Acrocanthosaurus

Acrocanthosaurus

Order: Saurischia

• Sub Order: Theropoda

The name Acrocanthosaurus atokensis (high-spined lizard from Atoka) was first assigned in 1950 to two very incomplete skeletons excavated from Atoka County in Oklahoma. Discovered in the Early Cretaceous Antlers Formation, this giant theropod predated T. rex by fifty million years.1 These early, fragmentary discoveries could tell paleontologists very few things – our knowledge began and ended with the basic facts that it was a carnivorous dinosaur with tall-spined vertebrae. The finer details of its anatomy and relationships still proved elusive. Many dinosaur species are known from only a single, partial specimen. Very few formations are as productive as the Hell Creek or Morrison, so our understanding of dinosaur diversity will always be stymied by an incomplete record.

In 1983, on private land in McCurtain County in southeastern Oklahoma, amateur collectors Cephis Hall and Sid Love began excavating a dinosaur specimen that would later be recognized as the best Acrocanthosaurus yet found. Once excavated, the unprepared specimen was eventually purchased by Allen and Fran Graffham of Geological Enterprises in Ardmore, Oklahoma, who also financed the initial preparation and restoration of this magnificent skeleton. A team of expert fossil preparators at Black Hills Institute invested thousands of hours in the painstaking process of cleaning the bones, restoring the skeleton and creating a museum quality mount. Bob Farrar of Black Hills Institute noted that “Without the Graffham’s willingness to invest in the purchase and preparation of this specimen, scientists would still know very little about this intriguing but poorly understood dinosaur genus.” The specimen was even nicknamed “Fran” in honor of the Graffhams’ contributions to paleontology.

Preparation of the specimen was difficult due to the abundant pyrite (iron disulfide) growing out of and around the bones, much like moss on a rock. The pyrite seemingly refused to part from the surface of the bone. This multiplied the number of hours needed for quality preparation as the pyrite had to be ‘rubbed’ off, often taking hours for one small patch. When the pyrite was removed, it released acids into the air. This safety hazard required either bones to be prepared in vacuum boxes or preparators to use respirators.

Despite these logistical challenges, the skeleton is very well preserved. The bones have been turned nearly jet black by minerals migrating via groundwater through the sediment. This specimen also has nearly complete arms and shoulder girdles. The arms and shoulders of Acrocanthosaurus are much larger and more heavily muscled than the arms of Tyrannosaurus rex. T. rex’s arms also only have two fingers, with a tiny vestigial digit present. In Acrocanthosaurus, each arm terminates in three wickedly curved, large claws, well designed for capturing and holding prey.

There is evidence in the skeleton of what was probably a near fatal hunting accident in a punctured shoulder blade and several broken ribs that have healed. Another, older injury was found on the skull. While preparing the specimen, a sheet of bone came off of the interior surface of the maxilla, exposing a tooth. This tooth, however, did not resemble the teeth associated with Acrocanthosaurus. Indeed, it didn’t resemble a theropod tooth at all – it was a crocodile tooth! Incidentally, the left maxilla of this specimen looked distinctly pathological. It appears probable that, at some point in its life, this individual had an altercation with a crocodilian and walked away with an injury it suffered from for the rest of its life.

Notably, this story is retold by a character in the Disney-Pixar film “The Good Dinosaur.” Pixar sought BHI President Peter Larson’s dinosaur expertise during production, and integrated the story into the film upon his suggestion. In December of 1997, the original fossil skeleton was purchased by the Friends of the North Carolina State Museum of Natural Sciences (Raleigh, North Carolina). The skull was then completely cleaned apart and inflated, revealing the beautiful interior bones never before seen in any other Acrocanthosaurus specimen. Preparators at Black Hills Institute noted this preparation as among the most difficult they had done. Their hard work clearly paid off, as “Fran” the Acrocanthosaurus remains a standout addition to its new home. “Even those of us involved in the preparation of this skeleton are awed by the sense of power we feel in the presence of the mounted skeleton.” explained Terry Wentz, chief preparator on the project.

The original skeleton was mounted (sans the skull) in BHIGR’s modular style with the bones strapped or cradled to leave them accessible for future study. The mount and skull were delivered to the North Carolina museum in November of 1999 and are now on display in the new museum building in Raleigh, North Carolina.

Footnotes 1. Stovall, J. Willis, and Wann Langston. “Acrocanthosaurus Atokensis, a New Genus and Species of Lower Cretaceous Theropoda from Oklahoma.” The American Midland Naturalist, vol. 43, no. 3, 1950, pp. 696–728. JSTOR, https://doi.org/10.2307/2421859.