Acrocanthosaurus

Acrocanthosaurus (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 and Texas, 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, approaching 12 meters (40 ft) in length, and weighing up to about 2.40 metric tons (2.65 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. However, there is still debate over its evolutionary relationships, with some scientists classifying it as an allosaurid, and others as a carcharodontosaurid. Acrocanthosaurus was the largest theropod in its ecosystem and likely an apex predator which possibly preyed on large sauropods and ornithopods.

Description
Although slightly smaller than gigantic relatives like Giganotosaurus, Acrocanthosaurus was still among the largest theropods ever to exist. The longest known individual measured 11.5 meters (38 ft) from snout to tail tip and weighed an estimated 2400 kilograms (5300 lb). Its skull alone was nearly 1.3 meters (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 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. 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. Rather than supporting a skin 'sail' as seen in Spinosaurus, 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 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 some more recent studies. Other scientists classify it as 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.

If Acrocanthosaurus was an allosaurid, it would have been most closely related to Jurassic genera like Allosaurus and Saurophaganax. Those cladistic analyses which find it to be a carcharodontosaurid usually place it in a basal position relative to the African Carcharodontosaurus and Giganotosaurus from South America. Neovenator, discovered in England, is often considered an even more basal carcharodontosaurid. 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 ακρα/akra ('high'), ακανθα/akantha ('thorn' or 'spine') and σαυρος/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), 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 private collectors, 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.

Acrocanthosaurus may be known from less complete remains outside of Oklahoma and Texas. 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.

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 backwards 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 backwards) 109° from the vertical, so that the humerus could actually be angled slightly upwards. Protraction (swinging forwards) was limited to only 24° past the vertical. The arm was unable 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 backwards) 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.

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, 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 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.

Paleoecology
Definite Acrocanthosaurus fossils have been found in the Twin Mountains Formation of northern Texas and the Antlers Formation of southern Oklahoma. 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 of Montana, 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 Paluxysaurus or possibly even the enormous Sauroposeidon, as well as large ornithopods like Tenontosaurus. The smaller theropod Deinonychus also prowled the area but at 3 meters (10 ft) in length, most likely provided only minimal competition for Acrocanthosaurus.