Acrocanthosaurus Atokensis

''Acrocanthosaurus'' ( ; ) is a genus of carcharodontosaurid dinosaur that existed in what is now North America during the Aptian and early Albian stages of the Early Cretaceous, from 113 to 110 million years ago. 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, suggesting a continent wide range.

''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 weighing approximately . 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.

Discovery and naming

''Acrocanthosaurus'' is named after 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''), 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 Langston had proposed the name "Acracanthus atokaensis" for the genus and species in his unpublished 1947 master's thesis, but the name was changed to ''Acrocanthosaurus atokensis'' for formal publication.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. 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. 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.

''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.

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). The original describers of this specimen, Currie and Carpenter, considered its mass to be "lighter than ''Tyrannosaurus''", and Carpenter later proposed a body mass estimate of . With other researchers, Henderson initially estimated the body mass of this specimen at , but he later moderated it at . Bates and his colleagues in 2009 considered this an underestimate and produced a larger body mass estimate of within the range of , but their method is not tested for animals of large body mass, so some researchers consider it an exaggeration. Subsequent studies have yielded mass estimates between based on various techniques.

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'' 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 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 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''. It was still regarded as an Allosaurid by some researchers researches, and is still occasionally placed within that family.

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.
The following cladogram after Novas ''et al.'', 2013, shows the placement of ''Acrocanthosaurus'' within Carcharodontosauridae. In 2011, Oliver Rauhut named a new genus of theropod dinosaur from the Tendaguru Formation in Tanzania named '' Veterupristisaurus'' and found it to be a sister taxon to ''Acrocanthosaurus'', further supporting its position as a carcharodontosaurid.

Paleobiology

Growth and longevity

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, ''Acrocanthosaurus'' needed 18–24 years to be mature.

Bite force

The bite force of ''Acrocanthosaurus'' was studied and compared with that of 33 other dinosaurs by Sakamoto ''et al.'' (2022). According to the results, its bite force at the anterior part of the jaws was 8,266 newtons, while the posterior bite force was estimated to be 16,894 newtons.

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 shoulder of ''Acrocanthosaurus'' was limited in its range of motion compared to that of humans. The arm could not swing in a complete circle, but could retract (swing backward) 109 ° from the vertical, so that the humerus could actually be angled slightly upwards. Protraction (swinging forward) was limited to only 24° past the vertical. The arm was unable to reach a vertical position when adducting (swinging downwards) but could abduct (swing upwards) to 9° above horizontal. Movement at the elbow was also limited compared to humans, with a total range of motion of only 57°. The arm could not completely extend (straighten), nor could it flex (bend) very far, with the humerus unable even to form a right angle with the forearm. The radius and ulna (forearm bones) locked together so that there was no possibility of pronation or supination (twisting) as in human forearms.

None of the carpals (wrist bones) fit together precisely, suggesting the presence of a large amount of cartilage in the wrist, which would have stiffened it. All of the digits were able to hyperextend (bend backward) until they nearly touched the wrist. When flexed, the middle digit would converge towards the first digit, while the third digit would twist inwards. The first digit of the hand bore the largest claw, which was permanently flexed so that it curved back towards the underside of the hand. Likewise, the middle claw may have been permanently flexed, while the third claw, also the smallest, was able to both flex and extend. After determining the ranges of motion in the joints of the forelimb, the study went on to hypothesize about the predatory habits of ''Acrocanthosaurus''. The forelimbs could not swing forward very far, unable even to scratch the animal's own neck. Therefore, they were not likely to have been used in the initial capture of prey and ''Acrocanthosaurus'' probably led with its mouth when hunting. On the other hand, the forelimbs were able to retract towards the body very strongly. Once prey had been seized in the jaws, the heavily muscled forelimbs may have retracted, holding the prey tightly against the body and preventing escape. As the prey animal attempted to pull away, it would only have been further impaled on the permanently flexed claws of the first two digits. The extreme hyperextensibility of the digits may have been an adaptation allowing ''Acrocanthosaurus'' to hold struggling prey without fear of dislocation. Once the prey was trapped against the body, ''Acrocanthosaurus'' may have dispatched it with its jaws. Another possibility is that ''Acrocanthosaurus'' held its prey in its jaws, while repeatedly retracting its forelimbs, tearing large gashes with its claws. Other less probable theories have suggested the forelimb range of motion being able to grasp onto the side of a sauropod and clinging on to topple the sauropods of smaller stature, though this is unlikely due to ''Acrocanthosaurus'' having a rather robust leg structure compared to other similarly structured theropods.

Brain and inner ear structure

In 2005, scientists reconstructed an endocast (replica) of an ''Acrocanthosaurus'' cranial cavity using computed tomography (CT scanning) to analyze the spaces within the holotype braincase (OMNH 10146). In life, much of this space would have been filled with the meninges and cerebrospinal fluid, in addition to the brain itself. However, the general features of the brain and cranial nerves could be determined from the endocast and compared to other theropods for which endocasts have been created. While the brain is similar to many theropods, it is most similar to that of allosauroids. It most resembles the brains of ''Carcharodontosaurus'' and ''Giganotosaurus'' rather than those of ''Allosaurus'' or ''Sinraptor'', providing support for the hypothesis that ''Acrocanthosaurus'' was a carcharodontosaurid.

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.

Possible footprints

The Glen Rose Formation of central Texas preserves many dinosaur footprints, including large, three-toed theropod prints. 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.

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. While interesting and plausible, this hypothesis is difficult to prove and other explanations exist. For example, several solitary theropods may have moved through in the same direction at different times after the sauropods had passed, creating the appearance of a pack stalking its prey. The same can be said for the purported "herd" of sauropods, who also may or may not have been moving as a group. 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.

Pathology

The skull of the ''Acrocanthosaurus atokensis'' holotype shows light exostotic material on the squamosal. The neural spine of the eleventh vertebra was fractured and healed while the neural spine of its third tail vertebra had an unusual hook-like structure.Molnar, R. E., 2001, Theropod paleopathology: a literature survey: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 337–363.

Paleoecology

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 possibly even 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 100 million years ago.

During this time, the area preserved in the Twin Mountains and Antlers formations was a large floodplain that drained into a shallow inland sea. A few million years later, this sea would expand to the north, becoming the Western Interior Seaway and dividing North America in two for nearly the entire Late Cretaceous. The Glen Rose Formation represents a coastal environment, with possible ''Acrocanthosaurus'' tracks preserved in mudflats along the ancient shoreline. As ''Acrocanthosaurus'' was a large predator, it is expected that it had an extensive home range and lived in many different environments in the area. Potential prey animals include sauropods like ''Astrodon'' ublished online/ref> 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, or even food, for ''Acrocanthosaurus''.

References

{{Taxonbar, from=Q131021

Carcharodontosaurids
Albian life
Aptian life
Early Cretaceous dinosaurs of North America
Fossil trackways
Cloverly fauna
Paleontology in Maryland
Paleontology in Oklahoma
Paleontology in Wyoming
Paleontology in Texas
Fossil taxa described in 1950
Taxa named by J. Willis Stovall
Taxa named by Wann Langston Jr.
Articles containing video clips
Apex predators