At more than 800 kg, ostrich-like dinosaurs found in Mississippi are among the largest ornithomimosaurs in the world

Ostrich-like dinosaurs, called ornithomisaurs, grew to enormous sizes in ancient eastern North America, according to a study published Oct. 19, 2022 in the open-access journal PLUS ONE by Chinzorig Tsogtbaatar of the North Carolina Museum of Natural Sciences and colleagues.

During the Late Cretaceous, North America was divided into two land masses by a seaway: Laramidia to the west and Appalachian Mountains to the east. However, fossils from the Appalachian Mountains are rare, and as a result ancient ecosystems from this region are poorly understood. In this study, Chinzorig and colleagues describe new ornithomisaurian dinosaur fossils from the Eutaw Formation of the Late Cretaceous of Mississippi.

Ornithomimosaurs, the so-called “bird mime” dinosaurs, were superficially ostrich-shaped with small heads, long arms, and powerful legs. The new fossils, including foot bones, are around 85 million years old, making them a rare glimpse into a little-known period of North American dinosaur evolution

By comparing the proportions of these fossils and the growth patterns within the bones, the authors determined that the fossils likely represent two different species of ornithomimosaurs, one relatively small and one very large. They estimate that the larger species weighed over 800 kg, and the individual studied was probably still growing when it died. This makes it one of the largest known ornithomimosaurs.

These fossils provide valuable insight into the otherwise poorly understood dinosaur ecosystems of late Cretaceous eastern North America. They also shed light on the evolution of ornithomisaurs; Huge body sizes and multiple coexisting species are recurring trends for these dinosaurs in North America and Asia. Hopefully further studies will elucidate the reasons for the success of these life strategies.

The authors add: “The coexistence of medium- and large-bodied ornithomimosaur taxa during the Late Cretaceous of Santon in North America not only provides important information about the diversity and distribution of North American ornithomimosaurs from the Appalachian landmass, but also suggests broader evidence for.” several cohabiting species of ornithominosaurian dinosaurs in the Late Cretaceous ecosystems of Laurasia.