Saurornitholestes

The foot of an assigned S. langstoni specimen

Two more complete, larger partial skeletons, RTMP 88.121.39 and MOR 660, dozens of isolated bones, and swathes of teeth are known from the badlands of Dinosaur Provincial Park. Most of these are housed at the Royal Tyrrell Museum of Palaeontology, located in Drumheller, Alberta, but they remain undescribed. The Alberta and Montana remains are usually attributed to the single species Saurornitholestes langstoni, but they come from a variety of rock formations indicating a wide span of time. For example, the Oldman Formation dated to about 77 million years ago[2], and the upper Two Medicine Formation dated to about 72 million years ago. Similar teeth are found in younger deposits, dated to about 70 to 69 million years ago,[3] but whether they represent S. langstoni or a different, related species is unknown. Neonate sized Saurornitholestes fossils have been reported in scientific literature.[4]

Fragmentary fossils of Saurornitholestes have also been found in the eastern half of the United States of America. A tooth found in the Mooreville Chalk Formation of Alabama was assigned to the genus. In 2015, Schwimmer et al. identified the existence of Saurornitholestes langstoni from the Tar Heel, Coachman, and Donoho Creek formations of North and South Carolina based on diagnostic teeth and a pedal ungual. This also makes S. langstoni the only dromaeosaurid taxon that is currently reported to have actually lived on the East Coast.[5]

Saurornitholestes sullivani is known from the Hunter Wash fauna of New Mexico’s Kirtland Formation, based on the frontal specimen SMP VP-1270. It differs from S. langstoni in the characters of the frontal.[6]

A well preserved skeleton of Saurornitholestes, named UALVP 55700, was discovered in 2014 and is currently under preparation by University of Alberta paleontologists working in Japan.[7] After examining the skull of the specimen, Currie and Evans announced in 2019 that the Zapsalis teeth from the Dinosaur Park Formation represented the second premaxillary tooth of S. langstoni.[8]

In 2006, Robert Sullivan named and described a second nominal species, Saurornitholestes robustus, based on holotype specimen named SMP VP-1955. The specimen is a left frontal, and the specific name refers to the great thickness of this bone, the only trait in which the species is known to differ from S. langstoni. The holotype was found in the Willow Wash fauna of the Kirtland Formation, dated to about 73 million years ago.[9] However, a subsequent overview of dromaeosaurid phylogeny asserted that S. robustus lacked any dromaeosaurid characters and should be considered an indeterminate theropod.[10] A study published in 2014 took the conclusion a step further by demonstrating that S. robustus was assignable to Troodontidae based on similarities with troodontids.[11]

Possible indeterminate fossils are known from the Hell Creek Formation of Montana, North Dakota, and South Dakota, dated to about 66 million years ago.[12]

Saurornitholestes sullivani is thought to have had a very keen sense of smell, due to its brain case showing that it had an unusually large olfactory bulb for its size.[6]

The second premaxillary teeth of Saurornitholestes, as well as those of Velociraptor and Bambiraptor, may have been structurally specialized for preening feathers. This may also have been the function of the unusual premaxillary teeth of the oviraptorosaurs Protarchaeopteryx and Incisivosaurus.[8]

An artistic restoration of an S. langstoni digging a multituberculate out of its burrow

Saurornitholestes' feeding habits were discovered to be typical of coelurosaurian theropods, with a characteristic "puncture and pull" feeding method. Studies of wearing patterns on its teeth by Angelica Torices et al. in a study regarding theropod feeding habits indicate that dromaeosaurid teeth share similar wearing patterns to those seen in tyrannosaurids and troodontids, respectively. However, micro wearing on the teeth indicated that Saurornitholestes likely preferred larger prey items than troodontids it shared its environment with. Such differentiations in its diet likely allowed it to inhabit the same environment as its more distant maniraptoran relatives. The same study also indicated that both Saurornitholestes and the related Dromaeosaurus likely included bones in their diet and were better adapted to handling the stress associated with attacking struggling prey, while troodontids, equipped with weaker jaws, preyed on softer animals and prey items, such as invertebrates and carrion. This feeding strategy and ability to handle struggling prey was also a feature dromaeosaurids shared with tyrannosaurids, such as Gorgosaurus, which was also analyzed in said study alongside these smaller theropods.[17][18]

A tooth of Saurornitholestes has been found embedded in the wing bone of a large pterosaur, probably a juvenile Quetzalcoatlus.[19] Because the pterosaur was so much larger than Saurornitholestes, Currie and Jacobsen suggest that the theropod was probably scavenging the remains of an already dead animal.[19]

In 2001, Bruce Rothschild and others published a study examining evidence for stress fractures and tendon avulsions in theropod dinosaurs and the implications for their behavior. They found that only two of the 82 Saurornitholestes foot bones checked for stress fractures actually had them. Two of the nine hand bones examined for stress fractures were found to have them as well.[20]

Aase Roland Jacobsen published a description of a Saurornitholestes dentary in 2001.[21] The dentary is about 12 cm long and preserves fifteen tooth positions, of which only ten preserve teeth.[22] Three toothmarks were visible on the inner "lingual" surface of the dentary.[22] Two of the three marks are series of grooves made by the serrations on the maker's teeth.[22] The striations are between 0.37 mm and 0.40 mm thick with cuboidal cross-sections.[23]

The shape of the preserved serrations are too different from those of Saurornitholestes for the marks to be the result of injuries incurred during intraspecific face biting behaviors.[24] Although the right shape for Dromaeosaurus tooth serrations, the preserved marks are too coarse to have been left by that genus.[24] Although a specific identification cannot be made, the most likely perpetrator would be a juvenile individual of one of the Dinosaur Park Formation's tyrannosaurids, like Gorgosaurus, or Daspletosaurus.[25]