Dakotaraptor

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

The first fossils of *Dakotaraptor* were found in South Dakota, USA, 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.

*Dakotaraptor* was about five to six metres long, which makes it one of the largest dromaeosaurids known. It had long arms. One of the lower arm bones shows bumps for feathers, proving that *Dakotaraptor* had wings. It also had long rear legs with a very large sickle claw on the second toe. The claw could be used to kill relatively large plant-eating dinosaurs. *Dakotaraptor* lived in the same time and area as *Tyrannosaurus rex*.

## Discovery

 File:Dakotaraptor sketch.jpg
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.

 File:Dakotaraptor lateral Wiki.jpg
The holotype, **PBMNH.P.10.113.T**, was found in a sandstone layer of the upper Hell Creek Formation, dating from the late Maastrichtian. It consists of a partial skeleton, lacking the skull, of an adult individual. It contains a piece of a back vertebra, ten tail vertebrae, a furcula, both upper arm bones, both ulnae, both radii, the first and second right metacarpals, three claws of the left hand, a right thighbone, both shinbones, a left astragalus bone, a left calcaneum, the left second, third and fourth metatarsal, the right fourth metatarsal, and the second and third claw of the right foot. Apart from the remains of the holotype, in the site bones were discovered that also belonged to *Dakotaraptor* but which represented a more gracile morph. These included the specimens PBMNH.P.10.115.T: a right shinbone; PBMNH.P.10.118.T: a connected left astragalus and calcaneum; and KUVP 152429: a furcula. Additionally four isolated teeth were referred, specimens PBMNH.P.10.119.T, PBMNH.P.10.121.T, PBMNH.P.10.122.T, and PBMNH.P.10.124.T. These fossils are part of the collection of The Palm Beach Museum of Natural History. Other referred fossils are NCSM 13170, a furcula, and KUVP 156045, an isolated tooth.

## Description

### Size

 File:Dakota raptor scale mmartyniuk.png
*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, *Utahraptor*. *Dakotaraptor* however, does not have the proportions and adaptations of *Utahraptor*, but more closely resembles smaller dromaeosaurids like *Deinonychus*.

### Distinguishing traits

Apart from the large size, the description of 2015 indicated some additional distinguishing traits. On the fourth foot claw, the boss that serves as an attachment for the tendon of the flexor muscle is reduced in size. The "blood groove" on the outer side of the fourth claw of the foot, towards the tip is fully enclosed over half of its length, forming a bony tubular structure. The second and third claws of the foot have sharp keels at their undersides. The second foot claw, the "sickle claw", equals 29% of the thighbone length. On the shinbone, the *crista fibularis*, the crest that contacts the calfbone, is long and lightly built with a height that does not exceed 9% of the crest length. The upper edge of this crest ends in a hook. On the second metacarpal, of the two condyles that contact the finger, the inner one is almost as large as the outer one. The outer side of the second metacarpal has but a shallow groove for the ligament that connects it to the third metacarpal. When the arm is seen in a flat position, of the second metacarpal the edge between the wrist joint and the upper shaft is straight in top view. The teeth have fifteen to twenty denticles per five millimetres on the rear edges and twenty to twenty-seven denticles on the front edges.

### Skeleton

#### Vertebral column

The vertebrae of the back are highly pneumatised, filled with trabecular bone that shows many air spaces. On the middle tail vertebrae the front joint processes, the prezygapophyses, are extremely elongated with an estimated intact length of seventy centimetres, spanning about ten vertebrae. This stiffens the tail.

#### Forelimb

Like virtually all other theropods, *Dakotaraptor* possessed a furcula, or “wishbone”, as part of the shoulder girdle. Theropod wishbones are quite varied and often different from the strongly U-shaped *furculae* most modern birds possess. *Dakotaraptor* had a furcula that was U- to V-shaped, similar to many other dromaeosaurids such as *Velociraptor*, and even the large spinosaurid theropod *Suchomimus*. In 2015, a study by Victoria Megan Arbour e.a. claimed that the presumed *Dakotaraptor* furculae in fact represented a part of a turtle armour, the entoplastron of *Axestemys splendida*, a member of the Trionychidae.

The wing of *Dakotaraptor* was given much attention in the describing article. Here, “wing” is used as an anatomical descriptive term not related to its functionality, since *Dakotaraptor* was flightless. This is similar to the term “wing” for the same appendages in ostriches, emus, and other flightless birds. It is meant to express that arm is equipped with long feathers. Many of the wing bones were discovered (humerus, radius, ulna, two of the three metacarpal wrist bones, and parts of the finger digits), so the wing is very complete. The humerus, the upper arm bone, is relatively long and slender. It is somewhat bent to the inside. The most notable anatomical feature is the row of very prominent bumps along a ridge on the lower edge of the ulna, one of the forearm bones. These are called ulnar papillae, or quill knobs. In birds and some other theropod dinosaurs, these bumps were spots for reinforced attachment of the remiges, or wing feathers. When quill knobs are present, this is considered a strong indication that the animal had long remiges on the wings. Since quill knobs are rare in the fossil record, paleontologists mainly relied on phylogenetic bracketing to determine if a species was likely to have had wing feathers - if a relative on a "higher" branch of the evolutionary tree had the feathers, and one on a "lower" branch down had them also, then a species in the middle position likely did as well. *Dakotaraptor*’s quill knobs show that the animal unequivocally had prominent wing feathers, making it the largest dromaeosaurid with confirmed plumage of that type. The quill knobs of *Dakotaraptor* have a diameter of about eight to ten millimetres, which shows that these feathers were rather large. It was estimated that a complete series might include fifteen of these *papillae ulnares*. The ulna is thirty-six centimetres long and the other lower arm bone, the radius, measures thirty-two centimetres. The hand bones show that their joints allowed for little mobility. The wingspan of *Dakotaraptor* was estimated at 120 centimetres, not taking into account possible primary remiges longer than the hand.

The second metacarpal of the metacarpus of the hand, the bone that primary remiges attach to, also had a flat bony shelf as its dorsal surface. The shelf made a perfect spot for the primary feathers to lay across in their life-attachment.

#### Hindlimb

Overall, the hindlimb is built lightly and with long elements, contrary to the robust, stocky hind limbs of *Utahraptor*. *Dakotaraptor* more closely resembles the agile, springy smaller dromaeosaurids and would have been well-suited at running and pursuit predation. The length of the thighbone is 558 millimetres. It is relatively shorter and more lightly built than that of *Utahraptor*. To the contrary the shinbone is rather elongated. The holotype shinbone is with a length of 678 millimetres the longest dromaeosaurid tibia known. It is 22% longer than the thighbone, indicating a good running capability. The shinbone's cnemial crest has a sharp corner pointing to the front. Its fibular crest ends in a hook-shaped process pointing to above, a condition that is unique in the entire Theropoda. The astragalus and calcaneum, the upper ankle bones, are fused just as in *Bambiraptor*. The top of the calcaneum has but a small contact facet for the calfbone, indicating that this fibula must have had a very narrow lower end.

 File:Claw Alt 2.jpg
The metatarsus has an estimated length of thirty-two centimetres, which makes it rather long relative to the remainder of the hindlimb.

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 sixteen centimetres and a length of twenty-four centimetres measured along the outer curve. This equals 29% of the length of the thighbone compared to 23% with *Deinonychus*. The claw is transversely flattened and has a droplet-shaped cross-section. The flexor tubercle, a large bump near the base, served as an attachment site for flexor muscles - the larger it was, the greater the slashing strength. *Dakotaraptor* has a flexor tubercle that is larger relative to overall claw size than it is in other discovered dromaeosaurids, potentially giving it the strongest slashing strength of any known member of this group. The flexor tubercle on the third claw of the foot is almost non-existent, very reduced in size compared to other dromaeosaurids, suggesting a more minimized use of that claw. As these are the bony cores of the claws, they would have been covered in a keratinous sheath that extended the "nail" and ended in a sharp tip. The third claw too is keeled but is much smaller with a tip to joint length of seven centimetres and a curve length of nine centimetres. The groove on its outer side towards the tip ends in a bone tunnel, a rare condition.

## Phylogeny

 File:Dakotaraptor and Utahraptor.jpg
*Dakotaraptor* was placed in the Dromaeosauridae. A cladistic analysis showed that is 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 *Deinonychus*, *Unenlagia*, and the *Achillobator* plus *Utahraptor* clade.

### Possible synonymy with *Acheroraptor*

*Acheroraptor temertyorum* is another theropod from the Hell Creek Formation, named in 2013 for a lower jaw, a maxilla, and some teeth. *Acheroraptor* was diagnosed by multiple features, including the possession of ridges on the teeth. Teeth are the only overlapping features between *Acheroraptor* and *Dakotaraptor*. However *Acheroraptor* is significantly smaller, and differs from *Dakotaraptor* only in that it possesses vertical ridges on the teeth crowns. Andrea Cau noted that though *Dakotaraptor* is known from individuals of differing sizes, some of the smaller specimens are also fully mature, and it is possible that the size difference means *Dakotaraptor* is simply a different species or size morph of *Acheroraptor*. A phylogenetic analysis presented by Cau, though relying on fragmentary specimens, did not result in a close relationship between the two.

## Paleobiology

 File:Dakotaraptor RPR.jpg
*Dakotaraptor* is the first medium-sized predator discovered in the Hell Creek Formation, intermediate in length between gigantic tyrannosaurids like *Tyrannosaurus* and smaller deinonychosaurians like *Acheroraptor*. Especially if it was a pack-hunter, cooperating in groups, it could have preyed upon larger herbivores, possibly competing with subadult tyrannosaurids in the six to nine metres length range. As its shinbone was longer than its thighbone, *Dakotaraptor* would have had a good running capacity, filling the niche of pursuit predator. The keeled claw of the second toe, the "sickle claw", was used to bring down prey and had a more robust flexor tubercle than that of *Utahraptor*. To the contrary, the third foot claw was relatively smaller in size than with other dromaeosaurids and seems not to have had an important function in attacking prey animals.

Two morphs, a robust and a gracile one, were present in the fossil material. A study of the bone histology showed that both morphs were adult, so the lighter build of some bones was not caused by a young age. Individual variety or pathologies might account for the difference but the simplest explanation is sexual dimorphism. It can as yet not be deduced which morph was male and which female.

On the ulna of the lower arm, wide *papillae ulnares* or quill knobs are present, attachment points for large pennaceous feathers. Adult *Dakotaraptor* individuals were much too heavy to fly. Despite this flightless condition, the feathers were not reduced. There is a variety of possible alternative functions for its wings, including shielding of eggs, display, intimidation and keeping balance while pinning down prey with the sickle claw. These functions however, do not necessitate quill knobs and the describing authors considered it likely that *Dakotaraptor* descended from a smaller flying ancestor which had them.

## References

1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 DePalma, Robert A.; Burnham, David A.; Martin, Larry D.; Larson, Peter L.; Bakker, Robert T. (2015). "The First Giant Raptor (Theropoda: Dromaeosauridae) from the Hell Creek Formation.". *Paleontological Contributions* (14).
2. ↑ DePalma, R. A. (2010). *Geology, taphonomy, and paleoecology of a unique Upper Cretaceous bonebed near the Cretaceous-Tertiary Boundary in South Dakota* (Master of Science thesis). University of Kansas. pp. 227 pp.
3. ↑ "The Open Access Dinosaurs of 2015". PLOS Paleo.
4. ↑ Arbour V.M., Zanno L.E., Larson D.W., Evans D.C., Sues H., 2015, "The furculae of the dromaeosaurid dinosaur *Dakotaraptor steini* are trionychid turtle entoplastra", *PeerJ PrePrints* **3**: e1957
5. ↑ 5.0 5.1 Evans, David C.; Larson, Derek W.; Currie, Philip J. (2013-11-19). "A new dromaeosaurid (Dinosauria: Theropoda) with Asian affinities from the latest Cretaceous of North America". *Naturwissenschaften* **100** (11): 1041–1049. ISSN 0028-1042. doi:10.1007/s00114-013-1107-5.
6. ↑ 6.0 6.1 Cau, A. (2015-10-31). "*Dakotaraptor*... a giant *Acheroraptor*?". Theropoda.