Disparity versus diversity in ankylosaurid dinosaurs: explored morphospace indicates two separate evolutive radiations

Marco Romano (1)
(1) - Dipartimento di Scienze della Terra, Sapienza Università di Roma, P.le Aldo Moro, 5, 00185, Rome, Italy. Corresponding author e-mail: marco.romano@uniroma1.it

Ankylosaurid ankylosaurs were armoured wide-bodied, rotund herbivores ornithischian dinosaurs notable for their weaponized tails and the presence of peculiar osteoderms. In this contribution I present the first diversity vs. disparity analysis in ankylosiaurids, using as a base the dataset from a recent published phylogeny for the clade. As already found recently in stegosaurs, the disparity as total variance results quite decoupled with respect to diversity through time, whereas the disparity obtained as sum of ranges appear very well coupled with the trend in diversity. Considering the disparity as sum of ranges as an indication of the total amount of the occupied morphospace the results indicate that, in the ankylosaurids, to an increase in the number of taxa corresponds a proportional increase in the explored morphospace. The study shows that the attainment of the maximum morphospace saturation in ankylosiaurids is at least bimodal. This indicates two separate evolutionary radiations for ankylosaurids, one between Albian and Cenomanian and a second one in the Campanian, separated by a crisis both in terms of number of species and of explored morphologies. Ankylosaurids were thus characterized by quite low diversity an morphological disparity well before the final extinction at the K-Pg boundary.

dinosaurs, disparity, morphospace, Mesozoic, evolutionary radiation, Cretaceous.