[Paleontology • 2015]
Dakotaraptor steini • The First Giant Raptor (Theropoda: Dromaeosauridae) from the latest Cretaceous (Hell Creek Formation, South Dakota)
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| Dakotaraptor steini
DePalma, Burnham, Martin, Larson & Bakker, 2015
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ABSTRACT
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
SYSTEMATIC PALEONTOLOGY
THEROPODA MARSH, 1881
MANIRAPTORA GAUTHIER, 1986
DROMAEOSAURIDAE MATTHEW & BROWN, 1922
DAKOTARAPTOR STEINI GEN. ET SP. NOV.
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).
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| 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. |
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| 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. http://hdl.handle.net/1808/18764
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| Dakotaraptor wing reconstruction.
Photo: R. DePalma
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| Dakotaraptor steini
Illustration: E. Willoughby | EWilloughby.DeviantArt.com
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| Dakotaraptor's Ornithomimus Dinner
Illustration: E. Willoughby | EWilloughby.DeviantArt.com
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CONCLUSION
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.
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