Saturday 31 October 2015

Dakotaraptor Steini: A giant, feathered Dromaeosaurid from the End Cretaceous Hell Creek Formation of South Dakota.

Dromaeosaurids were first described in the 1920s, but received relatively little attention from palaeontologists until the late 1960s. However in recent years it has been realized that the group were the closest non-avian relatives of the Birds, and they have become one of the best studied of all Dinosaur groups. The majority of Dromaeosaurids were small animals with wing-like forelimbs and feathers, interpreted as having lived much of their lives in the treetops, with some species thought to have been capable of true flight and others of gliding. However other members of the group retain feathered wings despite being clearly quite incapable of flying due to their size and shape, leading to speculation that the ‘wings’ may have had some other purpose, quite unrelated to flight.

Among these clearly non-flying Dromaeosaurids are four previously described ‘giant’ species, all from the Late Cretaceous, Deinonychus antirrhopus and Utahraptor ostrommaysi from North America, Achillobator giganticus from Asia and Austroraptor cabazai from South America. Rather than forming a distinct group these giant Dromaeosaurids appear to have arisen at least three times from more average-sized members of the group, suggesting that some factor favoured the evolution of larger Dromaeosaurids towards the end of the Cretaceous.

In a paper published in the journal Paleontological Contributions on 30 October 2015, Robert DePalma of the Department of Vertebrate Paleontology at The Palm Beach Museum of Natural History and the University of Kansas Biodiversity Institute, David Burnham and the late Larry Martin, also of the University of Kansas Biodiversity Institute, Peter Larson of the Black Hills Institute of Geological Research and Robert Bakker of the Houston Museum of Nature and Science describe a new species of giant Dromaeosaurid from the End Cretaceous Hell Creek Formation of Harding County, South Dakota.

The new species is named Dakotaraptor steini, where ‘Dakotaraptor’ refers to the state where the species was discovered and the Dakota people that gave it its name (‘raptor’ means ‘plunderer’ in Latin and is a common suffix for Dromaeosaurids, as well as being used as a collective name for modern Birds of Prey), and ‘steini’ honours palaeontologist Walter Stein. The species is described from an adult specimen comprising the right pedal unguals II & III, the right femur, the left and right tibiae, the left astragalus and calcaneum, the left metatarsals II-IV, the right metatarsal IV, one fragmentary dorsal centrum, 10 caudal vertebrae, the furcula, the left and right humeri, the left and right radii, the left and right ulnae, the right metacarpals I and II and three fragmentary left manual phalanges, plus an isolated left tibia, a left astragalus and calcaneum, two furcula and several teeth.

Quary map indicating the spatial relationship of the bones of the first Dakotaraptor specimen as they were found in the field. DePalma et al. (2015).

The ulna of Dakotaraptor shows a row of 10 oblong protuberances, interpreted as being quill knobs, or ulnar papilli; attachment points for flight feathers. These have been detected before in Theropod Dinosaurs, particularly Dromaeosaurids, as well as being known in modern and Mesozoic Birds. Importantly, in modern Birds, these structures only develop in species that place exceptional stresses on the wing feathers during flight. Since the size and shape of Dakotaraptor makes it highly unlikely that it was capable of any form of flight, it seems likely that it was placing stress on its wing feathers engaging in some other form of activity. This has been seen previously in other Dromaeosaurids deemed unlikely fliers (for example Velociraptor) and has led to a number of suggestions as to possible uses for wing feathers in Dromaeosaurids, such as brooding eggs and young or subduing prey. DePalma et al. feel that the brooding young explanation cannot explain the deep ulnar papilli seen in Dakotaraptor and other Dromaeosaurids, and therefore reject this hypothesis, but do feel they lend to support to the idea that strong feathers may have aided prey suppression in these animals.

Reconstructed Dakotaraptor wing and plumage, with Avian and Theropod comparisons. (A) Enlarged view of the quill knobs on the Dakotaraptor ulna, 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. Photograph (F) provided by Smalley’s Animal Hospital. DePalma et al. (2015).

Dakotaraptor is interpreted as being exceptionally large for a Dromaeosaurid, or indeed any form of Maniraptoran (the group of Theropod Dinosaurs that includes Dromaeosaurids and Birds, as well as other group such as Troodontids), reaching about 5.5 m in length and exceeding in size all other known Dromaeosaurids accept Utahraptor, which was similar in size and to which it is interpreted as being closely related. However while Utahraptor is interpreted as being a slow moving animal with a distinct morphology, Dakotaraptor appears to be essentially a scaled up version of the smaller Dromaeosaurids, and like them is thought likely to have been a highly active predator (though unlike them it is not thought to have been capable of any form of flight, as in larger flying animals the wing must be scaled up relative to the rest of the body, while Dakotaraptor retains its proportions).

Skeletal reconstruction of  the most intact Dakotaraptor specimen based on available material for Utahraptor, Dromaeosaurus, Deinonychus, and Achillobator, demonstrating overall proportions and the large size of the creature. Preserved elements shown in insert. DePalma et al. (2015).

As such Dakotaraptor presents a significant expansion of the known Hell Creek fauna, which previously, as well as a range of herbivorous Dinosaurs, included small Dromaeosaurid and large Tyranosaurid pedators, but no carnivores of intermediate size, something that would be expected in a similar modern fauna. DePalma et al. also note that the discovered specimens of Dakotaraptor, which are all interpreted as being adults group into two morphologies, a more robust form and a more gracile (slender) form. These specimens could represent variations within a population, but DePalma et al. feel that it is unlikely that only the more extreme variations would be preserved with no intermediate specimens, and therefore suggest that the two morphotypes may represent a sexually dimorphic species (species in which one sex is notably more robust than the other). They further note that this interpretation fits with the often-made suggestion that Dromaeosaurids may have been pack hunters, living permanently in some form of family group.

Interpretation of Dakotaraptor as a living animal. Emily Willoughby in DePalma et al. (2015).

See also…

Dromaeosaurid Dinosaurs are among the closest non-Avian relatives of the Birds and show many similarities to the earliest members of that group, making understanding Dromaeosaurs important for understanding the origin of Birds. In particulary...

Dromaeosaurs were small Therapod Dinosaurs, thought to have been the group most closely related to...

The Dromaeosaurs were a group of small, feathered dinosaurs closely related to the birds. They are commonly referred to as 'raptors' on account of an enlarged claw on each foot which was held clear of...

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