Do different disparity proxies converge on a common signal? Insights from the cranial morphometrics and evolutionary history of Pterosauria (Diapsida: Archosauria)

Abstract

Disparity, or morphological diversity, is often quantified by evolutionary biologists investigating the macroevolutionary history of clades over geological timescales. Disparity is typically quantified using proxies for morphology, such as measurements, discrete anatomical characters, or geometric morphometrics. If different proxies produce differing results, then the accurate quantification of disparity in deep time may be problematic. However, despite this, few studies have attempted to examine disparity of a single clade using multiple morphological proxies. Here, as a case study for this question, we examine the disparity of the volant Mesozoic fossil reptile clade Pterosauria, an intensively studied group that achieved substantial morphological, ecological and taxonomic diversity during their 145+ million‐year evolutionary history. We characterize broadscale patterns of cranial morphological disparity for pterosaurs for the first time using landmark‐based geometric morphometrics and make comparisons to calculations of pterosaur disparity based on alternative metrics. Landmark‐based disparity calculations suggest that monofenestratan pterosaurs were more diverse cranially than basal non‐monofenestratan pterosaurs (at least when the aberrant anurognathids are excluded), and that peak cranial disparity may have occurred in the Early Cretaceous, relatively late in pterosaur evolution. Significantly, our cranial disparity results are broadly congruent with those based on whole skeleton discrete character and limb proportion data sets, indicating that these divergent approaches document a consistent pattern of pterosaur morphological evolution. Therefore, pterosaurs provide an exemplar case demonstrating that different proxies for morphological form can converge on the same disparity signal, which is encouraging because often only one such proxy is available for extinct clades represented by fossils. Furthermore, mapping phylogeny into cranial morphospace demonstrates that pterosaur cranial morphology is significantly correlated with, and potentially constrained by, phylogenetic relationships.

The Good, the Bad, and the Ugly: The Influence of Skull Reconstructions and Intraspecific Variability in Studies of Cranial Morphometrics in Theropods and Basal Saurischians

“…long-snouted spinosaurids for basal Tetanurae), an increase of the interspecific variation even in rather closely related forms would also be expected. This is supported by several studies on crustacean, pterosaur and coelurosaur diversity for instance, which all show that disparity of larger taxonomic clades is higher than in the respective internal subclades (see 4,42–44). On the other hand, an overlap of intraspecific and interspecific variation in closely related taxa has also been demonstrated for instance in the cranial shape of recent Hominoidea [24], the osteology of skinks [45] or in molecular sequences of different bilaterian clades (e.g.…”

Slow and steady: the evolution of cranial disparity in fossil and recent turtles

“…A slow, steady increase of disparity in concert with a slow build-up of diversity, as opposed to an explosive, early increase of either, is indicative of the concordant evolution of these two diversity measures [68]. Delayed peaks of disparity have otherwise been found for pterosaurs [39,60,69,70] and the dentition of ungulates [71] and carnivoramorphan mammals [72], but most other animal groups previously investigated reach their maximum disparity early in evolution [68,73,74]. Global climate reconstructions point towards a gradual rise in temperatures from the Jurassic to the Palaeocene [66,67], which generally correlates with our observed increase in disparity within the same time window.…”

A Jurassic pterosaur from Patagonia and the origin of the pterodactyloid neurocranium

“…Pterosaurs first appeared in the Late Triassic and went on to achieve high levels of morphologic and taxonomic diversity during the Mesozoic, with more than 150 species recognized so far (Barrett et al, 2008; Butler, Benson & Barrett, 2013; Foth, Brusatte & Butler, 2012; Witton, 2013; Benson et al, 2014). They have traditionally been divided into two major groups, “rhamphorhynchoids” (a paraphyletic assemblage of basal pterosaurs) and pterodactyloids (Plieninger, 1901).…”