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Aundrea Westfall, Auburn University
The ornithomimids were a large and diverse group of dinosaurs that lived during the Cretaceous Period (145 to 66 million years ago). Their name means "bird mimic" and was chosen for their striking but superficial likeness to modern ostriches. Only one type of ornithomimid has been found in Alabama, and it has been provisionally identified as likely being the species Coelosaurus antiquus ("antique hollow lizard"). A large herd of the ornithomimid Gallimimus bullatus was featured in the 1993 film Jurassic Park.
Gallimimus bullatus
Ornithomimids roamed the giant supercontinent that paleontologists call Laurasia, which consisted of present-day North America and Asia, and some fossils have been found on the modern-day continent of Africa as well. Although they were most likely herbivorous, their close relatives include the large, infamous tyrannosaurids. Ornithomimids belong to the Coelurosauria, a grouping of dinosaurs that includes modern-day birds as well as most of the extinct dinosaurs found to have been feathered. They were not as large as their tyrannosaur cousins, but ornithomimids were by no means small: species ranged from 7 to 26 feet (2.2 to 8 meters) long as adults, with weights from about 450 to 1,550 pounds (200 to 700 kilograms). Despite their size, they were very lightly built, as most of their skeleton consisted of long, slender bones that enabled them to move quickly. Some ornithomimids evolved to be quite sizeable but maintained the structure of their hind limbs, thus retaining their speed and flexibility. Ornithomimids are thought to have been the fastest dinosaurs and may have reached speeds exceeding 30 miles (48 kilometers) per hour.
Gallimimus Skull
In addition to their large hind legs, ornithomimids resembled modern-day ostriches and other ratites, as these birds are known, in their elongated, keratin-covered snout and large eye orbits. This latter trait suggests a well-developed sense of vision, with eyes facing laterally outward from the sides of the animals' skulls, although with a more limited stereoscopic field of vision than that of animals with forward-facing eyes. Early analysts of ornithomimid fossils suggested they possessed eating habits ranging from filter feeding like flamingos to myrmecophagy, that is, eating ants and termites like aardvarks.The strongest modern arguments suggest that they were herbivorous; scientists believe this to be the case based on the animals' toothless beaks (although some of the earliest types had primitive teeth) and the stones found in the stomachs of several specimens. Many extant reptiles and birds have been known to swallow rocks, called gastroliths, to aid in the breakdown of tough plant material. Their large orbits lend more credence to the view that these animals were herbivores because these physical characteristics are still found in modern-day herbivores, such as horses and cows, and indicate a need to scan the environment widely for food and predators rather than focusing narrowly on a prey animal. They also had large, clawed forelimbs that some scientists suggest were used to pull down branches while grazing.
Struthomimus altus
The majority of ornithomimid fossils have been found across Asia and North America in deposits from seasonally wet habitats around rivers and lakes and have been nearly or totally absent from formations of drier environments. This characteristic differs sharply from ostriches, as the large modern birds prefer savannahs and near-desert climates. Most fossils have been from single individuals, thus providing little information about their social behavior. Thus far, only three multi-individual fossil sites containing groups of ornithomimids have been found, and they contained only three genera: Archaeornithomimus, Ornithomimus, and Struthiomimus. While these discoveries provide some evidence of gregarious behavior in ornithomimids, the differences in number and age distribution of individuals among the three fossil sites does not provide many answers or concrete details for paleontologists.
From their discovery in 1890 until the mid-twentieth century, ornithomimids were designated as "megalosaurs," a catch-all term given to almost all mid- to large-sized animals in the theropod, or "beast-footed," group of dinosaurs. As more fossils were uncovered and the distinctiveness of the ornithomimid family became clearer, paleontologists began reclassifying many of these dinosaurs as a group distinct from other dinosaurs. Debate among paleontologists, however, continues today about how to classify these animals.
The family consists of seven genera, five of which have been found in Asia and two of which have been found in North America, including Alabama. The most memorable to the general public is probably Gallimimus ("chicken mimic") for its appearance in the first Jurassic Park film; they are well documented in the fossil record. Other commonly found types are Ornithomimus ("bird mimic"), Struthomimus ("ostrich mimic"), and Archaeornithomimus ("ancient bird mimic"), so named because it is the oldest type found, at 70 million years old. Although several specimens have been discovered, the only ornithomimid identified thus far in Alabama is Coelosaurus, with one species described: Coelosaurus antiquus. The animals belonging to this genus are also sometimes identified as Ornithomimus antiquus, as there is still some debate about how they should be classified.
Additional Resources
Barrett, Paul M. "The Diet of Ostrich Dinosaurs (Theropoda: Ornithomimosauria)." Paleontology 48(2) (2005): 347-58.
Cullen, Thomas M., et al. "An Ornithomimid (Dinosauria) bonebed from the Late Cretaceous of Alberta, with Implications for the Behavior, Classification, and Stratigraphy of North American Ornithomimids." PloS ONE 8(3) (March 2013).
Longrich, Nicholas. "A New, Large Ornithomimid from the Cretaceous Dinosaur Park Formation of Alberta, Canada: Implications for the Study of Dissociated Dinosaur Remains." Paleontology 51(4) (July 2008): 983-97.
Thurmond, John T., and Douglas E. Jones. Fossil Vertebrates of Alabama. Revised edition.Tuscaloosa: University of Alabama Press, 2002.
Weishampel, David B., Peter Dodson, and Halszka Osmólska. The Dinosauria: Second Edition. Oakland, C.A.: University of California Press, 2007.
Published:  August 18, 2015   |   Last updated:  October 20, 2015