Protoceratops

Protoceratops (/ˌproʊtoʊˈsɛrətɒps/; lit. 'first horned face') is a genus of small protoceratopsid dinosaurs that lived in Asia during the Late Cretaceous, around 75 to 71 million years ago. The genus Protoceratops includes two species: P. andrewsi and the larger P. hellenikorhinus. The former was described in 1923 with fossils from the Mongolian Djadokhta Formation, and the latter in 2001 with fossils from the Chinese Bayan Mandahu Formation. Protoceratops was initially believed to be an ancestor of ankylosaurians and larger ceratopsians, such as Triceratops and relatives, until the discoveries of other protoceratopsids. Populations of P. andrewsi may have evolved into Bagaceratops rozhdestvenskyi through anagenesis.

Protoceratops were small ceratopsians, up to 2–2.5 m (6.6–8.2 ft) long and around 62–104 kg (137–229 lb) in body mass. While adults were largely quadrupedal, juveniles had the capacity to walk around bipedally if necessary. They were characterized by a proportionally large skull, short and stiff neck, and neck frill. The frill was likely used for display or intraspecific combat, as well as protection of the neck and anchoring of jaw muscles. A horn-like structure was present over the nose, which varied from a single structure in P. andrewsi to a double, paired structure in P. hellenikorhinus. The "horn" and frill were highly variable in shape and size across individuals of the same species, but there is no evidence of sexual dimorphism. They had a prominent parrot-like beak at the tip of the jaws. P. andrewsi had a pair of cylindrical, blunt teeth near the tip of the upper jaw. The forelimbs had five fingers of which only the first three bore wide and flat unguals. The feet were wide and had four toes with flattened, shovel-like unguals, which would have been useful for digging through the sand. The hindlimbs were longer than the forelimbs. The tail was long and had an enigmatic sail-like structure, which may have been used for display, swimming, or metabolic reasons.

Protoceratops, like many other ceratopsians, were herbivores equipped with prominent jaws and teeth suited for chopping foliage and other plant material. They are thought to have lived in highly sociable groups of mixed ages. They appear to have cared for their young. They laid soft-shelled eggs, a rare occurrence in dinosaurs. During maturation, the skull and neck frill underwent rapid growth. Protoceratops were hunted by Velociraptor, and one particularly famous specimen (the Fighting Dinosaurs) preserves a pair of them locked in combat. Protoceratops used to be characterized as nocturnal because of the large scleral ring around the eye, but they are now thought to have been cathemeral (active at dawn and dusk).

In 1900 Henry Fairfield Osborn suggested that Central Asia may have been the center of origin of most animal species, including humans, which caught the attention of explorer and zoologist Roy Chapman Andrews. This idea later gave rise to the First (1916 to 1917), Second (1919) and Third (1921 to 1930) Central Asiatic Expeditions to China and Mongolia, organized by the American Museum of Natural History under the direction of Osborn and field leadership of Andrews. The team of the third expedition arrived in Beijing in 1921 for the final preparations and started working in the field in 1922. During late 1922 the expedition explored the famous Flaming Cliffs of the Shabarakh Usu region of the Djadokhta Formation, Gobi Desert, now known as the Bayn Dzak region. On 2 September, the photographer James B. Shackelford discovered a partial juvenile skull—which would become the holotype specimen (AMNH 6251) of Protoceratops—in reddish sandstones. It was subsequently analyzed by the paleontologist Walter W. Granger who identified it as reptilian. On 21 September, the expedition returned to Beijing, and even though it was set up to look for remains of human ancestors, the team collected numerous dinosaur fossils and thus provided insights into the rich fossil record of Asia. Back in Beijing, the skull Shackelford had found was sent to the American Museum of Natural History for further study, after which Osborn reached out to Andrews and team via cable, notifying them about the importance of the specimen.

In 1923 the expedition again prospected the Flaming Cliffs, this time discovering even more specimens of Protoceratops and also the first remains of Oviraptor, Saurornithoides and Velociraptor. Most notably, the team discovered the first fossilized dinosaur eggs near the holotype of Oviraptor and given how abundant Protoceratops was, the nest was attributed to this taxon. This would later result in the interpretation of Oviraptor as an egg-thief. In the same year, Granger and William K. Gregory formally described the new genus and species Protoceratops andrewsi based on the holotype skull. The specific name, andrewsi, is in honor of Andrews for his prominent leadership during the expeditions. They identified Protoceratops as an ornithischian dinosaur closely related to ceratopsians representing a possible common ancestor between ankylosaurs and ceratopsians. Since Protoceratops was more primitive than any other known ceratopsian at that time, Granger and Gregory coined the new family Protoceratopsidae, mostly characterized by the lack of horns. The co-authors also agreed with Osborn that Asia, if more thoroughly explored, could solve many major evolutionary gaps in the fossil record. Although not stated in the original description, the generic name, Protoceratops, is intended to mean "first horned face" as it was believed that Protoceratops represented an early ancestor of ceratopsids. Other researchers immediately noted the importance of the Protoceratops finds, and the genus was hailed as the "long-sought ancestor of Triceratops". Most fossils were in an excellent state of preservation with even scleral rings (delicate ocular bones) preserved in some specimens, quickly making Protoceratops one of the best-known dinosaurs from Asia.

After spending much of 1924 making plans for the next fieldwork seasons, in 1925 Andrews and team explored the Flaming Cliffs yet again. During this year more eggs and nests were collected, alongside well-preserved and complete specimens of Protoceratops. By this time, Protoceratops had become one of the most abundant dinosaurs of the region with more than 100 specimens known, including skulls and skeletons of multiple individuals at different growth stages. Though more remains of Protoceratops were collected in later years of the expeditions, they were most abundant in the 1922 to 1925 seasons. Gregory and Charles C. Mook published another description of Protoceratops in 1925, discussing its anatomy and relationships. Thanks to the large collection of skulls found in the expeditions, they concluded that Protoceratops represented a ceratopsian more primitive than ceratopsids and not an ankylosaur-ceratopsian ancestor. In 1940, Barnum Brown and Erich Maren Schlaikjer described the anatomy of P. andrewsi in extensive detail using newly prepared specimens from the Asiatic expeditions.

In 1963, the Mongolian paleontologist Demberelyin Dashzeveg reported the discovery of a new fossiliferous locality of the Djadokhta Formation: Tugriken Shireh. Like the neighbouring Bayn Dzak, this new locality contained an abundance of Protoceratops fossils. During the 1960s to 1970s, Polish-Mongolian and Russian-Mongolian paleontological expeditions collected new, partial to complete specimens of Protoceratops at this locality, making this dinosaur species a common occurrence in Tugriken Shireh. Since its discovery, the Tugriken Shireh locality has yielded some of the most significant specimens of Protoceratops, such as the Fighting Dinosaurs, in situ individuals—a preservation condition also known as "standing" individuals or specimens in some cases—, authentic nests, and small herd-like groups. Specimens from this locality are usually found in articulation, suggesting possible mass mortality events.

Stephan N. F. Spiekman and colleagues reported a partial P. andrewsi skull (RGM 818207) in the collections of the Naturalis Biodiversity Center, Netherlands in 2015. Since Protoceratops fossils are only found in the Gobi Desert of Mongolia and this specimen was likely discovered during the Central Asiatic Expeditions, the team concluded that this skull was probably acquired by Delft University between 1940 and 1972 as part of a collection transfer.