Atrociraptor

Atrociraptor (meaning "savage thief" ) is a genus of saurornitholestine dromaeosaurid theropod dinosaur from the Late Cretaceous (Maastrichtian stage) of Alberta, Canada.

Discovery
In 1995, the fossil collector Wayne Marshall discovered a partial skull of a dromaeosaurid dinosaur in the Horseshoe Canyon Formation of Drumheller in Alberta, Canada, about 5 km west of the Royal Tyrrell Museum of Palaeontology. Marshall had previously worked in the museum's mounting shop, helping to construct exhibits until the museum opened in 1985, and reported fossils to museum staff as he found them over the years. Jaw fragments and teeth eroding from the hillside led to the discovery of the dromaeosaur specimen; Marshall collected the loose fossil fragments and delivered them to the palaeontologist Philip J. Currie at the museum, and the remaining parts were later collected. The specimen was in an isolated block of sandstone that was relatively hard, and preparation revealed the right maxilla (main bone of the upper jaw) exposed in outer side-view and the right dentary (tooth-bearing bone of the lower jaw) exposed in inner side view, and both of these bones were left in the block.

In 2004, Currie and the palaeontologist David Varricchio made the specimen (catalogued as RTMP 95.166.1) the holotype of the new genus and species Atrociraptor marshalli. The generic name is derived from the Latin words atroci, which means "savage", and raptor, Latin for "robber". The specific name references Marshall, the discoverer. The full name means “Wayne Marshall’s savage robber”. Since then, Marshall led the museum staff to many important finds. In all, the holotype preserves both premaxillae (frontmost bone of the upper jaw, freed from the rock matrix), a right maxilla, both dentaries (the left one is incomplete), associated teeth, and numerous bone fragments from the skull. Numerous isolated teeth from the Horseshoe Canyon Formation have since been assigned to Atrociraptor, many found in a bonebed containing Albertosaurus fossils.

Description
Atrociraptor was a relatively small dromaeosaurine, comparable to Velociraptor in size, and is estimated to have measured about 1.80 - 2 m in length and weighed 15 kg. As a dromaeosaur, it would have had large arms, a long tail with ossified tendons, and a large sickle-claw on the robust, hyper-extendible second toe. Fossils of other dromaeosaurs, such as Zhenyuanlong, show that even relatively large members of the group had pennaceous feathers, with large wings on the arms and long feathers on the tail.

Atrociraptor differs from Bambiraptor and other Dromaeosaurids in its more isodont dentition—the teeth have different sizes but the same form—and short deep snout. A skull opening, the maxillary fenestra, is relatively large and positioned right above another opening, the premaxillary fenestra, a condition not known from other species. The skull appears to have been unusually short and tall. The teeth are relatively straight, but they emerge from the tooth sockets at an angle to the jaw line, resulting in a strongly raked row of teeth.

Classification
When Atrociraptor was first described, it was assigned to the subfamily Velociraptorinae, a group known predominantly from the Late Cretaceous of Asia. This assignment was based on the difference in size between the anterior and posterior serrations on the maxillary teeth and the size of the second premaxillary tooth. This assignment was contested by Nick Longrich and Philip Currie in a 2009 analysis, which grouped it as a close relative of Saurornitholestes as part of a new subfamily: Saurornitholestinae.

The uncertainty around the classification of Atrociraptor would not be resolved for some time. In their description of Acheroraptor, David Evans and colleagues suggested that Atrociraptor was the sister taxon of the much older genus Deinonychus in a clade more derived than Saurornitholestinae but outside of both Velociraptorinae and Dromaeosaurinae. Notably, this analysis used the same phylogenetic data set as the earlier analysis by Longrich and Currie, but included more taxa which had been described in the interim. In this analysis, the taxon they were describing, Acheroraptor, was found to be a velociraptorine, although this would also be contested in the future. The analysis that accompanied the description of Dakotaraptor by Robert DePalma and colleagues recovered Atrociraptor as a member of Dromaeosaurinae, alongside Deinonychus, although there is no explanation in the description for this recovered classification. A similar result to DePalma and colleagues was later also recovered by William and Kristen Parsons in 2015.

Modern dromaeosaurid systematics is dominated by a few phylogenetic datasets which recover slightly different results. One of the most widespread is the so-called "TWiG matrix" (named for the Theropod Working Group), developed by Steven Brusatte, Andrea Cau, Mark Norell, and several other researchers, which contains data for a wide number of coelurosaur taxa and is updated regularly by new authors. Other matrices include the matrix published by Mark Powers, the matrix published by Scott Hartman and colleagues, and one produced by Philip Currie and David Evans. In many of the most updated analyses for each of these matrices, including data from recently described taxa, Atrociraptor has been consistently recovered as a member of Saurornitholestinae. The results of these analyses, including one of the most recent iterations of the various data sets, are shown below.


 * Evans, Larson, & Currie 2013


 * Czepiński 2022

Palaeobiology
The paleontologist Gregory S. Paul suggested in 2016 that Atrociraptor was able to attack relatively large prey, and wounding it more than usual for its relatives with its strong head and teeth. As a dromaeosaur, it would have used its sickle claw to deal with its prey.

Paleoenvironment
The holotype remains of Atrociraptor were found at a locality about 5 km West of the Royal Tyrrell Museum of Paleontology. This locality is part of the Horsthief Member of the Horseshoe Canyon Formation, which is the second-oldest member of the formation. The Horsethief Member ranges from around 72.2—71.5 million years ago, a timespan of about 700,000 years. Teeth from elsewhere in the Horseshoe Canyon Formation have been referred to Atrociraptor. If these referrals are valid, it would mean Atrociraptor had a much wider geographic and temporal distribution than initially suggested. These remains push the latest occurance of Atrociraptor to the Tolman Member of the formation (about 70.9—69.6 million years ago), suggesting the genus may have existed for over 2 million years.

In the early Maastrichtian, the continent of Laramidia (today North America) was about 8 degrees of latitude further North than North America is today. Despite this, the average temperature of the area was almost certainly much warmer than the region is today. The mean annual temperature in the Early Maastrichtian is estimated to have been around 10 C, compared to 4.5 C today.

The lower part of the Horseshoe Canyon Formation, where Atrociraptor was discovered, corresponds to "poorly-drained" sediments rich with organic material. The sediments of the Horsetheif Member are composed primarily of coal, shale which is high in organic material, sandstones, and a small number of mudstones. These are reflective of a highly saturated environment, likely a coastal plain or fluvial system which was on the margins of the Western Interior Seaway. This suggests that the region was very humid and composed primarily of wetlands with a high water table. This is futher corroborated by the presence of a wider variety of turtles in the lower members of the formation than in the higher members.

As the Maastrichtian stage progressed, the Western Interior Seaway began to shrink, which is reflected in the paleoclimatic reconstructions of the region. Later sediments are believed to have been further inland and less humid than those of the Late Campanian. This may be responsible for the apparent change in fauna during this period. How these climatic changed affected Atrociraptor is not clear, due to the rarity of its remains, but the discovery of teeth from the geologically younger Tolman Member suggests that the taxon may have survived through this period. Despite the apparent decline in temperature and humidity, it is known that major flooding events continued throughout this period. A bonebed in Dry Island Buffalo Jump Provincial Park, dated to one of the upper members of the Horseshoe Canyon Formation, preserves several Albertosaurus individuals, which are believed to have been killed and buried during a major storm.

Despite the regression of the Western Interior Seaway, Cretaceous Alberta became wetter and more humid in the uppermost part of the formation (around 68 million years ago), returning to conditions similar to those of the Horsethief Member. No remains from these youngest sediments have been referred to Atrociraptor, which suggests that the change in climate forced the small theropod to either move elsewhere or else become extinct.

Contemporary fauna
The Horsethief Member of the Horseshoe Canyon Formation constitutes the upper part of the so-called "Edmontosaurus regalis-Pachyrhinosaurus canadensis dinosaur zone". As the name suggests, two common terrestrial herbivores in these lower strata were Pachyrhinosaurus and Edmontosaurus. The lower part of Horseshoe Canyon has also preserves fossils of other ceratopsids including Anchiceratops, Arrhinoceratops, and several indeterminate specimens. Ankylosaurs are also well-represented in this member. The genera Edmontonia and Anodontosaurus have been found alongside indeterminate ankylosaur remains. Hadrosaurid remains are very common, but many of these are not confidently referred to any genera besides Edmontosaurus. Fragmentary remains of pachycephalosaurids have also been found.

Theropod remains are also common in the Horsetheif Member. Ornithomimus and Struthiomimus are known from several specimens, and other coelurosaurs are known from a few remains. These included the troodontid Albertavenator and the caenagnathids Apatoraptor and Epichirostenotes. Although their remains are not known directly from the Horsetheif Member, Dromaeosaurus, Paronychodon, and the tooth-taxon Richardoestesia are known from both older and younger deposits, so they are inferred to have existed at this time as well. The largest theropod in the Horsethief Member (and the Horseshoe Canyon Formation generally) was the tyrannosaurid Albertosaurus.

The holotype of Atrociraptor is the only fossil discovered from its locality, so it is not known for certain if any of these animals directly coexisted with it, but many of them are known to have been contemporaneous. Teeth found in the higher layers of the Horseshoe Canyon Formation (the Morrin and Tolman members) suggest that Atrociraptor may have also been a constituent of the younger “Hypacrosaurus altispinus-Saurolophus osborni dinosaur zone”. This period is typified by the presence of the hadrosaurids Saurolophus and Hypacrosaurus, in addition to numerous hadrosaur remains that have not yet been referred to a particular genus. This time interval extended from 71.5—69.6 million years ago and immediately followed the "Edmontosaurus regalis-Pachyrhinosaurus canadensis dinosaur zone". The Edmontosaurus, Pachyrhinosaurus, and Edmontonia that characterize the Horsethief Member appear to be completely absent from this zone, although these genera did persist elsewhere in Laramidia. The ceratopsids Anchiceratops and Arrhinoceratops persist into the Morrin Member and the early layers of the even younger Tolman Member, as does the ankylosaurid Anodontosaurus and the large predator Albertosaurus.

Small dinosaurs are also more common in the Tolman Member of the Horseshoe Canyon. These include the leptoceratopsid Montanoceratops, the pachycephalosaurid Sphaerotholus, the thescelosaurid Parksosaurus, and the alvarezsaurid Albertonykus in addition to the diverse assemblage of small theropods known from the Horsethief Member. The absence of these fossils from the older members does not necessarily mean that these taxa did not exist at that time, and may simply be reflective of the fossil bias which makes the preservation of small-bodied animals less likely.

Remains of smaller animals are also common in the Horseshoe Canyon. Fish are common fossils and are represented by sclerorhynchoids, guitarfish, sturgeons, paddlefishes, aspidorhynchids, osteoglossomorphs, elopiformes, ellimmichthyiformes, esocids, and acanthomorphs. Frogs and salamanders are also known from teeth found in these deposits alongside polyglyphanodontian lizards. Turtles were also very diverse in the warm and humid climate of the Early Maastrichtian. Fossils of macrobaenids, chelydrids, trionychids, adocids, and the giant genus Basilemys have been found from the parts of Horseshoe Canyon that correspond to wetlands. The choristodere genus Champsosaurus was also a resident of Alberta during this time.