The fossil record provides compelling examples of heterochrony at macroevolutionary scales such as the peramorphic giant antlers of the Irish elk. Heterochrony has also been invoked in the evolution of the distinctive cranial frill of ceratopsian dinosaurs such as Triceratops. Although ceratopsian frills vary in size, shape, and ornamentation, quantitative analyses that would allow for testing hypotheses of heterochrony are lacking. Here, we use geometric morphometrics to examine frill shape variation across ceratopsian diversity and within four species preserving growth series. We then test whether the frill constitutes an evolvable module both across and within species, and compare growth trajectories of taxa with ontogenetic growth series to identify heterochronic processes. Evolution of the ceratopsian frill consisted primarily of progressive expansion of its caudal and caudolateral margins, with morphospace occupation following taxonomic groups. Although taphonomic distortion represents a complicating factor, our data support modularity both across and within species. Peramorphosis played an important role in frill evolution, with acceleration operating early in neoceratopsian evolution followed by progenesis in later diverging cornosaurian ceratopsians. Peramorphic evolution of the ceratopsian frill may have been facilitated by the decoupling of this structure from the jaw musculature, an inference that predicts an expansion of morphospace occupation and higher evolutionary rates among ceratopsids as indeed borne out by our data. However, denser sampling of the meager record of early-diverging taxa is required to test this further.
Bibliographical noteFunding Information:
We are grateful to Leonardo Maiorino, Andrew Farke and Matt Lamana for providing digital images of and to Joseph A. Frederickson and Michael K. Brett‐Surman for sharing pictures of specimens. Carl Mehling, John Scannela, Patricia C. Burke, Peter Sheehan, Joshua Mathews, and Dallas Evans provided access to ceratopsian specimens under their care. Thanks also to Khishigjaw Tsogtbaatar for sharing locality data on MPC specimens of . Christian P. Klingenberg and Dean Adams provided advice on the use of MorphoJ and computation of the CR coefficient, respectively, while Andrew Meade kindly provided advice on BayesTraits. Ali Navabizadeh provided useful insights on ceratopsian temporal and jaw musculature. This project was supported by a Bass Postdoctoral Fellowship from the Field Museum of Natural History and the Ramón y Cajal program from the Ministry of Economy, Industry and Competitivity of Spain (RyC‐2015‐17388) presented to APM. Data collection was also supported by NSF EAR 0418648 to PJM. PJM is currently supported by NSF awards PLR 1341645 and FRES 1925884. JGP is supported by a Juan de la Cierva Fellowship from the Ministry of Economy, Industry and Competitivity of Spain (FJCI‐2014‐20380). Additional support was also provided by the CERCA Programme of the Generalitat de Catalunya. Protoceratops Centrosaurus P. andrewsi
© 2020 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd
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