More than 800 kg: The biggest ostrich-like-dinosaur of North America revealed

Dinosaurs from Mississippi that resemble ostriches are among the biggest in the world, weighing over 800 kg have been revealed. The understanding of dinosaur environments in eastern North America is also improved by the findings.

Published in PLOS ONE today, the new study — led by Chinzorig Tsogtbaatar of the North Carolina Museum of Natural Sciences and colleagues — shows that ostrich-like dinosaurs called ornithomimosaurs grew to enormous sizes in ancient eastern North America.

As stated, Laramidia to the west and Appalachia to the east were the two landmasses that made up North America during the Late Cretaceous Period. However, Appalachian fossils are uncommon, and as a result, little is known about this region's prehistoric ecosystems.

In this work, Chinzorig and associates describe fresh ornithomimosaur fossils from the Late Cretaceous Eutaw Formation of Mississippi.

Ornithomimosaurs, or "bird-mimic" dinosaurs, were ostrich-like in appearance, with small heads, long arms, and strong legs. The new fossils, which include foot bones, are approximately 85 million years old, providing a rare peek into a hitherto unknown period of North American dinosaur evolution.

Weighed over 800 kg

The authors established that the fossils likely represent two separate species of ornithomimosaurs, one very tiny and one quite huge, by comparing the proportions of these fossils and the patterns of growth inside the bones.

They estimate the larger species weighed more than 800kg, and the individual studied was most likely still developing when it died. This makes it one of the largest ornithomimosaurs yet discovered.

"The co-existence of medium- and large-bodied ornithomimosaur taxa during the Late Cretaceous Santonian of North America does not only suggest broader evidence of multiple cohabiting species of ornithomimosaurian dinosaurs in Late Cretaceous ecosystems of Laurasia, but it also provides key information on the diversity and distribution of North American ornithomimosaurs from the Appalachian landmass," said the authors.

Study abstract:

Reconstructing the evolution, diversity, and paleobiogeography of North America’s Late Cretaceous dinosaur assemblages requires spatiotemporally contiguous data; however, there remains a spatial and temporal disparity in dinosaur data on the continent. The rarity of vertebrate-bearing sedimentary deposits representing Turonian–Santonian ecosystems, and the relatively sparse record of dinosaurs from the eastern portion of the continent, present persistent challenges for studies of North American dinosaur evolution. Here we describe an assemblage of ornithomimosaurian materials from the Santonian Eutaw Formation of Mississippi. Morphological data coupled with osteohistological growth markers suggest the presence of two taxa of different body sizes, including one of the largest ornithomimosaurians known worldwide. The regression predicts a femoral circumference and a body mass of the Eutaw individuals similar to or greater than that of large-bodied ornithomimosaurs, Beishanlong grandis, and Gallimimus bullatus. The paleoosteohistology of MMNS VP-6332 demonstrates that the individual was at least ten years of age (similar to B. grandis [~375 kg, 13–14 years old at death]). Additional pedal elements share some intriguing features with ornithomimosaurs, yet suggest a larger-body size closer to Deinocheirus mirificus. The presence of a large-bodied ornithomimosaur in this region during this time is consistent with the relatively recent discoveries of early-diverging, large-bodied ornithomimosaurs from mid-Cretaceous strata of Laurasia (Arkansaurus fridayi and B. grandis). The smaller Eutaw taxon is represented by a tibia preserving seven growth cycles, with osteohistological indicators of decreasing growth, yet belongs to an individual approaching somatic maturity, suggesting the co-existence of medium- and large-bodiedornithomimosaur taxa during the Late Cretaceous Santonian of North America. The Eutawornithomimosaur materials provide key information on the diversity and distribution of North American ornithomimosaurs and Appalachian dinosaurs and fit with broader evidence of multiple cohabiting species of ornithomimosaurian dinosaurs in Late Cretaceous ecosystems of Laurasia.