Monday, 21 September 2015

Meet Mosaiceratops - the Mosaic ceratopsian

Although most of my posts are on pterosaurs, I did get my palaeontological start working on a ceratopsian dinosaur called Centrosaurus as an undergrad in Alberta. While I don't actively work on them anymore (other than trying to finish the manuscript!), they do still hold a special place in my heart, which is why I was very interested in a new species of ceratopsian named today. 

The evolution of ceratopsians (horned dinosaurs best known as animals like Triceratops and Centrosaurus) is not entirely understood. They are generally though of as being closely related to pachycephalosaurs (the dome-headed dinosaurs like Pachycephalosaurus), and are normally remembered for their large horns and bone frills, often well ornamented. The first ceratopsians com from the Upper Jurassic of China, while neoceratopsians first appear in the Lower Cretaceous, with most more derived species being found in the Upper Cretaceous. Although best known for the horns and frills, ceratopsians didn't always have crazy ornamentation. Basal ceratopsians and early neoceratopsians didn't have any horns and had only very little frills. A new species of basal neoceratopsian, Mosaiceratops azumai has been named from the Upper Cretaceous of China, by Zheng et al. (2015). 

Mosaiceratops is significant because of it's mosaic of features reflected in the name, meaning "mosaic horned-face", the mosaic made of a number of characters previously found only in some other groups. A single partial skeleton is known, including part of the skull and a large portion of the post-cranial skeleton. 
Mosaiceratops fossil (a), drawing (b), and skeletal reconstruction (c) from Zheng et al. 2015
The interesting part about Mosaiceratops is the mosaic of features. While it is still considered to be a basal neoceratopsian, it is found quite late, as most basal ceratopsians are found during the Lower Cretaceous. Despite this, it has several plesiomorphic ("ancestral") characters that are not found in other basal neoceratopsians. It also contains features that were previously only found in psittacosaurids, among the basal ceratopsians and basal neoceratopsians. Mosaiceratops is not a psittacosaurid, meaning these features are not diagnostic to that group. One of these features includes the lack of teeth on the premaxilla. This is something seen in more derived neoceratopsians, and psittacosaurids, but not in the rest of the basal neoceratopsians, suggesting that in ceratopsian evolution, there had to be reversals somewhere. Zheng et al. (2015) suggest that lack of premaxillary teeth is a diagnostic feature of the Psittacosauridae and Neoceratopsia, and the animals neoceratopsians that have premaxillary teeth actually underwent a reversal to the primitive condition found in basal ceratopsians. If this is the case, this is one of the few times there has been a major reversal like this in dinosaur evolution, although the authors recognise their analyses are not as well supported as they could be. Additionally, Mosaiceratops has features that are in-between psittacosaurids and more derived neoceratopsians, which places it in this position in a phylogenetic analysis. 
Phylogenetic position of Mosaiceratops from Zheng et al. 2015
Mosaiceratops is extremely important in helping us understand early ceratopsian evolution. We are slowly starting to fill the holes and figure out how this clade evolved. This is yet another case that shows that evolution does not happen in any specific order. Animals can have a mosaic of features that are shared with those more primitive AND those more derived than them, and there is no clear direction of traits or evolution. 

If you want to learn more about ceratopsians, and another fairly recently named basal neoceratopsian, Aquilops, check out this Palaeocast episode with Andy Farke.