Monday, May 8, 2017

Dakotaraptor skeleton

Dakotaraptor is a genus of large carnivorous theropod dromaeosaurid dinosaur from the Late Cretaceous of North America.

The first fossils of Dakotaraptor were found in South Dakota, United States, in 2005. In 2015, the genus Dakotaraptor received its name, meaning “plunderer of Dakota”, when the type species Dakotaraptor steini was described. The fossils contain an incomplete skeleton without a skull and some individual bones. They have been found in the Maastrichtian-age Hell Creek Formation, dated to the very end of the Cretaceous period, making Dakotaraptor one of the last surviving dromaeosaurids.

Size of Dakotaraptor steini (green) compared to a human. Author: Matthew Martyniuk

Dakotaraptor was about 5.5 metres (18 ft) long, which makes it one of the largest dromaeosaurids known. It had long arms. One of the lower arm bones shows quill knobs, demonstrating that it was most likely feathered. It also had long rear legs with a very large sickle claw on the second toe; this claw could be used to kill relatively large plant-eating dinosaurs. It lived in the same time and area as Tyrannosaurus.

Dakotaraptor is exceptionally large for a dromaeosaurid; it has an estimated adult length of 5.5 m (18 ft). This approaches the size of the largest known dromaeosaurid, UtahraptorDakotaraptor however, does not have the proportions and adaptations of Utahraptor, but more closely resembles smaller dromaeosaurids like Deinonychus.

In 2005, paleontologist Robert DePalma in Harding County, South Dakota discovered the skeleton of a large dromaeosaurid. Subsequently, the same site produced additional dromaeosaurid remains, as well as abundant other fossils in 2010 described by DePalma in his master’s thesis. In 2015, the type species Dakotaraptor steini was named and described by Robert A. DePalma, David A. Burnham, Larry Dean Martin, Peter Lars Larson and Robert Thomas Bakker. The generic name combines a reference to South Dakota and the Dakota people with a Latin raptor, “plunderer”. The specific name honours paleontologist Walter W. Stein. Dakotaraptor was one of eighteen dinosaur taxa from 2015 to be described in open access or free-to-read journals.

The known elements of Dakotaraptor and a reconstructed skeleton. From DePalma et al., 2015.

The foot claws of Dakotaraptor include a typical dromaeosaurid raptorial second claw, or “sickle claw”, which was used for killing or holding down prey. It is large and robust with a diameter of 16 centimetres (6.3 in) and a length of 24 centimetres (9.4 in) measured along the outer curve. This equals 29% of the length of the thighbone, compared to 23% in Deinonychus.

Dakotaraptor was placed in the Dromaeosauridae. A cladistic analysis showed that it was the sister species of Dromaeosaurus. They again formed a clade with Utahraptor, of which clade Achillobator was the direct side branch. Despite being related to other gigantic dromaeosaurids, Dakotaraptor was suggested to represent a separate fourth instance of dromaeosaurid size increase, besides DeinonychusUnenlagia, and the Achillobator plus Utahraptor clade.