Gallimimus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Ornithomimidae

Genus: Gallimimus

Species: Gallimimus bullatus

Introduction

Gallimimus was a large dinosaur that lived in Late Cretaceous Asia and represented a group of dinosaurs that evolved to fill an ecological niche similar to that of flightless birds. Gallimimus was an offshoot of the same evolutionary line that eventually led to the evolution of birds.

In the Triassic period, the continents of the world were united in a single supercontinent called Pangaea. The dinosaurs evolved during this period and spread across the world. In the Jurassic and Cretaceous, tectonic movement caused Pangaea to split into two continents, Laurasia in the north and Gondwana in the south.

Gallimimus represented a group of dinosaurs that evolved in Laurasia, after the continent split apart from Pangaea. Members of the group have been found across North America, Europe, and Asia. Late Cretaceous Asia contained a variety of rich ecosystems, from lush forests to open arid deserts, with a large number of unique dinosaur species.

Classification

Scientists use two primary methods to classify organisms into different groups. Linnaean taxonomy, or “systematics,” groups organisms heirarchically according to overall physical similarity, while an alternate system, “cladistics,” creates groups called “clades” to reflect the evolutionary relationships between species.

According to the Linnaean system, Gallimimus is a member of the order Saurischia, because of the “lizard-like” orientation of the animal's pelvis, and the suborder Theropoda, which comprises bipedal dinosaurs with reduced front limbs. The family Ornithomimidae includes theropods that evolved characteristics similar to flightless birds.

Cladistic analysis places Gallimimus in the clade Theropoda, based on the presence of a furcula, or “wishbone,” and features of the skull and leg bones. From there, the species is in the clade Coelurosauria, which had enlarged brains and protofeathers. The clade Ornithomimosauria includes coelurosaurs with smaller heads and elongated necks and the clade Ornithomimidae contains toothless coelurosaurs with beaks.

Anatomy

Gallimimus was a large dinosaur, reaching lengths of between 5 and 5.8 meters (18–26.2 ft) and standing at least 2 to 3 meters (6.5–9.8 ft) tall. The animal may have weighed approximately 440 kilograms (970 lbs). The species was bipedal, with long, powerful legs and three-toed feet ending in hoof-like claws. The animal also had a thick, stiff tail, providing balance.

Gallimimus had a small head, with large eyes located on the sides of its head and a long, wide bill on its snout. The neck was long and flexible, and the body narrow and streamlined, perhaps an adaptation to increase efficiency when running. The arms were much shorter than the legs, with short hands armed with three sharp claws. The species was not able to grasp with its hands, as the wrists were locked in position.

Paleontologists are uncertain about the animal's skin covering, but it likely had mosaic scales similar to skin impressions found for other dinosaur species. The animal may also have had feathers or protofeathers on part of its body, but paleontologists have not found impressions of feathers on Gallimimus fossils. As there were many carnivorous dinosaurs in Mongolia, Gallimimus may have evolved camouflage, like countershading, to help avoid predators.

Intelligence

Dinosaurs were less intelligent, on average, than mammals and birds filling similar ecological niches. A few groups of dinosaurs in the theropod suborder achieved higher intelligence, similar to small mammals or birds. Gallimimus most likely had high intelligence compared to other groups of dinosaurs and may have been similar in intelligence to a small mammal.

Reproduction and Population

Paleontologists believe that most dinosaurs were oviparous, or egg-laying, animals, like birds and most reptiles. Several juvenile specimens have been discovered that were much smaller than adult specimens. Some paleontologists have suggested that young Gallimimus would have required parental care after hatching.

An animal as large as Gallimimus was probably unable to sit on its nest and brood its eggs using body heat, in the manner of many modern birds. Instead, the species may have covered its nests in vegetation, which insulated the eggs against temperature fluctuation and gave off heat as the vegetation decomposed.

Diet

The diet of Gallimimus has been subject to debate among paleontologists. The presence of deep ridges on the end of the animal's bill suggested to some paleontologists that the animal may have been a filter feeder, living near lakes and rivers. Others have suggested that the ornithomimids may have used their short, downturned hands to dig into the nests of other dinosaurs, stealing and eating eggs.

Paleontologists have found that some ornithomimids swallowed small stones, called gastroliths, to aid in digesting tough vegetation, leading to the speculation that most ornithomimid species were herbivorous. Other paleontologists have suggested that ornithomimids were omnivores, primarily eating vegetation but also taking small prey when available.

Behavior

Paleontologists believe that ornithomimids may have been the fastest of all the dinosaurs. Gallimimus had long, powerfully built legs with exceptionally long femurs. Similar adaptations are seen in ostriches, flightless birds that are also known for their running ability. The species’ large eyes were set on the sides of the head, an adaptation seen in many herbivores living in predator-dense environments.

Paleontologists are uncertain whether Gallimimus displayed any social behavior, but groups of other ornithomimid fossils have been found that suggest that some ornithomimids gathered into groups. Some paleontologists believe that ornithomimids may have been herding animals, gathering in large groups to increase vigilance in watching for predators and to aid in protecting their nests and young.

Habitat and Other Life Forms

Late Cretaceous Mongolia contained a variety of habitats, from arid desert to moist fern forests. Gallimimus fossils have been discovered in the Nemegt Formation, which represents partially forested habitats in the Late Cretaceous, with arid plains and a small floodplain habitat. Paleontologists have discovered a variety of gymnosperm plants from this area, including several species of conifers.

In addition to Gallimimus, there were other ornithomimid dinosaurs living in the region, including the mid-sized Anserimimus, or “duck mimic,” and the smaller Garudimimus. The hadrosaurs, or “duck-billed” dinosaurs, were also present in Cretaceous Mongolia, including the genera Barsboldia and Gilmoreosaurus. Mongolia also supported a few species of ceratopsians, or horned dinosaurs.

There were both large and small predatory dinosaurs in Mongolia, including the swift small predators Velociraptor and Saurnitholestes. Large theropod predators like Therizinosaurus would have been most likely to pose a danger to the larger ornithomimids like Gallimimus. While the first true birds were then present in Mongolia, the skies were still dominated by flying reptiles, called pterosaurs, which were the dominant flying vertebrates throughout the Mesozoic.

Research

Gallimimus was first discovered in the Nemegt Formation of Mongolia and was described and named by paleontologists Halszka Osmolska and Rinchen Barsbold in 1972. Further expeditions have uncovered a few juvenile specimens of the same species. Barsbold and Osmolska also later theorized that the species may have been a filter feeding animal, based largely on features of the animal's bill and the presence of aquatic animal fossils in nearby fossil beds.

While Barsbold initially believed that, based on the study of an ornithomimid specimen in the 1990s, another species of Gallimimus had existed, it was determined upon reassessment in 2006 that differences in such features as the skull and metatarsals meant that the ornithomimid in question could not belong to the Gallimimus genus.

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