Acrocanthosaurus



Although slightly smaller than colossal relatives like Giganotosaurus, Acrocanthosaurus was still among the largest theropods ever to exist. The longest known individual measured 38 ft from snout to tail tip and weighed an estimated 13,000–15,000 lb. Its skull alone was nearly 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 was quite large, more than a quarter of the length of the skull and two-thirds of its height. The outside surface of the 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. 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 abeisaurids. 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 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 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, 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 neck and back vertebrae had prominent depressions (pleurocoels) on the sides, while the tail vertebrae bore smaller ones. This is more similar to carcharodontosaurids than to Allosaurus.