Utahraptor Temporal range: Early Cretaceous (Berriasian – Valanginian), 139–134.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Reconstructed skeleton in BYU Museum of Paleontology
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Dromaeosauridae
Clade:	†Eudromaeosauria
Subfamily:	†Dromaeosaurinae
Genus:	†Utahraptor Kirkland, Gaston & Burge, 1993
Type species
†Utahraptor ostrommaysi Kirkland et al., 1993

Utahraptor (meaning "Utah's predator") is a genus of large dromaeosaurid (a group of feathered carnivorous theropods) dinosaur that lived during the Early Cretaceous period from around 135 to 130 million years ago in what is now the United States. The genus was described in 1993 by an 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 was a heavily built, ground-dwelling, bipedal carnivore. It contains a single species, Utahraptor ostrommaysi, which is the largest-known member of the family Dromaeosauridae, measuring about 5.5–7 metres (18–23 ft) long and weighing around 250–500 kilograms (550–1,100 lb). 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.

Utahraptor is in the subfamily Dromaeosaurinae, which contained the biggest of the dromaeosaurs in the form of Utahraptor as well as Dakotaraptor and Achillobator. 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.

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 a find of a large claw by Carl Limone in October of 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.^[1] 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. [1]

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.^[1] From his description, Kirkland stated the meaning of genus name to be "Utah's predator,"^[1] but the Latin word raptor translates to 'robber' or 'plunderer', not 'predator'.^[2] Earlier, it had been intended to name the species "U. spielbergi" after film director Steven Spielberg, in exchange for him funding paleontological research, but no agreement could be reached on the amount of financial assistance.^[3]

In 2000, the specific name was emended by George Olshevsky to the plural genitive ostrommaysorum.^[4] 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 from two 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.^[5]

For instance, 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 et al. also suggested that the previously identified manual unguals of the specimens M184v.294, BYU 9438 and BYU 13068 are indeed pedal unguals.^[6] This suggestion was confirmed by Senter in 2007.^[7]

Description

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 56.5–60 cm (22.2–23.6 in).^[8][9]

Upon the description of Utahraptor, its dimensions were estimated as much as 7 m (23 ft) in length and somewhat less than 500 kg (1,100 lb), comparable in weight to a polar bear.^[1] Various authors agree in that Utahraptor is best estimated around 5.5 m (18 ft) long and weighing up to 250–350 kg (550–770 lb).^{[10] [11][12][13][14]}

Although feathers have never been found in association with Utahraptor specimens, there is strong phylogenetic evidence suggesting that all dromaeosaurids had them. The feathered genus Microraptor is one of the oldest-known dromaeosaurids and is phylo­genetically more primitive than Utahraptor.^[15] Since Microraptor and other dromaeosaurids possessed feathers, it is reasonable to assume that this trait was present in all of Dromaeosauridae. Feathers were very unlikely to have evolved more than once, so assuming that any given dromaeosaurid, such as Utahraptor, lacked feathers would require positive evidence that they did not have them.^[16] So far, there is nothing to suggest that feathers were lost in larger, more derived species of dromaeosaurs.^[17] The presence of quill knobs in Dakotaraptor evidenced that even larger dromaeosaurids had feathers.^[18]

According to Kirkland et al. in 1993, Utahraptor can be recognized by a few special autapomorphies. The claws on its hand are more specialized as cutting blades than in other dromaeosaurids. It has a lacrimal bone with distinctly parallel mesial and outer sides that gives it an elongate subrectangular appearance in top view and it has a base of the nasal opening on the premaxilla parallel to the premaxillary tooth row.^[1] In the revised diagnosis conducted by Turner et al. in 2012, Utahraptor differs from other dromaeosaurids in having an elongate nasal process of the premaxilla, a distal end of metatarsal III that is smooth, not ginglymoid, an L-shaped quadratojugal without a posterior process, the presence of a well-developed notch between the lesser trochanter and greater trochanter, and dorsal vertebrae that lack pleurocoels.^[19] Like other dromaeosaurids, Utahraptor had a large curved claw on the second toe of each foot. The second pedal ungual is preserved at 22 cm (8.7 in) in length and is estimated to reach 24 cm (9.4 in) in restoration.^[1]

Classification

Utahraptor is a member of the family Dromaeosauridae, a clade of theropod dinosaurs commonly known as "raptors". Utahraptor is the largest known genus in the family and belongs to the same clade of other notable dinosaurs such as Velociraptor, Deinonychus, or Dromaeosaurus. It is classified in the subfamily Dromaeosaurinae, which is found in the clade Eudromaeosauria.^[1]

In 2015, Utahraptor was found to be closely related to the smaller Dromaeosaurus and the giant Mongolian and North American dromaeosaurid genera Achillobator and Dakotaraptor:^[18]

The cladogram below is the result of a cladistic analysis conducted by Cau et al. in 2017.^[20]

Paleobiology

Predatory behavior

Kirkland et al. noted that given the huge size of Utahraptor, it was not as fast as Deinonychus and Velociraptor; instead, it would have had a similar speed to the contemporary iguanodonts, and was faster than sauropods. Additionally, the thickness of the tibia indicates that the animal possessed a significant leg force in order to kill prey. It was also suggested that lighter dromaeosaurids such as Velociraptor and Deinonychus relied on their hand claws to handle prey and retain balance while kicking it; in contrast to this, the heavily built Utahraptor may have been able to deliver kicks without the risk of losing balance, freeing the hands and using them to dispatch prey.^[1]

According to Gregory S. Paul, Utahraptor was not particularly fast and would have been an ambush hunter that preyed on large dinosaurs such as the contemporary iguanodonts and therizinosaurs. Its robust build and large sickle claw indicate it was well suited to hunting such prey. Like other dromaeosaurine dromaeosaurids, it may have also relied heavily on its jaws to dispatch prey—more so than other types of dromaeosaurids, such as velociraptorines.^[11]

Social behavior

In 2001, Kirkland et al. pursued a graduate student's discovery of a bone protruding from a 9-ton fossil block of sandstone in eastern Utah. It was determined to contain the bones of at least seven individuals, including an adult measuring about 4.8 m (16 ft), four juveniles, and a hatchling about 1 m (3.3 ft) long. Also fossilized with the Utahraptor pack are the remains of at least one possible iguanodont. Kirkland speculated that the Utahraptor pack attempted to scavenge carrion or attack helpless prey mired in quicksand, and were themselves mired in the attempt to feed on the herbivore. Similar sites such as the Cleveland-Lloyd Quarry and California's La Brea Tar Pits house such predator traps. Examination of the fossils are ongoing after a decade of excavation, but if Kirkland is correct, it may be one of the best-preserved predator traps ever discovered. The fossils may further reveal aspects into the behavior of Utahraptor, such as whether it might have hunted in groups like Deinonychus was believed to have done. Whether all the Utahraptor individuals were mired simultaneously or were drawn in, one-by-one is unclear.^[21] Further examination of the block suggests that the number of Utahraptor remains may be double the amount previously assumed.^[22]

While dinosaur behavior can only be theorized, it was later discovered in 2020 that Deinonychus may not have had complex social behavior seen in some pack-hunting animals alive in the modern era, based on differing dietary preferences in adults and juveniles indicating they did not live in long-term family groups. It is unknown if this lack of complex, cooperative social behavior also extended to Utahraptor.^[23]

Paleoenvironment

Utahraptor lived in the lower part of the Cedar Mountain Formation, a bed known as the Yellow Cat Member. According to the authors of its description, Utahraptor had an important ecological role as a major carnivore of the paleofauna of the present-day Arches region during the Early Cretaceous, and could probably attack prey larger than itself. Group hunting of individuals of at least 3.5 m (11 ft) and 70 kg (150 lb), if proven, could have killed 8 m (26 ft) prey of a weight of 1 to 2 t (0.98 to 2.0 long tons; 1.1 to 2.2 short tons). Additionally, sauropods ranging around 20 m (66 ft) may have been an important part of its diet.^[1] The paleontologist Thomas R. Holtz estimated that Utahraptor existed between 130 million and 125 million years ago.^[24] In multiple occasions, the Yellow Cat Member has been dated to Barremian-Aptian ages. Sames and Schudack (2010) proposed a reassignment of the estimated age, compromising Berriasian to Valanginian stages; however, this interpretation was not followed by most authors.^[25] Using advanced methods of radiometric and palynological dating, Joeckel et al. (2019) concluded that the Yellow Cat Member is indeed older than previous estimations. The deposition occurred between 139 ± 1.3 million to 134.6 ± 1.7 million years ago, or, Berriasian to Late Valanginian stages. Based on the presence of new palynoflora, Middle Berriasian–Early Hauterivian ages were provisionally assigned.^[26] However, the Yellow Cat Member is divided into distinct "lower" and "upper" layers, and Utahraptor fossils are only currently known within the upper Yellow Cat Member.^[27]

Utahraptor was unearthed from the Yellow Cat Member, which during the Berriasian to Late Valanginian was a semiarid area with floodplain prairies, riverine forests, and open woodlands predominated by conifers (Pinophyta), ferns (Polypodiopsida), hornworts (Anthocerotophyta) and other vascular plants.^[26] During the description of Mierasaurus, it was interpreted that there was also a waterlogged bog-like environment.^[28] There is believed to have been a short wet season. This is supported by the presence of charred spores and other carbonized plant debris in the pollen maceral that indicate the occurrence of ancient wildfires ignited during periods of low precipitation.^[11][26]

Paleofauna that were contemporaneous with the dromaeosaurid in the upper Yellow Cat Member included numerous dinosaurs, such as the medium-sized iguanodonts Hippodraco and Cedrorestes, the smaller theropods Martharaptor and Nedcolbertia, the nodosaurid Gastonia, and the sauropods Cedarosaurus and Moabosaurus.^[28][27][29] The only known mammal from the Upper Yellow Cat Member is Cifelliodon.^[30]

Other non-dinosaur or avian taxa known from the Member include the fish Ceratodus and Semionotus, the turtles Glyptops and Trinitichelys, Aquatilavipes (fossilized bird tracks), the rhynchocephalian Toxolophosaurus, and the indeterminate remains of hybodontid and polyacrodontid sharks.^[27]

Additional paleofauna was recovered, most of it being unnamed and/or indeterminate, including an isolated mesoeucrocodylian skull that measures 20 cm (7.9 in) in length.^[27] A neochoristodere unearthed from the Upper Yellow Cat Member, represented by a partial left femur,^[31] shows that aquatic paleofauna was present and diverse during the Early Cretaceous of the Cedar Mountain Formation.^[27] A large sail-backed iguanodont represented by large vertebrae and fragmentary remains,^[32] and an indeterminate eudromaeosaur known from a caudal vertebra and fragmented tail (UMNH VP 20209) were also present.^[33]

Cultural significance

Raptor Red was published in 1995, and features the fictionalized story of a female Utahraptor. Written by paleontologist Robert T. Bakker, it was positively regarded by mainstream reviewers, though updates to the science have rendered some of the story line facts presented untrue and the paleontology community was critical of fossil record inaccuracies.^[34][35] Bakker's anthropomorphosis of the titular Red was particularly praised.^[36][37][38]

In 2018, it was proposed by a 10-year-old elementary school student, Kenyon Roberts, that Utahraptor be the Utah state dinosaur, an act that was approved by the Senate.^[39] Initially Utahraptor would have replaced another dinosaur, Allosaurus, as the state's official fossil, but it was decided that Utahraptor would be another symbol of the state.^[40] In 2021, Steve Eliason^[41][42] successfully created a proposal for Utahraptor State Park where the block was discovered, proposed by the same Utah student, Kenyon Roberts. It was approved by the state House.^[43]