page_title: Acrocanthosaurus
section_title: Forelimb function
hierarchical_section_title: Acrocanthosaurus / Paleobiology / Forelimb function
caption_attribution_description: English: Mounted skeleton of Acrocanthosaurus specimen NCSM 14345 at North Carolina Museum of Natural Sciences
context_page_description: Acrocanthosaurus 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, 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, reaching 11.5 meters in length, and weighing up to 6.2 metric 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.
context_section_description: 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.
caption: Mounted skeleton seen from above
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