Crocodylomorph cranial shape evolution and its relationship with body size and ecology.

Pedro L Godoy

doi:10.1111/jeb.13540

Abstract

Crocodylomorpha, which includes living crocodylians and their extinct relatives, has a rich fossil record, extending back for more than 200 million years. Unlike modern semi-aquatic crocodylians, extinct crocodylomorphs exhibited more varied lifestyles, ranging from marine to fully terrestrial forms. This ecological diversity was mirrored by a remarkable morphological disparity, particularly in terms of cranial morphology, which seems to be closely associated with ecological roles in the group. Here, I use geometric morphometrics to comprehensively investigate cranial shape variation and disparity in Crocodylomorpha. I quantitatively assess the relationship between cranial shape and ecology (i.e. terrestrial, aquatic, and semi-aquatic lifestyles), as well as possible allometric shape changes. I also characterize patterns of cranial shape evolution and identify regime shifts. I found a strong link between shape and size, and a significant influence of ecology on the observed shape variation. Terrestrial taxa, particularly notosuchians, have significantly higher disparity, and shifts to more longirostrine regimes are associated with large-bodied aquatic or semi-aquatic species. This demonstrates an intricate relationship between cranial shape, body size and lifestyle in crocodylomorph evolutionary history. Additionally, disparity-through-time analyses were highly sensitive to different phylogenetic hypotheses, suggesting the description of overall patterns among distinct trees. For crocodylomorphs, most results agree in an early peak during the Early Jurassic and another in the middle of the Cretaceous, followed by nearly continuous decline until today. Since only crown-group members survived through the Cenozoic, this decrease in disparity was likely the result of habitat loss, which narrowed down the range of crocodylomorph lifestyles.

Highlights

The relationship between form and function has long been recognised (Cuvier, 1817; Russell, 1916; Lauder, 1981) and, given the phenotypic similarities generated by convergence, the incorporation of phylogenetic comparative methods has become almost imperative on analyses of evolutionary shape changes (Bookstein et al, 1985; Felsenstein, 1985; Harvey &Pagel, 1991; Rohlf, 2001, 2002; Losos, 2011; Monteiro, 2013)
Further examination of this relationship using an independent body size dataset provided very similar results, with a significant correlation between shape and size. These results indicate that body size is a strong predictor of cranial shape in the group
This is consistent with what was found in previous geometric morphometric studies (e.g. Pierce et al, 2008; Sadleir & Makovicky, 2008; Pierce et al, 2009; Piras et al, 2009; Young et al, 2010; Foth et al, 2017a; Wilberg, 2017; Godoy et al, 2018), indicating that this region of the skull has the highest morphological variation

Summary

Introduction

The relationship between form and function has long been recognised (Cuvier, 1817; Russell, 1916; Lauder, 1981) and, given the phenotypic similarities generated by convergence, the incorporation of phylogenetic comparative methods has become almost imperative on analyses of evolutionary shape changes (Bookstein et al, 1985; Felsenstein, 1985; Harvey &Pagel, 1991; Rohlf, 2001, 2002; Losos, 2011; Monteiro, 2013). With the current expansion of the use of geometric morphometrics techniques for analysing shape variation, studies that investigate the relationship between shape and either size or ecology (or both), while taking a phylogenetic approach, have become increasingly common (Adams et al, 2004; Zelditch et al, 2012). In this context, data collected from fossil organisms can yield essential information for a better comprehension of large-scale evolutionary shape changes. The crocodylomorph skull has received substantial attention in anatomical studies (Iordansky, 1973), which might explain the preference for this part of the skeleton as the source of morphological information in most works quantitatively investigating phenotypic evolution in the group (even though some important exceptions exist; e.g. Bonnan et al, 2008; Chamero et al, 2013, 2014; Stubbs et al, 2013;Walmsley et al, 2013; Toljagić & Butler, 2013; Gold et al, 2014)

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