Utahraptor

Utahraptor is a genus of large dromaeosaurid dinosaur that lived during the Early Cretaceous period from around 139 to 135 million years ago in what is now the United States. The genus was described in 1993 by American paleontologist James Kirkland and colleagues with the type species Utahraptor ostrommaysi, based on fossils that had been unearthed earlier from the Cedar Mountain Formation of Utah. Later, many additional specimens were described including those from the skull and postcranium in addition to those of younger individuals.
It is the largest known member of the family Dromaeosauridae, measuring about long and typically weighing around. As a heavily built, ground-dwelling, bipedal carnivore, its large size and variety of unique features have earned it attention in both pop culture and the scientific community. The jaws of Utahraptor were lined with small, serrated teeth that were used in conjunction with a large "killing claw" on its second toe to dispatch its prey. Its skull was boxy and elongated, akin to other dromaeosaurids like Dromaeosaurus and Velociraptor.
Being a carnivore, Utahraptor was adapted to hunt the other animals of the Cedar Mountain Formation ecosystem such as ankylosaurs and iguanodonts. Evidence from the leg physiology supports the idea of Utahraptor being an ambush predator, in contrast to other dromaeosaurs that were pursuit predators. Fossil remains of several individuals of various ages have been found together, suggesting that Utahraptor was gregarious and practiced degrees of post nestling care.

Discovery and naming

The first specimens of Utahraptor were found in 1975 by Jim Jensen in the Dalton Wells Quarry of Utah, near the town of Moab, but did not receive much attention. After the find of a large claw by Carl Limone in October 1991, James Kirkland, Robert Gaston and Donald Burge uncovered further remains of Utahraptor in 1991 in the Gaston Quarry in Grand County, Utah, within the Yellow Cat and Poison Strip members of the Cedar Mountain Formation. The holotype of Utahraptor, CEUM 184v.86, consists of a second pedal ungual, with potentially assigned elements from other specimens: pedal ungual CEUM 184v.294, tibia CEUM 184v.260 and premaxilla CEUM 184v.400. The holotype is housed in the paleontology collections of the Prehistoric Museum at Utah State University Eastern. Brigham Young University, the depository of Jensen's finds, currently houses the largest collection of Utahraptor fossils.
The type species, Utahraptor ostrommaysi, was named by Kirkland, Gaston and Burge in June 1993. The genus name Utahraptor is in reference to Utah, where the remains were found. The specific name, ostrommaysi, is in honor to John Ostrom for his investigations on Deinonychus and its relationships to birds, as well as Chris Mays, who helped in the research of Utahraptor by founding Dinamation. From his description, Kirkland stated the meaning of genus name to be "Utah's predator," but the Latin word raptor translates to 'robber' or 'plunderer', not 'predator'. Earlier, it had been intended to name the species "U. spielbergi" after film director Steven Spielberg. It was alleged that this was in exchange for Spielberg funding paleontological research, and that the name was changed because agreement couldn’t be reached on the amount of financial assistance. Kirkland, however, later expressed that this was a misquote. He had changed the name to U. ostrommaysi shortly before the publication of the paper, as Universal Studios has begun threatening to sue museums that have exhibits with the word "Jurassic" in their titles; Dinamation, Kirkland's employers with connections to various museums, thus implored him to change the name, which he did.
In 2000, the specific name was emended by George Olshevsky to the plural genitive ostrommaysorum. However, Thiago Vernaschi V. Costa and Normand David in 2019 criticized the use of the species name U. ostrommaysorum, since it has no clear justification or explanation. Although this spelling has been largely used by other authors, the genus Utahraptor was originally coined with the type species U. ostrommaysi and, given that the International Code of Zoological Nomenclature offers no provision for forming a genitive form for persons with different names, Costa and David conclude that the original spelling ostrommaysi has to be regarded as an arbitrary combination of letters and not a correctly formed genitive form. Under this reasoning, ostrommaysorum has no valid use and the original spelling ostrommaysi does not need to be emended. Other alternative and also invalid spellings were used in scientific literature, such as ostromaysi, ostromaysorum, ostromayssorum, ostromayorum and ostrommaysori.
Some elements were wrongly referred to the genus. The lacrimal bone of the specimen CEUM 184v.83 turned out to be a postorbital from the ankylosaur Gastonia. Britt and colleagues also suggested that the previously identified manual unguals of the specimens M184v.294, BYU 9438 and BYU 13068 are indeed pedal unguals. This suggestion was confirmed by Senter in 2007.

Description

Utahraptor was one of the largest dromaeosaurids, with the largest elements assigned to the genus being slightly larger than the same bones in Achillobator. It was fairly robust compared to other dromaeosaurs, with stocky limbs proportioned more like those of other large theropods than those of its close relatives. Unlike its cousins, U. ostrommaysi had a robust lower jaw with a downturned tip, and as a result, the teeth at the front were quite procumbent. The characteristic "killing claw" of dromaeosaurids may, in U. ostrommaysi, have measured about in length, and its feet appear to have had a very tight grip. Based on evidence of filamentation in other dromaeosaurs, U. ostrommaysi was likely feathered.

Size

Utahraptor was one of, if not the largest and heaviest of all dromaeosaurids, with the largest assigned specimen BYUVP 15465 having a femoral length of. Kirkland and colleagues originally estimated that it measured around in length and had a body mass of somewhere under, making it roughly the size of a modern polar bear, and subsequent authors have reported a lower length estimate of. A subsequent paper indicated that U. ostrommaysi was about the same size as Achillobator, and that the latter taxon was around in length and, ergo U. ostrommaysi was the same. Ruben Molina-Pérez and Asier Larramendi, in 2019, estimated that BYUVP 15465 was in length and weighed. Several mass estimates for U. ostrommaysi have placed it at around. In 2013, based on femoral length, Lindsay E. Zanno and Peter J. Makovicky estimated its body mass at around. In 2024, higher mass estimates were once again recovered by Romain Pintore and colleagues. The body masses of BYUVP 2536 and BYUVP 1833 were estimated around respectively, while BYUVP 7510-18078 was estimated to have weighed.

Skull and dentition

The skull elements preserved in Utahraptor's holotype consisted of a premaxilla, a lacrimal bone, and several teeth. Most of the holotype premaxilla was preserved, aside from the superior nasal process, the part at the top to which the upper part of the nasal bone would have articulated. The nasal process preserved in other specimens is somewhat elongated. According to Kirkland and colleagues in 1993, the premaxilla of U. ostrommaysi was incredibly large compared to that of other dromaeosaurids, perhaps 250% larger than that of Deinonychus. The main body of the bone was essentially square in shape. At the posterior margin of the naris, the subnarial process flared outward to join with the subnarial process of the nasal bone, and in doing so completely excluded the maxilla from the margins of the naris. The alveolar process or tooth row of the premaxilla was around in length. Utahraptor's maxilla had a rugose lateral surface covered in ridges and grooves which extended from neurovascular foramina, which would have supplied facial tissues with blood. The quadratojugal bone of U. ostrommaysi was L-shaped and lacked a posterior process. The mandible of U. ostrommaysi is known from several dentaries, recovered from the Utahraptor Block. One such specimen, UMNH VP 20501, comes from an adult specimen. It indicates that the teeth of Utahraptor's lower jaw were fairly procumbent anteriorly, meaning those towards the front would have visibly protruded forwards.
Utahraptor's teeth differed from those of most other dromaeosaurs in that the anterior and posterior denticles were only slightly different in density. They are most similar to those of Achillobator and Dromaeosaurus. Similarly, the counter-clockwise twisting of the dental carinae on the premaxillary teeth is like what is observed in Dromaeosaurus; consequently, these teeth were very asymmetric in cross-section.

Postcranial skeleton

The dorsal vertebrae of Utahraptor lacked pleurocoels, a series of hollow depressions found in the vertebrae of many dinosaurs which reduced their weight. As in other dromaeosaurs, the parapophyses were mounted on pedicles on each dorsal vertebra. The caudal vertebrae were similar to those of Deinonychus despite being considerably larger. They are platycoelous, meaning they are flat anteriorly and concave posteriorly. The chevrons and prezygapophyses of U. ostrommaysi's tail appear to have been shortened, which would have increased the tail's overall flexibility in comparison to that of many other dromaeosaurs. The specific condition of Utahraptor's tail vertebrae is dubbed a "hemicaudothecal" condition, and is also seen in Achillobator, suggesting that it may be a prerequisite for the evolution of a very large body size in dromaeosaurids.
The forelimbs of Utahraptor are largely unknown. The manual unguals, the bones which would have supported the hand claws, were broadly similar to those of other dromaeosaurs, bearing asymmetrical and prominent grooves along their inner and outer edges. However, they were more laterally compressed and were noted to have been more blade-like in construction, leading Kirkland and colleagues to suggest that U. ostrommaysi might have used them to slash at prey.
The femur of the largest known Utahraptor specimen measured, about 15 cm larger than that of Achillobator. There was a well-developed notch between the lesser and greater trochanters. What is known of the tibia is stout and has a straight shaft, with a circumference/length index of 40, between that of Ceratosaurus and Torvosaurus. It bore a prominent cnemial crest, twisting towards where the fibula would have sat. Its overall appearance was noted by Kirkland and colleagues to be somewhat reminiscent of Acrocanthosaurus, and in personal communications with the authors, Robert T. Bakker noted a further similarity in that the tibia may have been about as long as the femur, as in large non-dromaeosaur theropods. Like other dromaeosaurids, Utahraptor had a large curved claw on the second toe of each foot. The second pedal ungual, the bone on the second toe which would have supported this claw, is preserved in the holotype with a length when measured along its outside curve, and is estimated to have measured in total. The size of the flexor tubercles on U. ostrommaysi's pedal digits, to which digital flexors would have attached in life, suggests that it had a strong grip, perhaps more so than Deinonychus in proportion to body size.