Mississippi's Ostrich-Like Dinosaurs Weighed More Than 800 Kg, Study Finds

Ornithomimosaurs were superficially ostrich-shaped with small heads, long arms and strong legs. The new fossils discovered as part of the study are around 85 million years old.

Ostrich-like dinosaurs in ancient eastern North America grew to enormous sizes, a new study says. These dinosaurs are called ornithomimosaurs. Some of them weighed more than 800 kilograms. The study, led by Chinzorig Tsogtbaatar of the North Carolina Museum of Natural Sciences, was published October 19 in the journal PLOS ONE. 

What was North America like during the Late Cretaceous?

North America was split by a seaway into two landmasses during the Late Cretaceous Period (100.5 million to 66 million years ago). The landmass to the west was called Laramidia and the one to the east was called Appalachia. Since fossils from Appalachia are rare, ancient ecosystems from this region are poorly understood. Chinzorig and his colleagues describe new fossils of ornithomimosaur dinosaurs from the Late Cretaceous Eutaw Formation of Mississippi, in the new study. 

What does Ornithomimosaurs mean?

The word Ornithomimosaurs means "bird-mimic" dinosaurs. They were superficially ostrich-shaped with small heads, long arms and strong legs. The new fossils discovered as part of the study are around 85 million years old, and provide a rare glimpse into a poorly known interval of North American dinosaur evolution. 

Two different species of Ornithomimosaurs found

The authors compared the proportions of the fossils and patterns of growth within the bones, and determined that the fossils likely represent two different species of ornithomimosaurs. One of the species was relatively small and the other one was very large. The researchers estimate the larger species of ornithomimosaurs to have weighed more than 800 kilograms. The heavier individual was likely still growing when it died. The dinosaur was one of the largest ornithomimosaurs known, the study says. 

Ornithomimosaurs were also found in Asia 

The fossils provide significant details about the poorly understood dinosaur ecosystems of Late Cretaceous eastern North America, and also shed light on ornithomimosaur evolution. These dinosaurs were found across North America and Asia. They had giant body sizes, and lived side-by-side with multiple species. 

The individual studied was at least 10 years of age, the authors note in the study.

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