| Orkoraptor | |
|---|---|
 | |
| Artist's restoration | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | †Megaraptora |
| Family: | †Megaraptoridae |
| Genus: | †Orkoraptor Novas, Ezcurra & Lecuona, 2008 |
| Species: | †O. burkei |
| Binomial name | |
| †Orkoraptor burkei Novas, Ezcurra & Lecuona, 2008 | |
Orkoraptor is agenus of medium-sizedmegaraptorantheropoddinosaur from the lateCretaceous Period ofArgentina. It is known from incompletefossil remains including parts of the skull, teeth, tail vertebrae, and a partialtibia. The specialized teeth resemble those of somemaniraptoriform theropods, namely thedeinonychosaurs andcompsognathids. This and other anatomical features led the authors who described it (Novas, Ezcurra, and Lecuona) to suggest that it was amaniraptorancoelurosaur. However, subsequent studies found it to be amegaraptoran.[1][2] Found in theCerro Fortaleza Formation of SouthernPatagonia, it is one of the southernmost carnivorous dinosaurs known from South America.[3]
The nameOrkoraptor means "Toothed River thief", and was derived from theAoniken "Orr-Korr", the local name for theLa Leona River, located near the original fossil site. The species name honors Coleman Burke, an amateur paleontologist who supported the expedition that collected the original fossils.[3]
Orkoraptor was a medium-sized theropod, measuring 8.4 m (28 ft) long and weighing 1,400 kilograms (3,100 lb).[4][5] Theholotype,MPM-Pv 3457 was excavated in 2001 and consists of a rightpostorbital, a rightquadratojugal, a probable right coronoid, eight isolated teeth, the intercentrum of theatlas and its right neurapophysis, two proximal caudal vertebrae, the proximal half of the righttibia, eight fragmentary ribs, and three incomplete chevrons. It was initially believed to be acoelurosaur due to the upturned anterior process of the postorbital as well as specialized teeth which are unserrated on the mesial (front) side and have an 8-shaped cross section, similar to those ofdromaeosaurids andcompsognathids. However, many other features contradict those of these families and other coelurosaurs, so its original describers were unable to conclusively assign it to any specific family. They did note its similarity to a then-undescribed Patagonian theropod now known asMurusraptor.[3] More recent analyses have explained that these coelurosaur-like traits are also present in some megaraptorans. For example,Orkoraptor's postorbital is almost identical to that ofAerosteon[1] and the juvenileMegaraptor described in 2014 has teeth similar to those ofOrkoraptor.[2]
Orkoraptor was originally classified asManiraptoraincertae sedis. While the majority of phylogenetic trees recovered in its describers' analysis recovered it as acompsognathid, the describers considered this unlikely based on its much larger size and presence in much younger strata.[3]Orkoraptor was subsequently recovered inMegaraptora, a clade including several other enigmatic medium to large theropods, which has variously been considered to be a member ofAllosauroidea andTyrannosauroidea.[1][2][6] The phylogenetic trees in the publication ofGualicho shinyae have found megaraptorans to be either allosauroids or basal coelurosaurs.[7]
All known specimens ofOrkoraptor were collected from thePari Aike Formation, which is now considered by different authors to be either the middle section of theMata Amarilla Formation,[8] or merely an alternative name for theCerro Fortaleza Formation.[9][10] It was originally identified as coming from theMaastrichtian, and thus the youngest knownmegaraptoran, but more recently is considered to be from theCenomanian toSantonian.[8][6] The middle section of the Mata Amarilla Formation, whenceOrkoraptor was collected, contains a tuff layer that has been dated to 96.2 ± 0.7Ma, during theCenomanian.[8] However, the site whereOrkoraptor is known from, Cerro Hornos, was recently reinterpreted as coming from theCampanian.[11]
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