Acrocanthosaurus

***Acrocanthosaurus*** (/ˌækroʊˌkænθəˈsɔːrəs/ *ak-ro-**kan**-thə-**sawr**-əs*; meaning "high-spined lizard") is a genus of theropod dinosaur that existed in what is now North America during the Aptian and early Albian stages of the Early Cretaceous. Like most dinosaur genera, *Acrocanthosaurus*contains only a single species, *A. atokensis*. Its fossil remains are found mainly in the U.S. states of Oklahoma, Texas, and Wyoming, although teeth attributed to *Acrocanthosaurus* have been found as far east as Maryland.

*Acrocanthosaurus* was a bipedal predator. As the name suggests, it is best known for the high neural spines on many of its vertebrae, which most likely supported a ridge of muscle over the animal's neck, back and hips. *Acrocanthosaurus* was one of the largest theropods, reaching 11.5 m (38 ft) in length, and weighing up to 6.2 tonnes (6.8 short tons). Large theropod footprints discovered in Texas may have been made by *Acrocanthosaurus*, although there is no direct association with skeletal remains.

Recent discoveries have elucidated many details of its anatomy, allowing for specialized studies focusing on its brain structure and forelimb function. *Acrocanthosaurus* was the largest theropod in its ecosystem and likely an apex predator which preyed on sauropods, ornithopods, and ankylosaurs.

## Description

*Acrocanthosaurus* was among the largest theropods known to exist. The largest known specimen (NCSM 14345) is estimated to have measured 11.5 m (38 ft) from snout to tail tip and weighed 5.7 t (6.3 short tons)to 6.2 t (6.8 short tons), with an upper maximum weight of 7.25 t (7.99 short tons) within the realm of possibility for this specimen. Its skullalone was nearly 1.3 m (4.3 ft) in length.

The skull of *Acrocanthosaurus*, like most other allosauroids, was long, low and narrow. The weight-reducing opening in front of the eye socket (antorbital fenestra) was quite large, more than a quarter of the length of the skull and two-thirds of its height. The outside surface of the maxilla (upper jaw bone) and the upper surface of the nasal bone on the roof of the snout were not nearly as rough-textured as those of *Giganotosaurus* or *Carcharodontosaurus*. Long, low ridges arose from the nasal bones, running along each side of the snout from the nostril back to the eye, where they continued onto the lacrimal bones. This is a characteristic feature of all allosauroids. Unlike *Allosaurus*, there was no prominent crest on the lacrimal bone in front of the eye. The lacrimal and postorbitalbones met to form a thick brow over the eye, as seen in carcharodontosaurids and the unrelated abelisaurids. Nineteen curved, serrated teeth lined each side of the upper jaw, but a tooth count for the lower jaw has not been published. *Acrocanthosaurus* teeth were wider than those of *Carcharodontosaurus* and did not have the wrinkled texture that characterized the carcharodontosaurids. The dentary (tooth-bearing lower jaw bone) was squared off at the front edge, as in *Giganotosaurus*, and shallow, while the rest of the jaw behind it became very deep. *Acrocanthosaurus* and *Giganotosaurus* shared a thick horizontal ridge on the outside surface of the surangular bone of the lower jaw, underneath the articulation with the skull.

The most notable feature of *Acrocanthosaurus* was its row of tall neural spines, located on the vertebrae of the neck, back, hips and upper tail, which could be more than 2.5 times the height of the vertebrae from which they extended. Other dinosaurs also had high spines on the back, sometimes much higher than those of *Acrocanthosaurus*. For instance, the unrelated *Spinosaurus* had spines nearly 2 meters (6.5 ft) tall, about 11 times taller than the bodies of its vertebrae. The lower spines of*Acrocanthosaurus* had attachments for powerful muscles like those of modern bison, probably forming a tall, thick ridge down its back. The function of the spines remains unknown, although they may have been involved in communication, fat storage, or temperature control. All of its cervical(neck) and dorsal (back) vertebrae had prominent depressions (pleurocoels) on the sides, while the caudal (tail) vertebrae bore smaller ones. This is more similar to carcharodontosaurids than to *Allosaurus*.

Aside from its vertebrae, *Acrocanthosaurus* had a typical allosauroid skeleton. *Acrocanthosaurus* was bipedal, with a long, heavy tail counterbalancing the head and body, maintaining its center of gravity over its hips. Its forelimbs were relatively shorter and more robust than those of *Allosaurus* but were otherwise similar: each hand bore three clawed digits. Unlike many smaller fast-running dinosaurs, its femur was longer than its tibiaand metatarsals, suggesting that *Acrocanthosaurus* was not a fast runner. Unsurprisingly, the hind leg bones of *Acrocanthosaurus* were proportionally more robust than its smaller relative *Allosaurus*. Its feet had four digits each, although as is typical for theropods, the first was much smaller than the rest and did not make contact with the ground.

## Classification and systematics

*Acrocanthosaurus* is classified in the superfamily Allosauroidea within the infraorder Tetanurae. This superfamily is characterized by paired ridges on the nasal and lacrimal bones on top of the snout and tall neural spines on the neck vertebrae, among other features. It was originally placed in the family Allosauridae with *Allosaurus*, an arrangement also supported by studies as late as 2000. Most studies have found it to be a member of the related family Carcharodontosauridae.

At the time of its discovery, *Acrocanthosaurus* and most other large theropods were known from only fragmentary remains, leading to highly variable classifications for this genus. J. Willis Stovall and Wann Langston Jr. first assigned it to the "Antrodemidae", the equivalent of Allosauridae, but it was transferred to the taxonomic wastebasket Megalosauridae by Alfred Sherwood Romer in 1956. To other authors, the long spines on its vertebrae suggested a relationship with *Spinosaurus*.This interpretation of *Acrocanthosaurus* as a spinosaurid persisted into the 1980s, and was repeated in the semi-technical dinosaur books of the time.

Tall spined vertebrae from the Early Cretaceous of England were once considered to be very similar to those of*Acrocanthosaurus*, and in 1988 Gregory S. Paul named them as a second species of the genus, *A. altispinax*. These bones were originally assigned to *Altispinax*, an English theropod otherwise known only from teeth, and this assignment led to at least one author proposing that *Altispinax* itself was a synonym of *Acrocanthosaurus*. These vertebrae were later assigned to the new genus *Becklespinax*, separate from both*Acrocanthosaurus* and *Altispinax*.

Most cladistic analyses including *Acrocanthosaurus* have found it to be a carcharodontosaurid, usually in a basal position relative to the African *Carcharodontosaurus* and *Giganotosaurus* from South America. It has often been considered the sister taxon to the equally basal *Eocarcharia*, also from Africa. *Neovenator*, discovered in England, is often considered an even more basal carcharodontosaurid, or as a basal member of a sister group called Neovenatoridae. This suggests that the family originated in Europe and then dispersed into the southern continents (at the time united as the supercontinent Gondwana). If *Acrocanthosaurus* was a carcharodontosaurid, then dispersal would also have occurred into North America. All known carcharodontosaurids lived during the early-to-middle Cretaceous Period.

## Discovery and naming

*Acrocanthosaurus* is named for its tall neural spines, from the Greek *ɑκρɑ*/*akra* ('high'), *ɑκɑνθɑ*/*akantha*('thorn' or 'spine') and *σɑʊρος*/*sauros* ('lizard'). There is one named species (*A. atokensis*), which is named after Atoka County in Oklahoma, where the original specimens were found. The name was coined in 1950 by American paleontologists J. Willis Stovall and Wann Langston Jr. Langston had proposed the name "Acracanthus atokaensis" for the genus and species in his unpublished 1947 master's thesis, but the name was changed to *Acrocanthosaurus atokensis* for formal publication.

The holotype and paratype (OMNH 10146 and OMNH 10147), discovered in the early 1940s and described at the same time in 1950, consist of two partial skeletons and a piece of skull material from the Antlers Formation in Oklahoma. Two much more complete specimens were described in the 1990s. The first (SMU 74646) is a partial skeleton, missing most of the skull, recovered from the Twin Mountains Formation of Texas and currently part of the Fort Worth Museum of Science and History collection. An even more complete skeleton (NCSM 14345, nicknamed "Fran") was recovered from the Antlers Formation of Oklahoma by Cephis Hall and Sid Love, prepared by the Black Hills Institute in South Dakota, and is now housed at the North Carolina Museum of Natural Sciences in Raleigh. This specimen is the largest and includes the only known complete skull and forelimb. Skeletal elements of OMNH 10147 are almost the same size as comparable bones in NCSM 14345, indicating an animal of roughly the same size, while the holotype and SMU 74646 are significantly smaller.

The presence of *Acrocanthosaurus* in the Cloverly Formation was established in 2012 with the description of another partial skeleton, UM 20796. This specimen, consisting of parts of two vertebrae, partial pubic bones, a femur, a partial fibula, and fragments, represents a juvenile animal. It came from a bonebed in the Bighorn Basin of north-central Wyoming, and was found near the shoulder blade of a *Sauroposeidon*. An assortment of other fragmentary theropod remains from the formation may also belong to *Acrocanthosaurus*, which may be the only large theropod in the Cloverly Formation.

*Acrocanthosaurus* may be known from less complete remains outside of Oklahoma, Texas, and Wyoming. A tooth from southern Arizona has been referred to the genus, and matching tooth marks have been found in sauropod bones from the same area. Several teeth from the Arundel Formation of Maryland have been described as almost identical to those of *Acrocanthosaurus* and may represent an eastern representative of the genus. Many other teeth and bones from various geologic formations throughout the western United States have also been referred to *Acrocanthosaurus*, but most of these have been misidentified; there is, however, some disagreement with this assessment regarding fossils from the Cloverly Formation.