Acrocanthosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Carnosauria > Allosauroidea > Carcharodontosauria > Carcharodontosauridae

Overview: In the 1940’s, two partial skeletons belonging to a large theropod were unearthed in the state of Oklahoma. Recovered fossils included limb bones, parts of the skull, bits of pelvis, ribs and a series of vertebrae. The latter bones stood out due to their rather tall and fairly wide neural spines, which would later inspire the creature’s generic name – Acrocanthosaurus, the “high-spined lizard”. The holotype material came from Oklahoma’s Antlers Formation, so the fossils have been dated to the Early Cretaceous. Other similarly-aged geological formations in the United States, such as the Twin Mountains Formation and Cloverly Formation, have also yielded the fossilized remains of Acrocanthosaurus. It apparently had a wide range over North America during its heyday. Isolated teeth from as far east as Maryland could possibly belong to Acrocanthosaurus, originating from the Arundel Formation, though we can’t be certain if this is the case. In some older studies, Acrocanthosaurus was associated with the megalosaurids or even the spinosaurids, but in most modern studies, it’s classified as a basal member of the Carcharodontosauridae, making it an allosauroid theropod close to Carcharodontosaurus and Giganotosaurus.

Acrocanthosaurus was a massive theropod, with some specimens measuring up to eleven or more meters long and weighing multiple tons. Indeed, it’s often cited as the largest theropod known from North America prior to the appearance of Tyrannosaurus. Acrocanthosaurus was likely the top predator in North America at that time, going after juvenile sauropods, armored ankylosaurs and iguanodonts. It also coexisted with many smaller theropods, notably the dromaeosaurid genus Deinonychus. Its neural spines probably supported a ridge of muscle or fat along its back, possibly to store energy or to make it appear even larger than it already was. Its jaws were lined with blade-like teeth, while its arms were quite powerful despite their shorter length. Three clawed digits were sported by each hand. Allosauroid mega-theropods like Acrocanthosaurus were the top predators in most parts of the world during the Early Cretaceous and into the first half of the Late Cretaceous. Eventually, as dinosaurs like Acrocanthosaurus died out, the tyrannosaurs would begin to grow larger and fill in these now vacant niches (at least in the northern hemisphere). So far, Acrocanthosaurus is the only confirmed carcharodontosaurid known from North America.