Saturday, October 31, 2015

[Paleontology • 2015] Dakotaraptor steini • The First Giant Raptor (Theropoda: Dromaeosauridae) from the latest Cretaceous (Hell Creek Formation, South Dakota)

Dakotaraptor steini
DePalma, Burnham, Martin, Larson & Bakker, 2015

Most dromaeosaurids were small- to medium-sized cursorial, scansorial, and arboreal, sometimes volant predators, but a comparatively small percentage grew to gigantic proportions. Only two such giant “raptors” have been described from North America. Here, we describe a new giant dromaeosaurid, Dakotaraptor steini gen. et sp. nov., from the Hell Creek Formation of South Dakota. The discovery represents the first giant dromaeosaur from the Hell Creek Formation, and the most recent in the fossil record worldwide. A row of prominent ulnar papilli or “quill knobs” on the ulna is our first clear evidence for feather quills on a large dromaeosaurid forearm and impacts evolutionary reconstructions and functional morphology of such derived, typically flight-related features. The presence of this new predator expands our record of theropod diversity in latest Cretaceous Laramidia, and radically changes paleoecological reconstructions of the Hell Creek Formation.

Keywords: Maastrichtian, maniraptoran, Laramidia, flightless, ulnar papillae, paleoecology





Etymology.— Dakota, referring to the geographic location of the discovery as well as the Dakota First Nations Tribe, plus raptor, Latin for “plunderer”. The specific name honors paleontologist Walter W. Stein.

Locality and horizon.— Upper Hell Creek Formation (Upper Maastrichtian), no more than 20 m below the Cretaceous-Paleogene Boundary, Harding County, South Dakota, U.S.A. The fossils were discovered in medium- to fine-grained sandstone with clay-pebble laminae that was part of a low-energy stream channel facies. While the type strata were deposited in an active fluvial system, transport energy was sufficiently low that it was not uncommon to find bones >10 cm in length from various other taxa still articulated with their adjacent elements, and plant matter that bore virtually no taphonomic alteration. In several instances, articulated or associated small vertebrate skeletons were recovered. No other theropod bones were recovered from the type substratum except for Dakotaraptor. Flora at the study locality places the site within the HC III floral zone (Johnson, 2002). 

Figure 3. Wing elements of Dakotaraptor holotype (PBMNH.P.10.113.T) compared with Deinonychus (AMNH 3015, YPM 5220, YPM 5206).
A, Deinonychus wing compared to the reconstructed left wing of Dakotaraptor (B). C, left ulna of Dakotaraptor in dorsal (top) and lateral (bottom) views, and D, patch of reactive bone on the ulna caused by physical trauma. Arrow points to the deep radial sulcus. E, Dakotaraptor right metacarpal II (1) compared with that of Deinonychus (2), in (top to bottom) distal, medial, and dorsal views.

Figure 4. Reconstructed Dakotaraptor wing and plumage, with avian and theropod comparisons.
A, enlarged view of the quill knobs on the Dakotaraptor holotype ulna (PBMNH.P.10.113.T), compared with quill knobs in Velociraptor (B) and Concavenator (C); D, conservative reconstruction of the wing plumage for Dakotaraptor based on quill knob placement and comparison with other dromaeosaurid and bird wings; E, quill knobs on a modern Masked Booby (Sula dactylatra) ulna, and (F), X-ray of a modern Barred Owl (Strix varia) wing showing attachment of the remiges on the quill knobs. The flattened dorsal surface of Dakotaraptor’s metacarpal II would have provided a stable shelf for the primary remiges that laid across it, a possible driving force for evolving the flat surface.
B modified from Turner and others, 2007; C modified from Ortega, Escaso, and Sanz, 2010; F courtesy Smalley’s Animal Hospital.

Dakotaraptor wing reconstruction.
Photo: R. DePalma

Dakotaraptor steini
Illustration: E. Willoughby |

Dakotaraptor's Ornithomimus Dinner
Illustration: E. Willoughby |

A giant, feathered dromaeosaurid exhibited by gracile and robust morphotypes was unexpected and thus is an important addition to the Hell Creek fauna. It fills the gap in body size distribution between the small maniraptorans and large tyrannosaurids previously documented in the formation, while adding to the known diversity of Hell Creek maniraptorans. Dakotaraptor also records a fourth event in which dromaeosaurids achieved atypically large body size. Moreover, the presence of quill knobs, indicative of elongate, stiffened feathers on the forearms, is unprecedented in giant dromaeosaurids and requires a reexamination of trends in quill knob evolution. The functional morphology of the long feathers, possibly of modern avian aspect as implied by known Asian forms, must also be considered. Subsequent studies of Dakotaraptor and the Hell Creek fauna may aid in our understanding of the circumstances that prompted dromaeosaurids to retain ligamental architecture for feather attachment, and may also provide information critical to a more accurate understanding of the lost capacity for flight.

Robert A. DePalma, David A. Burnham, Larry D. Martin, Peter L. Larson and Robert T. Bakker. 2015. The First Giant Raptor (Theropoda: Dromaeosauridae) from the latest Cretaceous (Hell Creek Formation, South Dakota), Paleontological Contributions. 14.