Atrociraptor Marshalli

''Atrociraptor'' () is a genus of dromaeosaurid dinosaur that lived during the Late Cretaceous in what is now Alberta, Canada. The first specimen, a partial skull, was discovered in 1995 by the fossil collector Wayne Marshall in the Horseshoe Canyon Formation, about from the Royal Tyrrell Museum of Palaeontology where it was brought for preparation. In 2004, the specimen became the holotype of the new genus and species ''Atrociraptor marshalli''; the generic name is Latin for "savage robber", and the specific name refers to Marshall. The holotype consists of the (frontmost bones of the upper jaw), a (main bone of the upper jaw), the (tooth-bearing bones of the lower jaw), associated teeth, and other skull fragments. Isolated teeth from the same formation have since been assigned to ''Atrociraptor''.

Estimated to have measured about in length and weighed , ''Atrociraptor'' was a relatively small dromaeosaurid. As a dromaeosaurid, it would have had a large sickle-claw on the second toe and pennaceous feathers. ''Atrociraptor'' differs from its contemporary relatives in that its face is much deeper, and its teeth are more strongly inclined backwards than in most other dromaeosaurids and are almost all the same size. It also differed from most relatives in details of the skull, such as the part of the premaxilla below the nostril being taller than long, and in that its maxillary fenestra was larger. The fragmentary nature of the holotype has made the exact relations of ''Atrociraptor'' uncertain; it was initially thought to be a velociraptorine, but is now considered a saurornitholestine.

''Atrociraptor'' is thought to have been specialised for attacking larger prey than other dromaeosaurids, due to its deep snout. Various ideas for how dromaeosaurids used their sickle-claws have been proposed, and 21st-century studies suggest they used them to grasp and restrain struggling prey while dismembering them with the mouth. The holotype specimen is known from the Horsethief Member of the Horseshoe Canyon Formation, which dates to the Maastrichtian age, and ranges from around 72.2–71.5 million years ago. Assigned teeth from other parts of the formation indicate it survived for over 2 million years and across a wide geographic area.

Discovery

In 1995, the part-time fossil collector Wayne Marshall discovered a partial skull of a dromaeosaurid (commonly called "raptor") dinosaur in the Horseshoe Canyon Formation of Drumheller in Alberta, Canada, about 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. Much of the specimen was in a relatively hard block of sandstone, and preparation revealed the right (main bone of the upper jaw) exposed in outer side-view and the right (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 , which means , and , . The specific name references Marshall, the discoverer, and the full name can be translated as "Marshall's savage robber". Since then, Marshall has led the museum staff to many important finds. In all, the holotype preserves both (frontmost bone of the upper jaw, freed from the rock matrix), a right maxilla, both dentaries (the left one is incomplete), associated teeth (some having fallen out of their sockets prior to fossilisation), and numerous bone fragments from the skull. The specimen was not fully prepared by the time of its original description, so in 2022, the palaeontologist Mark J. Powers and colleagues used computed tomography to visualise further details of the skull. Numerous isolated teeth from the Horseshoe Canyon Formation (some of which had originally been assigned to ''Saurornitholestes'') have since been assigned to ''Atrociraptor''. Some of these were found in a bonebed in Dry Island Buffalo Jump Provincial Park, which also preserves multiple ''Albertosaurus'' individuals.

''Atrociraptor'' was featured in the 2022 film ''Jurassic World Dominion'', with the director describing it as more "brutish" than ''Velociraptor'' as it appears in the film. A ''/Film'' writer commented that while ''Atrociraptor'' "might sound like another made-up hybrid dinosaur invented for the ''Jurassic World'' films ... it's a very real dino with its own Wiki page and everything".

Description

''Atrociraptor'' was a relatively small dromaeosaurid, comparable to ''Velociraptor'' in size, and is estimated to have measured about in length and weighed . As a dromaeosaur, it would have had large arms, a long tail with vertebrae encased in rod-like extensions, and a large sickle-claw on the robust, hyper-extendible second toe. Fossils of other dromaeosaurids, 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'' does not have autapomorphies (unique diagnostic features) that can be used to distinguish it, but it has a unique combination of features that is so far unknown in relatives. ''Atrociraptor'' mainly differs from its contemporary relatives ''Bambiraptor'', ''Saurornitholestes'', and ''Velociraptor'' in that its face is much deeper, and in that its maxillary teeth are more strongly inclined backwards than in most other dromaeosaurids, and in the teeth being almost all the same size.

The relatively deep premaxillae have four teeth each, the same number as in other dromaeosaurids. The part of the premaxilla below the nostril is taller than long from front to back, as in ''Deinonychus'', ''Utahraptor'', and perhaps ''Dromaeosaurus'', while the opposite is the case for ''Bambiraptor'', ''Saurornitholestes'', and ''Velociraptor''. There is an elongated extension below the nostril, the subnarial process, that is wedged between the and the maxilla, as in other dromaeosaurids. The almost parallel subnarial and internarial (above the nostrils) processes are oriented more upwards than hindwards due to the depth of the snout, unlike in other dromaeosaurids where the opposite is true. The shallow depression on the side of the premaxilla that marks the lower front limit of the nostril opening is nested between the subnarial and internarial processes, while it extends further to the front in ''Velociraptor''.

The second tooth of the premaxilla is the largest of the four therein, based on the size of the , as in some relatives. As in ''Saurornitholestes'', the front is on the inner hind edge of the tooth, but more forwards positioned than the hind cutting edge. The teeth look more J-shaped than D-shaped in cross section. The on the front and hind cutting edges of the premaxillary teeth have almost the same basal diameter, though those on the front are taller. There are 2.3–3.0 serrations per .

The , an opening in the front of the antorbital fossa (a depression around the antorbital fenestra, the large opening in front of the eye), is positioned right under the maxillary fenestra, another opening. In relatives where both these fenestrae are known, the promaxillary fenestra is well in front of and below the maxillary fenestra. The maxilla is roughly triangular and relatively deeper than in other dromaeosaurids. The height between the maxillary fenestra and the tooth-bearing margin of the maxilla is more than twice the height of the largest tooth in ''Atrociraptor'', whereas it is less than twice that in other dromaeosaurids. If the teeth can be assumed to have had the same relative height among dromaeosaurids, the short and deep appearance of the maxilla of ''Atrociraptor'' could be due to the snout having an increased depth rather than because the snout was shortened.

The antorbital fenestra of ''Atrociraptor'' was relatively small compared to in other dromaeosaurids, with the portion in the maxilla taking up less than 43% its length, and the antorbital fossa also appears to have been relatively smaller. The rounded maxillary fenestra is larger than in relatives, and had a diameter of about when complete. In front of a depression on the side at the back of the maxilla, the margin of the antorbital fossa slopes forwards and up at a higher angle than in other dromaeosaurids. Right above the margin of the tooth sockets, the maxilla has a row of neurovascular foramina (which supplied blood). The lower margin of the maxilla is strongly convex when seen from the side.

The maxilla contains eleven teeth (comparable to most other dromaeosaurids), which are closely packed in their sockets with no gaps between them. The maxillary teeth are narrow from side to side, blade-like, and have a distinct inclination towards the back and down; only the teeth of ''Bambiraptor'' and ''Deinonychus'' are similarly inclined. The maxillary dentition is almost isodont (the teeth being of similar size), and, unusually for dromaeosurids, there are no gaps left by shed teeth in the holotype. The teeth vary little in overall height, while in ''Velociraptor'' every other tooth is noticeably longer than those next to it. The maxillary teeth have larger serrations on their hind cutting edge, 3–4.5 per mm, than their front, 5–8 per mm. The serrations on the hind edge have relatively straight shafts with hooked tips, and are taller than the serrations at the front. The front and hind cutting edges lie on the midline of the maxillary teeth like in relatives but unlike ''Dromaeosaurus'', and the teeth are generally comparable to those of ''Bambiraptor'', ''Deinonychus'', ''Saurornitholestes'', and ''Velociraptor''.

The dentary of ''Atrociraptor'' is similar to those of other dromaeosaurids. The upper and lower margins are almost parallel, though the height decreases somewhat towards the back of the tooth-bearing part. The (an opening at the side of the lower jaw) appears to have been small and set low as in relatives, and the dentary has two rows of nutritive foramina. The dentary is thin from side to side, the Meckelian canal

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