[D] Hesperonychus elizabethae [sG] [T]
Longrich & Currie 2009
Cretaceous Late Campanian
Saurischia Theropoda Tetanurae Coelurosauria Dromaeosauridae [Microraptorinae]
Dinosaur Park Formation, Alberta, Canada
50 cm < 1 m
The fossil assemblages of the Late Cretaceous of North America are dominated by large-bodied dinosaur species. Associated skeletons of small dinosaurs are exceedingly rare, and small (<10 kg) carnivorous theropods have not previously been reported from these beds. Here, we describe a small dromaeosaurid from the 75-millionyear-old Dinosaur Park Formation of Alberta, Canada.
Hesperonychus elizabethae gen. et sp. nov. is represented by a pelvic girdle from an animal weighing 1,900 g. Despite its size, the pubes and ilia are coossified, indicating that the animal was somatically mature. This is the smallest carnivorous, nonavian dinosaur known from North America. Phylogenetic analysis of Hesperonychus reveals that it is not closely related to previously described North American dromaeosaurids. Instead, Hesperonychus is a member of the dromaeosaurid clade Microraptorinae, a group containing the 4-winged Microraptor and the feathered Sinornithosaurus, both from the Lower Cretaceous Jehol Group of China.
Hesperonychus is the youngest known member of this lineage, extending the temporal range of the clade by 45 million years, and it is the first microraptorine known from North America, providing further evidence for an affinity between the dinosaur faunas of North America and Asia. Study of fossil collections from the Dinosaur Park and Oldman formations of Alberta has revealed numerous isolated bones of small, basal dromaeosaurids, which are tentatively referred to Hesperonychus. These fossils suggest that small dromaeosaurids were a significant component of the carnivore community in this Late Cretaceous biota.
UALVP (University of Alberta Laboratory for Vertebrate Palaeontology, Edmonton) 48778, a partial pelvic girdle comprising the pubes and ilia.
A number of isolated pedal phalanges are tentatively referred to Hesperonychus (see Specimen Data in SI Appendix and Fig. 2). Phalanx II-1 is represented by TMP (Royal Tyrrell Museum of Palaeontology, Drumheller, AB, Canada) 1989.116.65, TMP 1966.19.22, and UALVP 50687. Phalanx II-2 is represented by TMP 1992.36.61 and TMP 1983.67.7. Phalanx II-3 is represented by TMP 1979.10.6, TMP 1980.16.1880, TMP 1990.107.15, TMP 1995.092.0009, TMP 2000.12.100, and UALVP 50686
The name Hesperonychus derives from hesperus (Latin, west) and onychos (Greek, claw). The specific epithet elizabethae honors the late Dr. Elizabeth Nicholls, who discovered the holotype.
Horizon and Locality
The holotype was collected from exposures of the Dinosaur Park Formation located on the south side of the Red Deer River, 20 km east of Dinosaur Provincial Park, Alberta. The Dinosaur Park Formation was deposited during the middle of the late Campanian, between 76.5 and 74.8 Ma (29). Referred material was collected from Dinosaur Provincial Park and surrounding badlands and other locations in southern Alberta, including Devil’s Coulee, Manyberries, Onefour, Irvine, and Sandy Point (Specimen Data in SI Appendix); all are within 200 km of the type locality. All specimens for which precise stratigraphic data are available come from a narrow chronostratigraphic interval that encompasses the Dinosaur Park Formation and coeval beds of the uppermost Oldman Formation (Specimen Data in SI Appendix). This interval represents no more than 1.7 million years of time.
Small dromaeosaurid characterized by the following autapomorphies: pubic peduncle of ilium with medial surface deeply excavated; posterior wing of ilium with medial shelf split to form anterior and posterior processes; lateral tubercles of pubis wing-like and curving anteriorly; pubis with fossa on lateral surface ventral to acetabulum; pubic apron shifted onto posterior surface of pubis; pubic symphysis teardrop-shaped in lateral view; ischiadic process of pubis reduced to a narrow lamina