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

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.

Angel sharks (Squatinidae): A review of biological knowledge and exploitation

“…Whilst angel sharks are a morphologically conservative group (Compagno, 1984;López-Romero et al, 2020), morphological characteristics that are typically used to distinguish angel shark species include the structure of barbels on the nasal flap, the shape and relative size of the pectoral fins, and dental formula (e.g., Acero Castro-Aguirre et al, 2006;Milessi et al, 2001;Vooren & Da Silva, 1991), as well as colouration and a range of morphological relationships (e.g., the eye-spiracle distance). The structure of dermal denticles along the median line of the dorsal surface (which may be enlarged and thorn-like) have often been used as identification features, but there can also be ontogenetic differences in these denticles (e.g., Vaz & de Carvalho, 2013), which may limit their utility as diagnostic features.…”

Evolutionary trends in the elasmobranch neurocranium

“…We tested for evolutionary integration/modularity using the geomorph functions phylo.modularity, phylo.integration and globalIntegration [24]. We tested six different hypotheses of modularity, ranging from 2-5 modules, adapting module partitions presented by López-Romero et al [13]. Details of these partitions can be found in the supplementary materials (Supplementary Table S3).…”

“…Thus, determining the ecomorphological relevance of neurocranial shape variation in extant forms can inform paleobiological interpretations, and aid in better understanding the paleoecology of extinct elasmobranch taxa. Unfortunately, existing ecomorphological studies of neurocranial evolution encompass less than 5% of extant elasmobranch diversity [5,13]. These studies are by no means representative of Elasmobranchii as a whole, and thus further studies incorporating additional data are necessary.…”

The genetics‐morphology‐behavior trifecta: Unraveling the single greatest limitation affecting our understanding of chondrichthyan evolution

“…Of particular relevance to morphological evolution, lack of integration between genetic and morphological studies constrains understanding of the extent to which evolutionary modularity/integration influences chondrichthyan morphological evolution. Both are known to have played important roles in other clades, and recently studies have begun to assess the potential for modular/integrated morphological evolution in some chondrichthyan taxa (Klingenberg, 2008 ; López‐Romero et al, 2020 ). It is now understood that the extent of evolutionary integration in morphological structures depends upon several factors including the extent of shared gene regulatory networks, genetic linkage at loci underlying morphological variation, and ecological trait correlations (Brakefield, 2006 ; Lande & Arnold, 1983 ; Saltz et al, 2017 ).…”