Patterns of divergence in the morphology of ceratopsian dinosaurs: sympatry is not a driver of ornament evolution

Abstract

Establishing the origin and function of unusual traits in fossil taxa provides a crucial tool in understanding macroevolutionary patterns over long periods of time. Ceratopsian dinosaurs are known for their exaggerated and often elaborate horns and frills, which vary considerably between species. Many explanations have been proposed for the origin and evolution of these 'ornamental' traits, from predator defence to socio-sexual dominance signalling and, more recently, species recognition. A key prediction of the species recognition hypothesis is that two or more species possessing divergent ornamental traits should have been at least partially sympatric. For the first time to our knowledge, we test this hypothesis in ceratopsians by conducting a comparison of the morphological characters of 46 species. A total of 350 ceratopsian cladistic characters were categorized as either 'internal', 'display' (i.e. ornamental) or 'non display'. Patterns of diversity of these characters were evaluated across 1035 unique species pairs. Display characters were found to diverge rapidly overall, but sympatric species were not found to differ significantly in their ornamental disparity from non-sympatric species, regardless of phylogenetic distance. The prediction of the species recognition hypothesis, and thus the idea that ornamentation evolved as a species recognition mechanism, has no statistical support among known ceratopsians.

Competition structured a Late Cretaceous megaherbivorous dinosaur assemblage

“…It may be further possible to test whether more closely related (i.e., more ecomorphologically similar) taxa are more likely to go extinct as a result of sympatry, bolstering the character displacement hypothesis. Megaherbivore coexistence was evidently not a major evolutionary driver of cranial ornamentation during the Late Cretaceous 161 , but its role in promoting ecomorphological disparity via competition cannot yet be discounted.…”

Morphological innovation and the evolution of hadrosaurid dinosaurs

“…An important question in the evolutionary history of dinosaurs, particularly in the hadrosaurids, is how and why the great disparity of exaggerated cranial structures evolved (Hopson 1975; Evans et al 2009; Evans 2010; Prieto-Márquez and Wagner 2013; Gates et al 2016; Knapp et al 2018). Comparative anatomy has revealed the anatomical transformations involved in producing these complex structures.…”

“…Ceratopsids also possessed conspicuous and highly disparate cranial ornamentation in the form of nasal and postorbital horns and caudodorsally expanded parieto-squamosal frills (Brown and Henderson 2015). These structures likely functioned as displays in socio-sexual signaling (Hone et al 2011; Brown and Henderson 2015; Knapp et al 2018), as well as in combat and for the attachment of jaw musculature (Maiorino et al 2017). As in hadrosaurids, the postcranial skeleton in neoceratopsians is morphologically conservative (Dodson et al 2004).…”

A subadult individual of Styracosaurus albertensis (Ornithischia: Ceratopsidae) with comments on ontogeny and intraspecific variation in Styracosaurus and Centrosaurus.

“…parietals. It is worth noting that these differences in patterns of ontogenetic development (i.e., heterochrony) between these closely related taxa are largely restricted to the most diagnostic features, which have been hypothesized to be evolving under sociosexual selection (Sampson 1997(Sampson , 1999Padian and Horner 2011;Knell et al 2013;Knapp et al 2018).…”