The protoceratopsids were a mainly Asian branch of the ceratopsians (with one species also known from Europe), splitting off shortly after the leptoceratopsids. Although still fairly small, they looked a lot more like a typical ceratopsian than their ancestors, with relatively big heads for their body size and large neck frills.
The best-known member of this group was the 1.8m long (6′) Protoceratops (“first horned face”), which lived in Mongolia during the Late Cretaceous (~75-71 mya). It was a very common animal, with hundreds of fossils representing all different stages of growth, and large numbers found together suggest it may have lived in herds.
The size and shape of its frill differed a lot between individuals, with some being short and compact and others being much longer or wider. Some paleontologists have considered this to be evidence of sexual dimorphism, but further studies show no clear distinctions between the proposed “male” and “female” morphs – it was probably just natural genetic variation among the population.
For many years a fossilized clutch of eggs was believed to belong to Protoceratops, with a nearby Oviraptor with a crushed skull thought to be an egg-eating thief killed while trying to raid the nest. The later discovery of an identical egg containing an Oviraptor embryo revealed that the story was the wrong way around – the Oviraptor was the one brooding and protecting its nest all along.
Another famous fossil preserves a fight between a Protoceratops and a Velociraptor, the pair dying together before being buried in sand. The predator’s sickle claw was lodged into the Protoceratops’ neck, while the ceratopsian was crushing the Velociraptor’s arm in its beak.
(Oh, and Protoceratops fossils also probably didn’t actually inspire legends about griffins.)