The remarkable convergence of skull shape in crocodilians and toothed whales

Abstract

The striking resemblance of long-snouted aquatic mammals and reptiles has long been considered an example of morphological convergence, yet the true cause of this similarity remains untested. We addressed this deficit through three-dimensional morphometric analysis of the full diversity of crocodilian and toothed whale (Odontoceti) skull shapes. Our focus on biomechanically important aspects of shape allowed us to overcome difficulties involved in comparing mammals and reptiles, which have fundamental differences in the number and position of skull bones. We examined whether diet, habitat and prey size correlated with skull shape using phylogenetically informed statistical procedures. Crocodilians and toothed whales have a similar range of skull shapes, varying from extremely short and broad to extremely elongate. This spectrum of shapes represented more of the total variation in our dataset than between phylogenetic groups. The most elongate species (river dolphins and gharials) are extremely convergent in skull shape, clustering outside of the range of the other taxa. Our results suggest the remarkable convergence between long-snouted river dolphins and gharials is driven by diet rather than physical factors intrinsic to riverine environments. Despite diverging approximately 288 million years ago, crocodilians and odontocetes have evolved a remarkably similar morphological solution to feeding on similar prey.

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

“…Snout length can provide useful insights into ecological specializations (Taylor, 1987; Busbey, 1995; Brochu, 2001; McHenry et al ., 2006; Pierce et al ., 2008; Walmsley et al ., 2013) and recent examinations of crocodylomorph cranial functional morphology further indicate strong ecological selective pressures on the snout, particularly those arising from feeding behaviour (McHenry et al ., 2006; Gignac & O’Brien, 2016; McCurry et al ., 2017 a ; Ballel et al ., 2019; Gignac et al ., 2019). Longer snouts are traditionally associated with a piscivorous diet, since the tip of the snout moves faster through water, facilitating the capture of small prey such as fish (Thorbjarnarson, 1990; McHenry et al ., 2006; Walmsley et al ., 2013; McCurry et al ., 2017 b ).…”

“…Previous works that use geometric morphometrics for studying crocodylomorph cranial shape have mostly focused on specific subgroups, especially crocodylians (Monteiro et al ., 1997; Pierce et al ., 2008; Sadleir & Makovicky, 2008; Piras et al ., 2009, 2010, 2014; Pearcy & Wijtten, 2011; Watanabe & Slice, 2014; Okamoto et al ., 2015; Clarac et al ., 2016; Salas-Gismondi et al ., 2016, 2018; Iijima, 2017; McCurry et al ., 2017 a ; Foth et al ., 2017; Bona et al ., 2018; Fernandez Blanco et al ., 2018; Morris et al ., 2019), but also thalattosuchians (Pierce et al ., 2009; Young et al ., 2010) and notosuchians (Godoy et al ., 2018). One important exception is the recent work of Wilberg (2017), that assessed cranial shape variation across Crocodyliformes (which is only slightly less inclusive than Crocodylomorpha; Irmis et al ., 2013), sampling a large number of species.…”

Convergence and functional evolution of longirostry in crocodylomorphs

“…In line with biomechanical evidence, a recent study demonstrates that the convergent evolution of long snouts in crocodilians and river dolphins was driven by dietary specialization (McCurry et al . ). These ideas led to the interpretation of many extinct longirostrine taxa as piscivores.…”

Trait independence primes convergent trait loss

“…; McCurry et al. ). However, developmental dependencies among traits can contribute to convergent evolution by limiting the phenotypic variation available to selection (Arthur ; Beldade et al.…”

“…The repeated, independent evolution of traits (convergent evolution) is often attributed to shared environmental selection pressures (Rüber and Adams 2001;Harmon et al 2005;Rosenblum 2005;Edwards et al 2012;McCurry et al 2017). However, developmental dependencies among traits can contribute to convergent evolution by limiting the phenotypic variation available to selection (Arthur 2002;Beldade et al 2002;Kavanagh et al 2013).…”