Description and Classification

Arkansaurus belongs to the group Ornithomimosauria, often called “ostrich-mimic” dinosaurs due to their resemblance to modern ostriches. Like other ornithomimosaurs, Arkansaurus was likely a bipedal (walked on two legs) dinosaur with a slender build, long legs adapted for swift running, a long neck, and a small head, possibly with a toothless beak. Although only foot bones have been discovered, scientists can infer its general appearance by comparing it to more complete relatives such as Struthiomimus and Ornithomimus. Based on the size of its foot, Arkansaurus is estimated to have been a medium-sized ornithomimosaur, perhaps around 2 to 4 meters (6.5 to 13 feet) long. It would have been a lightly built animal, consistent with a fast-moving lifestyle.

As an ornithomimosaur, Arkansaurus is classified within Theropoda, the larger group of mostly carnivorous dinosaurs that also includes giants like Tyrannosaurus rex and agile hunters like Velociraptor. However, many ornithomimosaurs are thought to have been omnivores or herbivores. The exact placement of Arkansaurus within Ornithomimosauria is still subject to study, but it represents an important find for understanding the diversity of these dinosaurs in North America during the Early Cretaceous.

Distinguishing Features

Since Arkansaurus is known only from a partial right foot (including three metatarsals, some toe bones, and three claws), its distinguishing features are primarily based on the anatomy of these bones. Some features noted by paleontologists include:

• The third metatarsal (the middle long bone of the foot leading to a toe) is relatively long and slender, indicating an adaptation for running.
• The articular surface at the far end of the third metatarsal, where it connected to the toe bone, is less “pulley-shaped” (ginglymoid) compared to some more advanced ornithomimosaurs.
• The claws (unguals) are fairly straight and not strongly curved, which differs from the more sharply curved claws seen in some other theropods.
• Overall, the foot structure shows a combination of features seen in different groups of ornithomimosaurs, suggesting it might be a relatively basal (early diverging) member of the group or represent a distinct lineage.

Paleoenvironment and Diet

Arkansaurus lived during the Early Cretaceous epoch. The fossils were discovered in rocks belonging to the Trinity Group, specifically the Holly Creek Formation. At that time, Arkansas was part of a large coastal plain bordering the early Gulf of Mexico. The environment was likely warm and humid, characterized by rivers, deltas, and forests. This habitat would have supported a diverse ecosystem.

Arkansaurus shared its environment with other dinosaurs, including the large theropod Acrocanthosaurus, giant long-necked sauropods like Sauroposeidon, and various turtles and crocodiles. While direct evidence of its diet is absent, most ornithomimosaurs are thought to have been omnivores, eating a mix of plants, insects, small animals, and possibly eggs. Its speed would have been useful for both escaping larger predators and catching small, quick prey.

Significance and Ongoing Research

Arkansaurus is a significant discovery because it is one of the few ornithomimosaur specimens found in eastern North America from the Early Cretaceous period. Its fossils provide valuable information about the geographic distribution and evolution of ornithomimosaurs. The presence of Arkansaurus in Arkansas helps paleontologists understand the types of dinosaurs that lived in this part of the world during a time when the continents were arranged differently and sea levels were higher.

In 2017, Arkansaurus fridayi was officially designated as the State Dinosaur of Arkansas, recognizing its importance to the state’s natural heritage. Ongoing research involves comparing the known fossils of Arkansaurus with those of other ornithomimosaurs to better understand its evolutionary relationships. The limited nature of the current fossil material means that future discoveries of more complete skeletons are eagerly anticipated, as they would provide much more detail about this dinosaur’s anatomy, lifestyle, and place in the dinosaur family tree. Such finds would further enrich our understanding of dinosaur diversity in the American South during the Cretaceous period.