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. 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. but the name was changed to Acrocanthosaurus atokensis for formal publication.
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. 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. 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.
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, 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.
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, 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.
Cau (2024) recovered similar results for Acrocanthosaurus. His results are shown below.
In the description of the carcharodontosaurid Tameryraptor, it is noted the specimen NCSM 14345 possesses several differences from the type specimen of Acrocanthosaurus atokensis, seen listed below:
- The antorbital fossa is separated dorsally and ventrally by an anterior expansion of the lateral lamina.
- A narrow orbital margin with an antorbital fossa that is restricted to the antorbital part of the jugal.
- A wide, dorsally bifurcated supraoccipital crest.
- A wide angle between the occiput in the braincase and the posterior skull roof.
- The absence of a ridge extending from the dorsal rim of the paroccipital process medially below the posterior exit of the mid-cerebral vein.
- A completely split trigeminal foramen.
- Wider pneumatic pockets next to the occipital condyle.
- Ventral indentation of the articular surface of the occipital condyle.
- The distal shaft of the tibia is straight.
While beyond the scope of their paper, the authors note that these differences may be grounds for a reexamination of North American carcharodontosaurid material, and may suggest a much richer diversity than once thought.
Paleobiology
Growth and longevity
thumb|Restoration of a pair of Acrocanthosaurus engaging in [[courtship behavior|alt=]]
From the bone features of the holotype OMNH 10146 and NCSM 14345, it is estimated that Acrocanthosaurus required at least 12 years to fully grow. This number may have been much higher because in the process of bones remodeling and the growth of the medullary cavity, some Harris lines were lost. If accounting for these lines, then Acrocanthosaurus needed 18–24 years to be mature.
Forelimb function
Like those of most other non-avian theropods, Acrocanthosaurus forelimbs did not make contact with the ground and were not used for locomotion; instead, they served a predatory function. The discovery of a complete forelimb (NCSM 14345) allowed the first analysis of the function and range of motion of the forelimb in Acrocanthosaurus. The study examined the bone surfaces which would have articulated with other bones to determine how far the joints could move without dislocating. In many of the joints, the bones did not fit together exactly, indicating the presence of a considerable amount of cartilage in the joints, as is seen in many living archosaurs. Among other findings, the study suggested that, in a resting position, the forelimbs would have hung from the shoulders with the humerus angled backward slightly, the elbow bent, and the claws facing medially (inwards).
The brain was slightly sigmoidal (S-shaped), without much expansion of the cerebral hemispheres, more like a crocodile than a bird. This is in keeping with the overall conservatism of non-coelurosaurian theropod brains. Acrocanthosaurus had large and bulbous olfactory bulbs, indicating a good sense of smell. Reconstructing the semicircular canals of the ear, which control balance, shows that the head was held at a 25° angle below horizontal. This was determined by orienting the endocast so that the lateral semicircular canal was parallel to the ground, as it usually is when an animal is in an alert posture. The most famous of these trackways was discovered along the Paluxy River in Dinosaur Valley State Park, a section of which is now on exhibit in the American Museum of Natural History in New York City, although several other sites around the state have been described in the literature. It is impossible to say what animal made the prints, since no fossil bones have been associated with the trackways. However, scientists have long considered it likely that the footprints belong to Acrocanthosaurus. A 2001 study compared the Glen Rose footprints to the feet of various large theropods but could not confidently assign them to any particular genus. However, the study noted that the tracks were within the ranges of size and shape expected for Acrocanthosaurus. Because the Glen Rose Formation is close to the Antlers and Twin Mountains Formations in both geographical location and geological age, and the only large theropod known from those formations is Acrocanthosaurus, the study concluded that Acrocanthosaurus was most likely to have made the tracks.
thumb|Digital fly-through over the Glen Rose trackway, reconstructed from photographs
The famous Glen Rose trackway on display in New York City includes theropod footprints belonging to several individuals which moved in the same direction as up to twelve sauropod dinosaurs. The theropod prints are sometimes found on top of the sauropod footprints, indicating that they were formed later. This has been put forth as evidence that a small pack of Acrocanthosaurus was stalking a herd of sauropods. At a point where it crosses the path of one of the sauropods, one of the theropod trackways is missing a footprint, which has been cited as evidence of an attack. However, other scientists doubt the validity of this interpretation because the sauropod did not change gait, as would be expected if a large predator were hanging onto its side.
Paleoecology
thumb|left|Acrocanthosaurus carrying a [[Tenontosaurus carcass away from a pair of Deinonychus]]
Definite Acrocanthosaurus fossils have been found in the Twin Mountains Formation of northern Texas, the Antlers Formation of southern Oklahoma, and the Cloverly Formation of north-central Wyoming and the Arundel Formation in Maryland. These geological formations have not been dated radiometrically, but scientists have used biostratigraphy to estimate their age. Based on changes in ammonite taxa, the boundary between the Aptian and Albian stages of the Early Cretaceous has been located within the Glen Rose Formation of Texas, which may contain Acrocanthosaurus footprints and lies just above the Twin Mountains Formation. This indicates that the Twin Mountains Formation lies entirely within the Aptian stage, which lasted from 125 to 112 million years ago. The Antlers Formation contains fossils of Deinonychus and Tenontosaurus, two dinosaur genera also found in the Cloverly Formation, which has been radiometrically dated to the Aptian and Albian stages, suggesting a similar age for the Antlers. Therefore, Acrocanthosaurus most likely existed between 125 and 99.6 million years ago. or possibly even the enormous Sauroposeidon, as well as large ornithopods like Tenontosaurus. The smaller theropod Deinonychus also prowled the area but at in length, most likely provided only minimal competition for adult Acrocanthosaurus, and may have been preyed upon.