Horned dinosaurs carried a beak bone found in no other animal
Every ceratopsian, from tiny Psittacosaurus to Triceratops, had a toothless bone at the tip of its upper jaw called the rostral, which no other animal possesses. Paired with a matching bone below, it formed a parrot-like beak for cropping plants, and it lets palaeontologists recognise the group from a skull alone.
Ceratopsia means horned faces, though purists pointed out in the 1960s that Ceratopia would be the correct Greek; almost nobody uses it. The group began in Asia, home to the oldest known member, Yinlong downsi, dated to roughly 161.2 to 155.7 million years back. Early forms walked on two legs and were small, as little as a metre long and 23 kilograms. Later giants such as Triceratops walked on all fours, stretched beyond 9 metres and weighed over 9 tonnes. Teeth that seem to belong to the group turn up in North America from 112 to 100 million years ago, showing they had crossed the land bridge where the Bering Strait now lies.
The famous frills may have shielded the neck, anchored big jaw muscles, helped shed heat or simply shown off, perhaps all of these. Small bones around the frill edge, misleadingly named epoccipitals, were purely ornamental and differed from species to species. Unusually for dinosaurs, skulls are the parts most often preserved, and several species are known from nothing else. Growth series from embryo to adult survive for Psittacosaurus and Protoceratops.
The first finds confused everyone. Teeth collected in Montana in 1855 were filed under duck-billed dinosaurs, and only in the early 20th century were some recognised as ceratopsian. In 1888 and 1889 Othniel Charles Marsh described Ceratops and Triceratops, and in 1890 he coined the name Ceratopsia, prompting his rival Edward Drinker Cope to realise several of his own specimens belonged to the same family.
How fast they moved is debated. Most reconstructions give them semi-sprawling front legs, suggesting a slow gait, but one 2000 study argued the larger species had upright forelimbs and might have matched a rhino's 56 kilometres per hour. The last of the line, Triceratops prorsus, vanished in the mass extinction 66 million years ago.
Source: Ceratopsia
