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.

Keywords: Ceratopsia; Dinosauria; evolution; ornamentation; species recognition; sympatry.

Figure 1.

Line drawings of ceratopsian skulls in simplified phylogeny to illustrate morphological diversity of cranial ornaments within the clade. (a) Liaoceratops yangzigouensis; (b) Protoceratops andrewsi; (c) Centrosaurus apertus; (d) Achelousaurus horneri; (e) Pachyrhinosaurus canadensis; (f) Chasmosaurus belli; (g) Triceratops horridus. Node 1 represents the clade Coronosauria, containing all taxa with enlarged frills. Node 2 represents the clade Ceratopsoidea, encompassing Centrosaurinae (orange branch) and Chasmosaurinae (blue branch), within which the majority of cranial ornamental diversity, and all horned taxa, are found. Lower image: full-body illustration of Styracosaurus albertensis (Centrosaurinae) with highlighted examples of the three different character classes used in this study (refer to the electronic supplementary material for full list of characters).

Figure 2.

Pairwise comparison plots for sympatry categories (rows a–e) and character classes (columns i–iii). Species pairs that do not fall into sympatry categories are shown in light grey in rows b to e for reference. Second-order polynomial regressions fitted with confidence intervals are set at 95%.

Figure 3.

Distributions of second-order polynomial model output parameters for 10 000 randomly sampled n species pairs of internal (a and d), display (b and e), and other visual (c and f) character classes. Randomly sampled values are shown in grey. Values calculated for sympatry categories are overlaid in coloured points for each character class (green: sympatric; orange: allopatric; red: pseudo-sympatric; blue: contemporary).