paleontologists. Nevertheless, the early evolution of the paravians, the group that contains birds and their closest non-avian dinosaur relatives, remains very poorly known. Even the most basal members of one paravian lineage, the Dromaeosauridae, already show a body plan that differs substantially from their closest non-paravian relatives. Recently, the dromaeosaurid Halszkaraptor escuilliei was described from the Cretaceous of Mongolia. Halszkaraptor possesses numerous unserrated premaxillary teeth, a platyrostral rostrum with a developed neurovascular system, an elongate neck, bizarrely-proportioned forearms, and a foreword-shifted center of mass, differing markedly from other paravians. A reevaluation of the anatomy, taphonomy, environmental setting, and phylogenetic position of H. escuilliei based on additional comparisons with other maniraptorans suggests that, rather than indicating it was a semiaquatic piscivore, the body plan of this dinosaur bears features widely distributed among maniraptorans and in some cases intermediate between the conditions in dromaeosaurids and related clades. I find no evidence for a semiaquatic lifestyle in Halszkaraptor. A phylogenetic reevaluation of Halszkaraptorinae places it as the sister clade to Unenlagiinae, indicating the bizarre features of unenlagiines previously interpreted as evidence of piscivory may also represent a mosaic of plesiomorphic, derived, and intermediate features. The anatomy of Halszkaraptor reveals that dromaeosaurids still possessed many features found in more basal maniraptoran and coelurosaur clades, including some that may have been tied to herbivory. Rather than being a semiaquatic piscavore, Halszkaraptor was a basal dromaeosaurid showing transitional features.

Figure 1

Rostral anatomy of select theropods. Rostrum of Halszkaraptor after Cau et al. in lateral (A) and dorsal (B) views. Rostrum of Erlikosaurus after Lautenschlager et al. in (C) lateral and dorsal (D) views. Rostrum of Jianchangosaurus after Pu et al. in (E) lateral view. Rostrum of Harpymimus after Kobayashi and Barsbold in (F) lateral view. Rostrum of Tsaagan (cast) in (G) lateral view. Rostrum of Velociraptor in (H) lateral and (I) ventral views. Rostrum of Gorgosaurus (cast) in (J) lateral view nar, naris; pmf, premaxillary foramina; max, maxilla; pmd, premaxillary dentition; md, maxillary dentition. Scale bar = 9 mm (A,B), 100 mm (C,D), 50 mm (E–F), 10 mm (G), 20 mm (H–I).

Figure 2

Comparative anatomy of the cervical series in selected theropods. Cervical series of Halszkaraptor (A) after Cau et al., Falcarius (B) after Zanno, and Struthiomimus (C) after Osborn.

Figure 3

Comparative anatomy of the forearm of Halszkaraptor and selected tetrapods. Forelimb of Halszkaraptor (A), forelimb of Muraenosaurus (B) after Andrews, manus of Tanystropheus (C) after Nosotti, manus of Araripemys (D) after Meylan, generalized manus of a therizinosauroid (E) after^,, and (F) manus of Deinocheirus (pers. obs. of Deinocheirus cast at AMNH). hum, humerus; r&u, radius and ulna; ru, rounded unguals/ultimate phalanges; pm, paddle-like morphology; recu, recurved unguals.

Figure 4

Comparative anatomy of the ilium in selected tetrapods. Ilium of Halszkaraptor (A) after Cau et al., ilium of Anchiornis (B) after Xu et al., ilium of Tyrannosaurus (C), and ilium of Deinonychus (D). ace, acetabulum; sac, supraacetabular crest; stp, supratrochanteric process; pp, posterior process.

Figure 5

Comparative pedal anatomy of Halszkaraptor. (A) Left and right pes of Halszkaraptor after Cau et al.. (B) Right pes of Allosaurus. (C) metatarsus of Struthiomimus in lateral view, (D) pes of Deinonychus in medial view. Generalized tyrannosaur metatarsus in (E) dorsolateral and (F) medial views. mt II, metatarsal II; mt III, metatarsal III; mt IV, metatarsal IV; pd II-3, pedal ungual II-3.

Figure 6

Phylogenetic relationships of Halszkaraptor and rostral coverings in maniraptorans. Strict consensus topology (A) recovered from the phylogenetic analysis of Coelurosauria. Clade diets follow Zanno and Makovicky (red = inferred carnivory; green = inferred herbivory). Tree length = 3306; Consistency index = 0.329; Retention index = 0.762. Silhouettes by the author. Dentaries of (A) Deinonychus, (B) Dromaeosaurus, (C) Halszkaraptor after Cau et al., (D) Velociraptor, (E) “Bambiraptor,” (F) Tyrannosaurus, and (G) Struthiomimus in lateral view. Arrows point to downturned ‘chins’ at the dentary symphysis. dr, lateral dentary ridge scale bar = 20 mm (B–F).