Acrocanthosaurus

*Acrocanthosaurus atokensis*

* Timothy Bradley Artwork
* Robert F Walters Artwork

***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.

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 Langston had proposed the name "Acracanthus atokaensis" for the genus and species in his unpublished 1947 master's thesis, but the name was changed to *Acrocanthosaurus atokensis* for formal publication.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

In 1950, a team of scientists, led by paleontologists J. Willis Stovall and Wann Langston, Jr, was excavating in the deserts of Oklahoma when they came across very large fossilized bones and tall vertebrae. They were taken to a nearby museum and were found to belong to a new species of very large theropod dinosaur. It wasn't long afterward that it was officially named *Acrocanthosaurus atokensis*, or "Atoka's high-spined lizard", after the tall spines along its back and the area in Oklahoma where it was first found. However, it was first discovered in 1940s but hadn't gained any attention until its rediscovery a decade later. Since then, several more *Acrocanthosaurus* specimens have been found, and even a very well preserved fossil trackway of it chasing down a *Sauroposeidon*.

It's been found in several states along the western half of America, including Oklahoma, Texas, Wyoming, Arizona, and even as far east as Maryland some specimens have been found, suggesting *Acrocanthosaurus* was a very successful predator. And in 2012, the pubic bones, vertebrae, and fibula of a juvenile Acrocanthosaurus were found in the Cloverly Formation in Wyoming, suggesting this theropod was the only large predator in this formation during the time of the dinosaurs.

## Description

*Acrocanthosaurus* was a large carnivorous theropod that had a large roughly triangular head, numerous sharp serrated teeth, strong forelimbs, and powerful hind legs. It lived alongside the small raptor *Deinonychus*, some small ankylosaurs, and hunted large sauropods. *Acrocanthosaurus* had one feature that distinguished it from other theropods; the tall neural spine which ran along its neck, back, and tail.

Some of these spines were over a foot tall. It may represent an evolutionary link between the Late Jurassic *Allosaurus* and the gigantic Early Cretaceous *Carcharodontosaurus*. It was one of the largest bipedal carnivorous dinosaurs, with one mounted specimen reaching a length of possibly 11.5 meters (38 feet) and an estimated weight of 5 to 6 tonnes. Many of its vertebrae had high neural spines, though nowhere as high as those of *Spinosaurus*.

Scientists have debated why *Acrocanthosaurus* had a sail or a ridge along its back. Some think it released heat when the animal was too hot. It also may have been used for display to make the animal look bigger when it faced rivals for territories or mates. This is much like a house cat fluffing up its fur and arching its back to make itself look bigger. There are other large tall-spined dinosaurs from Europe and Africa, but scientists don't know why Acrocanthosaurus had it. An interesting take on the usage of Acrocanthosaurus' sail was as a technique of hunting prey. It is possible that the tall vertebral spines were attachment points for strong muscles, providing a great form of anchorage for the neck while the animal attacked large prey.

Acrocanthosaurus was an early carcharodontisaurid with an exceptionally large sail. Acrocathosaurus is estimated to be 5-7 metric tons (most reliable estimate being 5.8 metric tons), 11-12 meters long, and 3 meters tall at the hip.

Scientists have found footprints probably made by *Acrocanthosaurus* in several places in Texas. In one place, it looks like an *Acrocanthosaurus* stalked a large sauropod across a mud flat, possibly *Sauroposeidon*. When the sauropod footprints changed direction, so did those of *Acrocanthosaurus*. The outcome of this discovery shows that the *Acrocanthosaurus'* jaws were hanging onto one of the thighs of the sauropod while the it was running.

A study in 2021 estimated a bite force of 17651 Newtons (~1.765 tonnes) for Acrocanthosaurus.

## Classification

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.

## Specimens

* OMNH 10146- The holotype, known from a partial skull, cervical, dorsal, and caudal vertebrae, 9 dorsal spines, two chevrons, partial ischium and pubis, tibias and fibulas, 5 ribs, coracoid, and one gastralia. 10 m long.
* OMNH 10147- The paratype, found in 1940 with the holotype, consists of 4 dorsal spines, 5 caudal spines, 2 dorsal vertebrae, 13 caudal vertebrae, 2 chevrons, ribs, pubis, and partial leg bones. 9-10 m long.
* NCSM 14345- A large adult skeleton found in 1982 has a complete skull, left arm, right leg, ilium, ischium, 10 chevrons, 18 caudal vertebrae, 9 spines, 10 ribs, 2 neck ribs, and gastralia. Was 11 m long.
* SMU 74646- partial jaw and squamosal bones, near complete neck, near complete ribcage, scapula, pubis ischium, 2 femurs, and caudal elements. about 9 m long.
* UM 20796- youngest individual found, known from only 1 caudal and dorsal vertebrae, right femur and partial right tibia, and partial pubis. about 4 m long.

## Paleobiology

### 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*.

## JPInstitute.com Description

Called Acro for short, this dinosaur is a therapod (a two-legged meat-eater) which had 68 long, knife-like teeth in its 5 foot long head. At 40 feet long, this dinosaur ruled the land about 35 million years before there were any T. rexes. Unlike T. rex, this dinosaur had larger, more powerful arms that could grab, hold and crush other dinosaurs. It could probably lift a small car off the ground!

There is one very interesting feature about the Acro, and that is the high spines along its back and neck. In fact the name Acrocanthosaurus means 'high spined lizard' in Greek (the last part of its name, atokensis, refers to Atoka County in Oklahoma where the only full skeleton was discovered). The spines are not nearly as big as those on some dinosaurs that have 'sails' on their back, like Spinosaurus. Instead, Acro's spines, the longest of which is about 2 feet, were anchors to which huge muscles were attached. This made Acro very strong and powerful. And Acros probably needed to be very strong because there is evidence that it hunted huge sauropod dinosaurs. In Texas, there are dinosaur tracks that show a large Acro tracking and hunting a large brachiosaur called Pleurocoelus. Some modern animals have a similar type of spine along their back to give them strength: horses, elephants and buffaloes all have this type of muscle suspension.

Acro had relatively small feet for a large dinosaur. This was because it lived in a drier climate and did not need a large surface area to keep it from sinking in mud. Although similar in size and weight to T. rex, its feet were only about half the size. T. rex lived in a much wetter environment and probably spent a lot of time walking through marshy ground. The teeth of an Acro were typical of the large meat-eaters of the late Jurassic and Early Cretaceous. They were long (as much as 9 or 10 inches if you include the root) and fairly thin, curving slightly inward with serrations along the front and back edges. This means that Acro did not necessarily use its teeth as a killing tool. Teeth like this were most likely used to rip big pieces of flesh from prey which it was holding with its powerful arms and feet. Unlike T. rex, Acro teeth could not have been used to crunch bone as they would break too easily. When its teeth did break, Acro had new teeth waiting. Like many dinosaurs, it was constantly shedding old teeth and replacing them with new.

Acrocanthosaurus is in the same family of dinosaurs as the Allosaurus, which was a very successful family of dinosaurs. It was most likely the dominant North American predator of the Early Cretaceous. It is known primarily from one specimen, nicknamed 'Fran' which was discovered in Atoka County, Oklahoma by Cephis Hall and Sid Love in 1983. The specimen showed a number of interesting pathologies, including a hole in the right scapula (shoulder blade) that had become infected. The ribs beneath and in front of this scapula appear to have been broken and healed, indicating that the animal had been injured. Scattered remains of Acrocanthosaurus have been found in Oklahoma, Texas, Utah and Arizona.

## Appearance in other media

### *Jurassic Park*

* *Acrocanthosaurus* appears in the Vivendi Universal video game *Jurassic Park: Operation Genesis* for PC and console platforms. It can live with *Carcharodontosaurus* without a fight in the game.
* It appears in Jurassic Park Builder as a silver-tier dinosaur.
* It appears in Jurassic World: The Game as a VIP creature.
* *Acrocanthosaurus* was added to *Jurassic World: Evolution* on April 17 in the Carnivore Dinosaur Pack. It inaccurately has a very thick head, body and teeth, making it look more like a tyrannosaurid. It also nearly lacks the spine down its back that it was named after, though it is there in the model.

## Links

http://web.archive.org/web/20031006000916fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_acroc.html
https://web.archive.org/web/20080516230746/http://kids.yahoo.com/dinosaurs/2--Acrocanthosaurus

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