| Saurornitholestes | |
|---|---|
 | |
| Skeletal reconstruction ofS. langstoni | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Dromaeosauridae |
| Clade: | †Eudromaeosauria |
| Subfamily: | †Saurornitholestinae |
| Genus: | †Saurornitholestes Sues, 1978 |
| Type species | |
| †Saurornitholestes langstoni Sues, 1978 | |
| Other species | |
| Synonyms | |
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Saurornitholestes ("lizard-bird thief") is agenus of carnivorousdromaeosauridtheropoddinosaur from the lateCretaceous ofCanada (Alberta andSaskatchewan[3]) and theUnited States (Montana,New Mexico,Alabama, andSouth Carolina). Two species have been named:S. langstoni in 1978 andS. sullivani in 2015.Saurornitholestes was a small, bipedal, meat-eating dinosaur, equipped with a sickle-like claw on each foot. A three possible species, "S."robustus are in dispute.
In 1974, Canadian amateur paleontologist Irene Vanderloh discovered the skeleton of a small theropod nearSteveville in Alberta. She showed it toJohn Storer of theProvincial Museum of Alberta, who brought it to the attention ofHans-Dieter Sues. In 1978, Sues named and described the specimen as the type speciesSaurornitholestes langstoni. The generic name is in reference to theSaurornithoididae, due to the resemblance with this group that is today seen as part of theTroodontidae, and combines their name with a Greeklestes, "thief". Thespecific name honoursWann Langston, Jr.[4]
Theholotype specimen, TMP 1974.10.5, was uncovered in a layer of theDinosaur Park Formation dating to the lateCampanian. It consists of a very fragmentary skeleton including teeth, skull elements, two vertebrae, ribs, tail elements and a part of the hand. Also threeparatypes were assigned: CMN 12343, CMN 12354, and UA 5283, all frontals.[4]
Two more complete and larger partial skeletons (RTMP 88.121.39 and MOR 660), dozens of isolated bones, and scores of teeth are known from the badlands ofDinosaur Provincial Park in Alberta; most of these are housed at theRoyal Tyrrell Museum of Palaeontology, inDrumheller,Alberta and remain undescribed. The Alberta and Montana remains are usually attributed to the single speciesSaurornitholestes langstoni, though they come from a variety of rock formations indicating a wide span of time; for example, theOldman Formation (dated to about 77 million years ago[5]) and the upperTwo Medicine Formation (about 72 million years ago). Similar teeth are found in younger deposits, dated to around 70 to 69 million years ago,[6] but whether they representS. langstoni or a different, related species is unknown. Neonate-sizedSaurornitholestes fossils have been reported in thescientific literature.[7]
Fragmentary fossils ofSaurornitholestes are also known from the eastern half of North America, which formed the landmass ofAppalachia during the Late Cretaceous. A tooth found in theMooreville Chalk ofAlabama has been assigned to the genus. In 2015, Schwimmer et al. identified the existence ofSaurornitholestes langstoni from theCoachman andDonoho Creek formations ofSouth Carolina based on diagnostic teeth and a pedal ungual. This also makesS. langstoni currently the only dromaeosaurid taxon reported with certainty from the East Coast.[8]
Saurornitholestes sullivani is known from the Hunter Wash fauna of theKirtland Formation in New Mexico, based on the frontal SMP VP-1270. It differs fromS. langstoni in the characters of the frontal.[9]
A well-preserved skeleton ofSaurornitholestes (specimen UALVP 55700) discovered in 2014 is currently under preparation byUniversity of Alberta paleontologists working in Japan.[10] After examining the skull of that specimen, Currie and Evans announced in 2019 that theZapsalis teeth from the Dinosaur Park Formation represented the second premaxillary tooth ofS. langstoni.[11]
In 2006,Robert Sullivan named and described a second nominal species,Saurornitholestes robustus, based on holotype SMP VP-1955, a left frontal. The specific name refers to the great thickness of this bone, the only trait in which the species is known to differ fromS. langstoni. The holotype was found in the Willow Wash fauna of theKirtland Formation in New Mexico, dated to about 73 million years ago.[12] However, a subsequent overview of dromaeosaurid phylogeny asserted thatS. robustus lacked dromaeosaurid characters and should be considered an indeterminate theropod,[13] and a study published in 2014 took the conclusion a step further by demonstrating thatS. robustus was assignable toTroodontidae based on similarities with troodontids.[14]
Possible indeterminate fossils are known from theHell Creek Formation inMontana,North Dakota, andSouth Dakota, dated to about 66 million years ago.[15]
Saurornitholestes was a small dromaeosaur, with the type speciesS. langstoni measuring about 1.3–1.8 m (4 ft 3 in – 5 ft 11 in) long and weighing approximately between 5 and 22.5 kg (11 and 50 lb).[16][17][18][19] At the hip it stood 60 cm (2 ft) tall. Like othertheropods in the Dromaeosauridae,Saurornitholestes had a long, curving, blade-like claw on the second toe.Saurornitholestes was more long-legged and lightly built than other dromaeosaurids such asVelociraptor andDromaeosaurus. It resemblesVelociraptor in having large, fanglike teeth in the front of the jaws.Saurornitholestes most closely resemblesVelociraptor, although the precise relationships of the Dromaeosauridae are still relatively poorly understood.
In 1978, Sues assignedSaurornitholestes to theDromaeosauridae. Later studies most often found it a member of the dromaeosauridVelociraptorinae, but acladistic analysis byPhilip J. Currie in 2009 recovered a position in a more basal dromaeosauridclade that was named theSaurornitholestinae.
The cladogram below is the result of a 2019 analysis byPhilip J. Currie and David C. Evans. Currie and Evans recoveredSaurornitholestes as thesister taxon ofAtrociraptor.[11]
| Eudromaeosauria |
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Saurornitholestes sullivani is thought to have had a keen sense of smell, due to its skull suggesting an unusually large olfactory bulb.[9]
The secondpremaxillary teeth of (at least)Saurornitholestes,Velociraptor, andBambiraptor may have been structurally specialized for preening feathers. This may also have been the function of the unusual premaxillary teeth of theoviraptorosaursProtarchaeopteryx andIncisivosaurus.[11]
Saurornitholestes' feeding habits were discovered to be typical of coelurosaurian theropods, with a characteristic "puncture and pull" feeding method. Studies of wear patterns on the teeth of this animal by Angelica Toriceset al. in a study regarding theropod feeding habits indicate that dromaeosaurid teeth share similar wear patterns to those seen in the Tyrannosauridae and Troodontidae, respectively. However, micro-wear on the teeth indicated thatSaurornitholestes likely preferred larger prey items than the troodontids it shared their environment with. Such differentiations in its diet likely allowed the theropod to inhabit the same environment as its more distant maniraptoran relations. The same study also indicated that bothSaurornitholestes and the relatedDromaeosaurus (also analyzed in the study) likely included bone in their diet and were better adapted to handle the stresses 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 that these two dromaeosaurids shared with tyrannosaurids such asGorgosaurus, which was also analyzed in said study alongside these smaller theropods.[20][21]
A tooth ofSaurornitholestes has been found embedded in the wing bone of a largepterosaur, possibly a juvenileQuetzalcoatlus.[22] Because the pterosaur was so much larger thanSaurornitholestes, Currie and Jacobsen suggest that the theropod was probably scavenging the remains of an already dead animal.[22]
In 2001,Bruce Rothschild and others published a study examining evidence forstress fractures andtendon avulsions intheropod dinosaurs and the implications for their behavior. They found that only two of the 82Saurornitholestes foot bones checked for stress fractures actually had them. Two of the nine hand bones examined for stress fractures were found to have them.[23]
Aase Roland Jacobsen published a description of aSaurornitholestesdentary in 2001.[24] The dentary is about 12 cm long and preserves fifteen tooth positions, of which only ten preserve teeth.[25] Three toothmarks were visible on the inner"lingual" surface of the dentary.[25] Two of the three marks are series of grooves made by the serrations on the maker's teeth.[25] Thestriations are between 0.37 mm and 0.40 mm thick withcuboidal cross-sections.[26]
The shape of the preserved serrations are too different from those ofSaurornitholestes for the marks to be the result of injuries incurred duringintraspecific face biting behaviors.[27] Although the right shape forDromaeosaurus tooth serrations, the preserved marks are too coarse to have been left by that genus.[27] Although a specific identification cannot be made, the most likely perpetrator would be a juvenile individual of one of theDinosaur Park Formation's tyrannosaurids, likeGorgosaurus, orDaspletosaurus.[28]
Saurornitholestes was found on both sides of theWestern Interior Seaway. Alberta, the location ofSaurornitholestes langstoni, had a habitat similar to the United States Middle West being plains[29] and floodplain swamps.[30] In its eastern range,Saurornitholestes lived alongsidehadrosaurs likeEotrachodon andHypsibema, large theropods likeAppalachiosaurus andDryptosaurus, an unidentifiedornithomimosaur, and another unidentified small theropod that was likely either adromaeosaurid or atroodontid.Saurornitholestes appears to have been the most common small theropod in Dinosaur Provincial Park, and teeth and bones are much more common than those of its more robust contemporary,Dromaeosaurus.[8]
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