Acheroraptor

Credit: Emily Willoughby.

*Acheroraptor* is an extinct genus of dromaeosaurid dinosaur that lived in Late Cretaceous North America.

## History

David C. Evans, Derek W. Larson and Philip J. Currie (2013) described *Acheroraptor*, named after the Greek Ἀχέρων (Acheron), "underworld", in referencing the provenance in the Hell Creek Formation and the Latin raptor, "thief". The species epithet refers to James and Louise Temerty, the chairman and ROM Board of Governor and wife, who supported the Royal Ontario Museum for years.

## Description

ROM 63777, a full right maxilla with several articulated and isolated teeth, is the holotype. A referred specimen is ROM 63778, a mandible. Both, housed in the ROM, are from the upper Hell Creek, Montana, collected ~4 meters apart in the same mixed faunal bonebed. This dates to the Latest Maastrichtian, just before the Chicxulub impactor. The holotype was collected on August 28, 2009 by commercial fossil hunters, one of which collected the dentary some years later. They provided detailed geographic data from GPS and photos of the specimen in situ the day of discovery. Both were purchased by the ROM from a private collector.

## Paleoecology

*Acheroraptor* is the youngest dromaeosaurid. The Hell Creek Formation has been dated to 66 ± 0.07 million years ago, around the time of te K-PG mass extinction. The flora and fauna reported is rich, with Acheroraptor reported from the *Tyrannosaurus*-*Triceratops* fauna. Evans et al. found it was the only Hell Creek dromaeosaur, with teeth once referred to *Dromaeosaurus* and *Saurornitholestes* reclassified as Acheroraptor. They also found it was probably the only dromaeosaur. However, in 2015, this was disproven with the discovery of *Dakotaraptor*, a far larger animal.

Tyrannosaurids, ornithomimids, troodontids, birds and caenagnathids are reported. Tyrannosaurids are *Tyrannosaurus*, and the likely junior synonym *Nanotyrannus*. *Struthiomimus*, *Ornithomimus* (now thought to be a different, closely-related ornithoimosaur) and "Orcomimus" are ornithomimids. *Avisaurus*, *Brodavis baileyi*, two unnamed hesperornithoforms and maybe *Potamornis* are present birds. *Anzu*, *Leptorhynchos* and a possible undescribed form (similar to *Gigantoraptor* are reported oviraptorosaurs. However, as of 2016, only footprints are known, but were reassigned to tyrannosaurs. Troodontids reported are *Troodon* (invalid), *Pectinodon* and *Paronychodon* (dubious?). *Richardoestesia* is the only coleurosaur.

Ornithischians are abundant. *Ankylosaurus* (ankylosaurid) and *Denversaurus* (nodosaurid) are known, with *Edmontonia* being a possibility. A leptoceratopsid (*Leptoceratops*), *Nedoceratops* and *Tatankaceratops* (chasmosaurines; dubious?) and *Triceratops* (chasmosaurine} are ceratopsians. Hadrosaurs include *Edmontosaurus* and a *Parasaurolophus*-like taxon. *Sphaerotholus* and *Pachycephalosaurus* are reported, with *Stygimoloch* and *Dracorex* being proposed (or possibly a new species of *Pachycephalosaurus*), with another undescribed specimen from North Dakota.

Mammals are plentiful in Hell Creek: *Paracimexomys*, *Paressonodon*, *Meniscoessus*, *Essonodon*, *Cimolemys*, *Cimolodon*, *Cimexomys*, *Mesodma*, *Neoplagiaulax*, *Alphadon*. *Protalphodon*, *Turgidodon*, *Pediomys*, *Protolambda*, *Leptalestes*, *Didelphodon*, *Nancuris*, *Nortedelphys*, *Glasbius*, *Alostera*, *Protungulatum*, *Cimolestes*, *Batodon*, *Gypsonictops* and possibly *Paranyctoides* are reported.

## Classification

Evans et al. (2013) explored the phylogenetic placement of *Acheroraptor* with several data matrices. Both pecimens were coded as a single taxa in the Turner et al. (2012) data matrix. They recovered *Acheroraptor* as a member of the clade featuring eudromaeosauria and microraptoria, confirming it was a dromaeosaur, and possibly a eudromaeosaur. However, the taxa were in a large polytomy, because of the limited codings available.

Evans et al. (2013) also coded the specimens, both together and separate, into an updated, smaller dataset of Longrich and Currie (2009). *Velociraptor osmolskae* and *Balaur* were added, with *Itemirus* excluded (due to Miyashita and Currie (2009) finding it was a tyrannosauroid) and the *Adasaurus* codings were separated into the holotype and IGM 100/23 (following Turner et al. (2012). Additionally, some characters were rescored and modified, with two maxillary characters added into Turner et al. (2012)'s matrix. They found a topology more resolved, placing the taxon as a relatively basal velociraptorine, which was thought to only include Asian taxa. They found the following cladogram:

One unpublished thesis places *Acheroraptor* in Saurornitholestinae. Records of *Sauroritholestes* teeth from Hell Creek may be explained by this[*citation needed*].

## References

https://en.wikipedia.org/wiki/Acheroraptor