Acrocanthosaurus ( ; ) is a genus of carcharodontosaurid dinosaurs that existed in what is now North America during the Barremian and early Cenomanian stages of the Early Cretaceous and Late Cretaceous, from 125 to 99.6 million years ago. Like most dinosaur genera, Acrocanthosaurus contains only a single species, A. atokensis. It had a continent-wide range, with fossil remains known from the U.S. states of Arizona, Oklahoma, Texas, Utah, and Wyoming in the west and Maryland in the east. However, most of these remains are assigned to the species based on the assumption that Acrocanthosaurus is the only large carcharodontosaurid from North America during this time, and the possibility exists that some referred specimens could represent distinct taxa.
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, with the largest known specimen reaching in length and in weight. Large theropod footprints discovered in Texas may have been made by Acrocanthosaurus, although no direct association with skeletal remains has been found. 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 that preyed on sauropods, ornithopods, and ankylosaurs.
Discovery and naming
Acrocanthosaurus is named after its tall neural spines, from the Greek ἄκρος/ákros (high), ἄκανθος/ákanthos (thorn or spine), and σαῦρος/saũros (lizard). The lone named species (A. atokensis), is 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 Langston had proposed the name "Acracanthus atokaensis" for the genus and species in his unpublished 1947 master's thesis, but the name was changed to Acrocanthosaurus atokensis for formal publication.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. The specimen is the largest and includes the only known complete skull and forelimb. The skeletal elements of OMNH 10147 are almost the same size as comparable bones in NCSM 14345, indicating an animal 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). The 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.
Unlike many other dinosaur genera, much less large theropods, Acrocanthosaurus inhabited both the western and eastern regions of the North American continent. The presence of the genus in the Arundel Formation of Maryland (roughly concurrent with the western formations) had long been suspected, with teeth almost identical to Acrocanthosaurus previously known from the formation. In 2024, an incomplete theropod skeleton (USNM 466054) from the Arundel Formation was identified as that of a subadult Acrocanthosaurus, referred to as A. cf. atokensis, marking the first definitive record of the genus from eastern North America. This skeleton, the most completely known theropod specimen from the formation despite its fragmentary nature, had been previously identified as an ornithomimosaur until this study and also represents the smallest known individual of the genus.
Acrocanthosaurus may be known from incomplete remains outside Oklahoma, Texas, Wyoming, and Maryland. 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. Many other teeth and bones from various geologic formations throughout the western United States have also been referred to as Acrocanthosaurus, but most of these have been misidentified; there is, however, some disagreement with this assessment regarding fossils from the Cloverly Formation.
Description
Acrocanthosaurus was among the largest theropods known to exist, with an estimated skull length of and body length of , based on the largest known specimen (NCSM 14345). Researchers have yielded body mass estimates for this specimen between based on various techniques.
A 2017 biomechanical study of the running ability of Tyrannosaurus by the biologist William I. Sellers and colleagues suggested that skeletal loads. Using a calculated weight estimate of 7 tons, the model showed that speeds above would have probably shattered the leg bones of Tyrannosaurus. The finding may mean that running was also not possible for other giant theropod dinosaurs like Giganotosaurus, Mapusaurus and Acrocanthosaurus.
Skull
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 nor 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, no prominent crest was on the lacrimal bone in front of the eye. The lacrimal and postorbital bones 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.
Postcranial skeleton
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 African genus Spinosaurus had spines nearly 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, muscle, 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 tibia and 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 the Allosauridae, but it was transferred to the Megalosauridae, a wastebasket taxon, 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 Carcharodontosaurus of Africa 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. The following cladogram after Novas et al., 2013, shows the placement of Acrocanthosaurus within Carcharodontosauridae.
Cau (2024) recovered similar results for Acrocanthosaurus. His results are shown below.