was a primitive ceratopsian dinosaur (the ceratopsian dinosaurs were quadruped dinosaurs with distinctive horns and frills at the back of their skulls), discovered in Mongolia in the 1920s by American palaeontologist Roy Chapman Andrews
. They are well known to scientists as a large number of specimens have been discovered, often well preserved and in groups. This was due to the environment in which they lived, a desert with rolling sand dunes, where they had a good chance of being rapidly buried (not always after they died) and then preserved as fossils.
Protoceratops lived between about 75 and 71 million years ago in the interior of north-east Asia. Unlike more advanced members of the ceratopsid group, Protoceratops lacked large horns and a well developed neck frill. It was also much smaller than later ceratopsians, about the size of a sheep; by comparison the better known Triceratops could reach 9 m in length and stood 3 m at the shoulder, more than twice the size of a White Rhinoceros. There are currently two recognized species of Protoceratops, the original Mongolian species, P. andrewsi, named after its discoverer, and a slightly larger species from Inner Mongolia (to the south), P. hellenikorhinus, which had a slightly larger frill, better developed (though still small) horns and premaxillary teeth (which P. andrewsi lacks). Some scientists consider Protoceratops to be a single, sexually dimorphic species, with the apparent difference in distribution being due to sampling error (i.e., not enough samples have been discovered to show that the two types coexisted in the same space).
The Protoceratops nest. The arrow gives the direction of the prevailing wind, and the scale bar is in centimeters.
Sedimentologists (geologists that study sedimentary rocks) are able to determine wind direction in ancient eolian (wind blown) sediments from the shape of the dunes. Dunes form as grains of sand are blown up the windward slope, leading to a gentle slope in the direction the wind is coming from. However this cannot climb infinitely, so on the leeward (away from the wind) side there is a sharper slope, where the grains reach the top of the dune, then collapse down the other side in a series of small avalanches (occasionally trapping small dinosaurs).
A sand-dune migrating with the wind has a distinct shape.
The three dimensional shape is hard to spot in sedimentary rocks, but the distinctive cross section is easily recognized.
Identifying infant dinosaurs is not always easy. The distinctive features of animals like Protoceratops are not often present in infant animals. As such it would be difficult to distinguish a juvenile Protoceratops from the juvenile of another primitive ceratopsian, such as Psittacosaurus, Liaoceratops or Auroroceratops. However none of these species have the same physical or temporal distribution (i.e., they were not found in the same time and place), so Fastovsky et al. have tentatively assigned the specimens to the species Protoceratops andrewsi.
Ruling out P. hellenikorhinus was even more difficult, as this species lived at the same time and (relatively) close by. The differences in size would not show in a juvenile of unknown age and development, and the horns and frills are less developed than in an adult of either species. This leaves the possibility of cutting into a skull to try to detect developing premaxillary teeth, which would be diagnostic of P. hellenikorhinus. It was deemed unwise to damage the fossils in this way, as the absence of teeth would not be evidence for the specimens being P. andrewsi, since it is unclear at what stage the teeth start to develop. It is possible that in future the specimens could be x-rayed, since the presence of teeth would determine that the infants were in fact P. hellenikorhinus, though this would be an expensive and difficult process for a relatively academic point.
The delicate skulls of the juvenile dinosaurs. Scale bar in centimeters.
Whatever the exact classification of these dinosaurs, it is clear that they were primitive ceratopsians, and that they had stayed in the nest after hatching, which has profound implications for our understanding of how these animals lived. This is because an immature dinosaur would be highly unlikely to remain in the nest unless it was receiving parental care, something that has long been a debating point among palaeontologists.
Birds, the only living dinosaurs, typically show extensive care for their young, with most species incubating their eggs with their own body-heat, and many continuing to feed the young in the nest for some time after they hatch. Crocodilians, the closest living relatives of the dinosaurs, neither incubate their eggs nor feed their young, but (unlike most other reptiles) most species attend their young at the time of hatching.
Nests containing the eggs of many species of Mesozoic dinosaur have been discovered in the past, including some with adults apparently incubating the eggs (also overrun by Mongolian sand dunes). However these incubators have always been from Therapod species closely related to birds, leaving room for speculation about the degree of parental care among dinosaurs as a whole. For some species, notably the larger sauropods, it is difficult to see how the gigantic adults could have cared for the tiny infants, though the animals are known to have travelled in packs containing individuals of differing sizes and therefore ages.
Citipati osmolskae, an Oviraptid Therapod closely related to the earliest birds, discovered apparently incubating eggs when it was overrun by a sand dune in Cretaceous Mongolia.
Ceratopsians are advanced Ornithischian dinosaurs, about as distantly related to birds as it is possible to be and still be a dinosaur. There are two possible interpretations that can be made from the discovery of parental care in Ceratopsians. Either caring for young was a primitive trait in dinosaurs (i.e. a trait found in the earliest members of the group, and persisting in their descendants - though it would still be possible for some groups to loose the trait) or it evolved separately in advanced Therapods and primitive Ceratopsians, suggesting that the trait was possible and advantageous, and therefore likely to have evolved in other groups as well. In either event this discovery is likely to fuel further research into parental care among the palaontological community.