Acrocanthosaurus

Chapter • Meet the dinosaur

Overview

Acrocanthosaurus was one of the largest predators ever to walk in North America. At roughly 11–11.5 metres long and an estimated 4.4–6.2 tonnes, it rivalled Tyrannosaurus in bulk but was built along different lines: a narrower skull, longer arms, and a back profile that makes it instantly recognisable. The neural spines of its neck, back and tail vertebrae were dramatically elongated, in places about two and a half times the height of the vertebral body itself. Scars for tendon attachment show that these spines almost certainly supported a thick, muscular ridge rather than a thin skin sail. The skull was about 1.2–1.3 metres long and lined with laterally compressed, serrated teeth that worked like flesh-slicing blades. No skin or feather impressions are known for Acrocanthosaurus or its close carcharodontosaurid relatives, so large predators of this kind are normally reconstructed with scaly hide. It walked on two legs with the body held horizontally and the stiffened tail extended behind as a counterweight — no dragging tail, no upright kangaroo pose. CT scanning of the braincase of the specimen nicknamed "Fran" produced one of the best endocasts known for any large theropod: the brain was more crocodile-like than bird-like in shape, the olfactory bulbs were large, and the habitual head posture was tilted slightly downwards. The arms were short but extremely powerful, and the shoulder and elbow joints allowed them to be swept inwards toward the chest rather than raised high, a motion often interpreted as a grip for holding struggling prey while the jaws did the killing. Bone histology reveals growth rings suggesting that the animal reached close to adult size at around 18–19 years of age. As the apex predator of its ecosystem, Acrocanthosaurus had plenty to hunt. The same rock units preserve the giant sauropod Sauroposeidon, the armoured Sauropelta and the abundant plant-eater Tenontosaurus. Jaw mechanics and tooth shape point to a bite-and-tear strategy, slicing away large mouthfuls of flesh rather than crushing bone as tyrannosaurs did. Scavenging carcasses was almost certainly part of the diet as well. The first bones were dug up in Atoka County, Oklahoma, in the late 1940s, and in 1950 J. Willis Stovall and Wann Langston Jr. named Acrocanthosaurus atokensis — roughly "high-spined lizard from Atoka". The material stayed fragmentary for decades, until a nearly complete skeleton was discovered in 1983 in the Antlers Formation of Oklahoma. Nicknamed "Fran" after its finder, it was described scientifically by Philip Currie and Kenneth Carpenter in 2000 and is now displayed at the North Carolina Museum of Natural Sciences, making Acrocanthosaurus one of the best-known giant Cretaceous predators of North America.

Chapter • Where it lived

Habitat

Acrocanthosaurus lived in the Early Cretaceous, about 113–110 million years ago, in what is now the southern and central United States. Fossils come from the Antlers Formation of Oklahoma, the Twin Mountains and Paluxy formations of Texas, and referred material from Arkansas, Wyoming and Maryland. The landscape was one of broad river floodplains and deltas draining toward the shallow seaway that would later expand into the Western Interior Seaway, with meandering channels, wetlands and forests of conifers, ferns and cycads. Along the Paluxy River in Texas, the Glen Rose Formation preserves famous trackways where large theropod feet of the Acrocanthosaurus type crossed the same lime mud as sauropod trails.

Chapter • Wild behavior

Behavior & Ecology

Did you know?

Chapter • Scientific place

Taxonomy