The first specimen, a partial skull, was discovered in 1995 by the fossil collector Wayne Marshall in the Horseshoe Canyon Formation, about 5 km (3 mi) from the Royal Tyrrell Museum of Palaeontology where it was brought for preparation.

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.

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.

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 5 km (3 mi) west of the Royal Tyrrell Museum of Palaeontology.

[13] 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".

[14] Atrociraptor was a relatively small dromaeosaurid, comparable to Velociraptor in size, and is estimated to have measured about 1.8–2 m (5.9–6.6 ft) in length and weighed 15 kg (33 lb).

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.

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.

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.

The external mandibular fenestra (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.

For this reason, some studies fully omit Atrociraptor from the presentation of their findings in order to improve the clarity of the interrelationships they found in their phylogenetic analyses.

[20] When Atrociraptor was first described in 2004, it was found to be closely related to Deinonychus and assigned to the dromaeosaurid subfamily Velociraptorinae, a group known predominantly from the Late Cretaceous of Asia.

[23] By 2012, the palaeontologist Alan H. Turner and colleagues stated that the three phylogenetic analyses featuring Atrociraptor so far had such disparate results that there was no consensus regarding its affinity to other dromaeosaurids.

[19] In their 2013 description of Acheroraptor, the palaeontologist David Evans and colleagues suggested that Atrociraptor was the sister taxon of the much older genus Deinonychus in a clade more advanced than Saurornitholestinae but outside of both Velociraptorinae and Dromaeosaurinae.

[22] One of these datasets is the so-called "TWiG matrix" (an abbreviation for the Theropod Working Group), developed by the palaeontologists Steven Brusatte, Andrea Cau, Mark Norell, and several other researchers, which contains data for most named coelurosaurian taxa and is updated regularly by new authors.

[22] In many of the most updated analyses for each of these matrices, including data from recently described taxa, Atrociraptor has been consistently found to be a member of Saurornitholestinae.

[34] The lower part of the Horseshoe Canyon Formation, where the holotype was discovered, corresponds to poorly-drained sediments which reflect a depositional setting with a lot of standing water.

These sediments are rich with organic materials, which reflect a highly saturated and humid environment, likely a coastal plain or fluvial system which was on the margins of the Western Interior Seaway.

[11] How these climatic changes 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.

The Albertosaurus bonebed which contains teeth referred to Atrociraptor, dated to one of the upper members of the Horseshoe Canyon Formation, was believed to have been deposited during a major storm.

[12] 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 assigned to Atrociraptor, which suggests that either the change in climate forced the small theropod to move elsewhere or it became extinct.

Although their remains are not known directly from the Horsethief Member, Dromaeosaurus, Paronychodon, and the poorly-understood taxon Richardoestesia (known only from teeth) are known from both older and younger deposits, so they are inferred to have existed at this time as well.

[1] 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 zone.

Edmontosaurus, Pachyrhinosaurus, and Edmontonia that characterise the Horsethief Member appear to be completely absent from this zone, although these genera did persist elsewhere in Laramidia.

[1] The aforementioned teeth, which have been assigned to Atrociraptor, suggest that it directly shared its environment in the Tolman Member with Albertosaurus, Hypacrosaurus, one or more troodontids, ornithomimids, and possibly other dromaeosaurids.

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.

Fish are common fossils and are represented by sclerorhynchoids, guitarfish, sturgeons, paddlefishes, aspidorhynchids, osteoglossomorphs, elopiformes, ellimmichthyiformes, esocids, and acanthomorphs.

Fossils of macrobaenids, chelydrids, trionychids, adocids, and the giant genus Basilemys have been found from the parts of Horseshoe Canyon Formation that correspond to wetlands.

Uniquely, fossils of true cycads – which are otherwise very abundant among Mesozoic flora – appear to be completely absent from the Horseshoe Canyon Formation.