800-Kilogram Ostrich-Like Dinosaurs Stomped Around Mississippi 85 Million Years Ago

The "bird-mimics" were some of the largest ever to exist.


Rachael Funnell


Rachael Funnell

Digital Content Producer

Rachael is a writer and digital content producer at IFLScience with a Zoology degree from the University of Southampton, UK, and a nose for novelty animal stories.

Digital Content Producer

This art was inspired by an Aepyornithomimus specimen from the Djadokhta Formation in Mongolia, but this new research is shedding light on ornithomimosaurian evolution with specimens from North America. Image credit: Masato Hattori, via Wikimedia Commons

North America was once home to enormous ostrich-like dinosaurs called ornithomimosaurs, new research has found, and some of the largest known to have existed were stomping around what’s now called Mississippi. Clocking in at 800 kilograms (1,764 pounds) in weight, the “bird-mimic” dinosaurs had little heads on top of long necks as well as long legs and arms with round bodies.

New specimens that lived around 85 million years ago are teaching us new things about the existence of ornithomimosaurs in North America, a place which, during the Late Cretaceous, was separated into two landmasses by the ocean. To the west, we had Laramidia, while to the east sat Appalachia, which is where the focal specimens of a new study once lived.


The retrieved specimens were studied by a research team led by Chinzorig Tsogtbaatar of the North Carolina Museum of Natural Sciences. They were taken from the Santonian Eutaw Formation of Mississippi and subjected to morphological and histological investigations.

Such research represents an exciting opportunity to fill in gaps in our knowledge of the animals that lived in eastern North America during the Late Cretaceous, a period in time that has gone somewhat understudied here owing to the rarity of sediment deposits containing vertebrate fossils in the region, write the researchers.

Excitingly, their findings “suggest the presence of two taxa of different body sizes, including one of the largest ornithomimosaurians known worldwide.”

Paleohistological transverse sections of the large- and medium-bodied Eutaw ornithomimosaurs, and their comparative size within known ornithomimosaur taxa through a geological time. Image credit: Tsogtbaatar et al., CC BY 4.0

The researchers were able to reach their conclusions by comparing growth patterns within the specimens’ fossilized bones which told them they were looking at two types of ornithomimosaurs of different sizes. The bigger of the two would’ve weighed more than 800 kilograms – more than an adult giraffe – and was still growing, likely placing it among the largest ornithomimosaurs on the planet (that we know about, you never know when the next true Big Bird might pop up).


The discovery of the asymmetric pair lends to existing research that has found ornithomimosaurs weren’t afraid of living next door to different species, and often these cohabiting species were very different in size with some being very, very large. The researchers hope next to further investigate why such different animals were well suited to their shared environment.

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

The study is published in PLOS ONE.


  • tag
  • animals,

  • dinosaurs,

  • fossils,

  • extinct,

  • late cretaceous