It’s time for another month of themed blog posts, and this August features one of the most iconic groups of dinosaurs: the “horn-faced” ceratopsians!
Existing for almost 100 million years, from the Late Jurassic all the way up to the K-Pg mass extinction, ceratopsians originated in Asia and were part of a group called marginocephalians, sharing a common ancestor with the closely related pachycephalosaurs. The earliest members barely resembled their more famous descendants, lacking showy headgear and looking more like fairly generic basal neornithischians – but by the time of the Late Cretaceous their descendants had migrated across to North America and evolved into large quadrupeds, with some forms like Triceratops being so incredibly common that they must have been the dominant herbivores in their environments.
So let’s start right at the beginning of the group with…
Yinlong (meaning “hidden dragon”) was the earliest ceratopsian that we currently know about. Living in China during the Late Jurassic (~161-156 mya), it measured around 1.2m long (4′) and was a transitional form between the basic neornithischian body plan and the later more specialized ceratopsians.
There was only a very small ridge of raised bone at the back of its skull, and its skeleton shared several important anatomical features with pachycephalosaurs. This suggests that many of the characteristics thought to be unique to pachys were actually ancestral to all marginocephalians, and the ceratopsians lost those features early on in their evolution.
It also had enlarged canine-like teeth at the front of its snout, a feature very similar to heterodontosaurids – which could be the result of convergent evolution, or could mean that heterodontosaurids were much more closely related to marginocephalians than previously thought.
We don’t yet know whether early ceratopsians like Yinlong were fluffy, as depicted above, but the presence of extensive fuzz in basal neornithischians means it was at least a possibility.