Evolutionary trends of the conserved neurocranium shape in angel sharks (Squatiniformes, Elasmobranchii)

Zerina Johanson,Sebastian Stumpf,Jürgen Kriwet,Cathrin Pfaff,Giuseppe Marramà,Faviel A López-Romero

doi:10.1038/s41598-020-69525-7

Zerina Johanson, Sebastian Stumpf + Show 4 more

Open Access

https://doi.org/10.1038/s41598-020-69525-7

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Abstract

Elasmobranchii (i.e., sharks, skates, and rays) forms one of the most diverse groups of marine predators. With a fossil record extending back into the Devonian, several modifications in their body plan illustrate their body shape diversity through time. The angel sharks, whose fossil record dates back to the Late Jurassic, some 160 Ma, have a dorsoventrally flattened body, similar to skates and rays. Fossil skeletons of this group show that the overall morphology was well established earlier in its history. By examining the skull shape of well-preserved fossil material compared to extant angel sharks using geometric morphometric methods, within a phylogenetic framework, we were able to determine the conservative skull shape among angel sharks with a high degree of integration. The morphospace occupation of extant angel sharks is rather restricted, with extensive overlap. Most of the differences in skull shape are related to their geographic distribution patterns. We found higher levels of disparity in extinct forms, but lower ones in extant species. Since angel sharks display a highly specialized prey capture behaviour, we suggest that the morphological integration and biogeographic processes are the main drivers of their diversity, which might limit their capacity to display higher disparities since their origin.

Highlights

IntroductionElasmobranchs (sharks, skates, and rays) represent a lineage of vertebrates with a fossil record extending over 400 million years[1,2]
Elasmobranchs represent a lineage of vertebrates with a fossil record extending over 400 million years[1,2]
The goals of this study are to (1) explore the disparity of both †Pseudorhina and Squatina, and among the clades within extant Squatina (Fig. 1A), (2) observe whether clade arrangement is reflected in morphospace occupation, (3) whether the neurocranial shape bears a phylogenetic signal for the shape variation, and (4) whether noticeable morphological evolutionary rate changes occurred or did the neurocranial shape remain constant through time

Summary

Introduction

Elasmobranchs (sharks, skates, and rays) represent a lineage of vertebrates with a fossil record extending over 400 million years[1,2]. Most modern sharks display rather conservative overall body s hapes[5], for most of the pelagic species, and distinguishable from the benthic species[6], some specific morphological traits like the neurocranium may vary strongly in shape, as seen in the hammerhead and sawsharks[7,8,9,10] Another example of extreme morphology is displayed by angel sharks (Squatiniformes), which are characterized by highly dorsoventrally compressed bodies similar to batomorphs (i.e., skates and rays). We tested whether the neurocranium could be divided into multiple modules to investigate which of these might contribute to the diversity observed in extant clades, and we examined whether the level of integration among modules restricts morphological evolution Taken together, this will provide a better understanding of diversification patterns within this group of unusual sharks

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