Protoceratops

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Protoceratopsidae

Genus: Protoceratops

Species: Protoceratops andrewsi

Introduction

Protoceratops was a small herbivorous dinosaur that lived in Late Cretaceous Mongolia. The species represented a primitive lineage of the ceratopsians, or horned dinosaurs, one of the largest and most diverse dinosaur groups in the Cretaceous Northern Hemisphere.

In the Triassic period, the continents were united in a single land mass known as Pangaea. Tectonic movement eventually caused Pangaea to break into two supercontinents, Laurasia in the north and Gondwana in the south. Dinosaurs first appeared in the Triassic and spread across the globe, radiating into a variety of species and further dividing into distinct northern and southern groups as the continents divided.

Protoceratops represents a lineage that first radiated into Asia in the Jurassic period (201 to 145 million years ago), and later also occupied North America. Unlike the large ceratopsians in North America, those in Asia were smaller and maintained primitive characteristics similar to their ancestor species. Judging from the frequency of Protoceratops fossils, paleontologists suspect that the species was widespread and generally common throughout its range.

Classification

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

Systematic analysis places Protoceratops into the order Ornithischia, based on the “bird-like” orientation of the hips. From there, it is placed in the suborder Cerapoda, based on features of the skull, and then into the family Proceratopsidae, a lineage of primitive ceratopsians lacking fully developed skull horns.

A cladistic analysis places Protoceratops in the Ornithischia clade, then under the clade Marginocephalia, based largely on the shelf of bone extending from the rear of the skull. Next, the dinosaur falls under the Ceratopsia clade, based on its cranial specializations (including snout structure). The dinosaur is next placed in the clade Coronosauria, the members of which had enlarged cranial frills and were obligate quadrupeds..

In 2017 Matthew Baron and his colleagues posited a common ancestor for ornithischians, such as Protoceratops, and theropods, such as Allosaurus, and proposed they be reclassified into a new clade, Ornithoscelida.

Anatomy

Protoceratops was a small ceratopsian, approximately 1.8 meters (5.9 feet) long and standing 0.6 meters (1.9 feet) at the hip. Paleontologists estimate that the animal weighed between 240 to 245 kilograms (529 to 540 pounds). The animal had thick, sturdy legs terminating in hoof-like clawed feet.

Protoceratops had a large, hooked beak at the end of its skull and grinding teeth in its cheeks. The animal also had a short, thick frill extending from the rear of the skull, with two large fenestrae in the skull that were probably covered by skin. The size and shape of the frill varied among individuals.

Protoceratops was most likely covered in small, mosaic scales, similar to impressions found for other ceratopsians and hadrosaurs. The animal had large eye sockets, which indicates a wide field of vision.

Intelligence

The ceratopsians had mid-range EQ values, generally from 1 to 1.5 on the EQ scale. Some paleontologists believe that the smaller ceratopsians were armed with greater intelligence, perhaps enabling them to better avoid predation, whereas the larger ceratopsians utilized increased size for protection and had less need of advanced intelligence.

Reproduction and Population

Paleontologists believe they have found groups of shallow nests for several ceratopsian species, indicating that in some cases, ceratopsians gathered in social nesting grounds. Protoceratops might similarly have gathered into communal nesting grounds, aiding in their defense of eggs through group vigilance.

Variation in the size and shape of the skull frill has led some paleontologists to speculate that the species may have displayed sexual dimorphism. For example, scientists believe that female protoceratopsids were smaller than males. Alternatively, differences in skull shape might be due to individual variation or other intraspecific variation or might have varied with age. Some paleontologists posit that the frill, which grew in size over time, was an ornamental display feature used to attract mates and intimidate rivals or to indicate social position. Others think they aided in species recognition.

Diet

Paleontologists believe that Protoceratops was an herbivore, using its parrot-like beak to slice through tough vegetation and woody plants. Though angiosperms, or flowering plants, had begun to appear in the Late Cretaceous, the dominant plants were still the gymnosperms, including a variety of conifer species. In addition to conifers, another dominant group of gymnosperms were the cycads, a group of palm-like plants with woody trunks.

Along with its powerful beak, Protoceratops had rows of crushing teeth in its jaws that were used to pulverize plant matter before swallowing. The evolution of teeth and chewing in herbivores was one of the key developments in the cerapod group that enabled them to achieve more efficient digestion. Paleontologists believe that Protoceratops and other ceratopsians had fleshy cheeks on the sides of the mouth, preventing the loss of food when chewing.

Behavior

Because groups of Protoceratops fossils have been found in close association to one another, paleontologists now suspect that the dinosaur may have gathered into herds. It is difficult to prove social behavior from fossil evidence, and some paleontologists have suggested that Protoceratops might have gathered around a communal food or water source but did not otherwise live or travel together. Others cite the burial of four midsized P. andrewsi juveniles atop one another as indicative of age-segregated groupings in response to predation pressures.

Protoceratops fossils have been found in areas that were dry and arid in the Late Cretaceous, and the animal's eggs also show adaptations to dry conditions. Paleontologists now believe that Protoceratops may have lived on the periphery of the desert, relying on the vegetation that gathered in transitional zones.

Given its wide ocular orbit, probable foraging times, and difficulty with thermoregulation, Protoceratops was likely active at various times of day. One now-famous Protoceratops fossil was discovered in combat with the small, nocturnal theropod hunter Velociraptor, indicating that this species may have preyed upon Protoceratops. There were a variety of predator species in the environment, and Protoceratops likely used its bony beak and perhaps social cooperation to avoid predation.

Habitat and Other Life Forms

Late Cretaceous Mongolia was a patchwork of varying habitats, ranging from desert and scrub to tropical forests. Many of the Mongolian fossils studied to date have been uncovered from the Gobi Desert, much of which was still arid and dry during the Cretaceous. Paleontologists have discovered a large variety of small lizards and other reptiles living in the environment.

The first true birds appeared in the Cretaceous and there were several species in Cretaceous Mongolia, including Hollanda luceria, a long-legged bird resembling a modern road-runner. The oviraptorosaurids, close relatives of the birds, were also present in Cretaceous Mongolia. The species Oviraptor occupied the same environment as well as the medium-sized Tochisaurus. Most of the dinosaurs in the environment were smaller than those in North America, but there were a few large predatory species like the theropod Dinocheirus living near the Gobi.

In addition to Protoceratops, the ceratopsians were represented by a few other small species like Bagaceratops and a variety of pachycephalosaurids, related members of the same marginocephalia clade, like Prenocephale and Homalocephale. Another common herbivore group in Mongolia were the ornithomimids (such as Anserimimus), bipedal herbivores and omnivores that developed bodies similar to modern ostriches.

Research

Protoceratops was described after the species was discovered during a 1922 expedition to the Gobi Desert; the type species Protoceratops andrewsi is named after famed explorer Roy Chapman Andrews. A number of further specimens were discovered in subsequent decades.

When the predatory species Oviraptor was discovered in the mid-1920s in close proximity to a nest of eggs, it was believed that the animal had been feeding on the eggs of a Protoceratops, thus inspiring the name Oviraptor or “egg thief.” However, a therapod nest discovery around the 1990s strongly suggested that the eggs in the other nest also contained a brood of oviraptorids, raising questions about how Protoceratops actually nested and parented. The 2014 description of a nest containing fifteen P. andrewsi juveniles of the same age, size, and developmental stage might suggest some amount of parental involvement after hatching, although no adult was found nearby.

Also in the 1990s, Researcher Kenneth Carpenter described a specimen of Protoceratops that appeared to have died while in mortal combat with a Velociraptor. The find was taken as substantial evidence that Velociraptor was a predator of Protoceratops.

In the twenty-first century, further expeditions to Mongolia and elsewhere in Asia have revealed additional specimens in the genus. In addition to P. andrewsi, the species P. hellenikorhinus is considered a member of Protoceratops.

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