Shark mandible evolution reveals patterns of trophic and habitat-mediated diversification

Three-dimensional fossils of a Cretaceous collared carpet shark (Parascylliidae, Orectolobiformes) shed light on skeletal evolution in galeomorphs

A rich fossil record of teeth shows that many living shark families' origins lie deep in the Mesozoic. Skeletal fossils of the sharks to whom these teeth belonged are far rarer and when they are preserved are often flattened, hindering understanding of the evolutionary radiation of living shark groups. Here we use computed tomography to describe two articulated Upper Cretaceous shark skeletons from the Chalk of the UK preserving three-dimensional neurocrania, visceral cartilages, pectoral skeletons and vertebrae. These fossils display skeletal anatomies characteristic of the Parascylliidae, a family of Orectolobiformes now endemic to Australia and the Indo Pacific. However, they differ in having a more heavily mineralized braincase and a tri-basal pectoral fin endoskeleton, while their teeth can be attributed to a new species of the problematic taxon Pararhincodon. Phylogenetic analysis of these new fossils confirms that Pararhincodon is a stem-group parascylliid, providing insight into the evolution of parascylliids' distinctive anatomy during the late Mesozoic-Cenozoic shift in orectolobiform biodiversity from the Northern Atlantic to the Indo Pacific. Meanwhile both Pararhincodon and extant parascylliids have a distinctive vertebral morphology previously described only in Carcharhiniformes, contributing a skeletal perspective to the picture emerging from macroevolutionary analyses of coastal, small-bodied origins for galeomorphs.

Drivers of diversification in sharks and rays (Chondrichthyes: Elasmobranchii)

Elasmobranchs (sharks and rays) are a charismatic lineage of unquestionable ecological importance in past and present marine ecosystems. Represented by over 1200 species, elasmobranchs have undergone substantial shifts in taxonomic diversity since their origin. Quantifying these diversification trends and their underlying causes improves our understanding of macroevolutionary processes and the factors influencing community composition through deep time. Studies addressing drivers of diversification in Elasmobranchii have yielded conflicting results; some report clear relationships between specific traits and diversification events, whilst others fail to find support for such relationships. There is also some evidence to suggest that biotic interactions or environmental factors (global climatic change and tectonic events) have shaped elasmobranch diversification dynamics. In this review, we summarise the diversification dynamics of elasmobranchs over their evolutionary history, before considering the evidence for the three principal hypothesised drivers of diversification in this clade: trait evolution, biotic interactions, and environmental change. Finally, we discuss major limitations in the field, and how discordant methodologies and data sources hamper our current understanding of diversification in Elasmobranchii. Whilst future studies will undoubtedly be required to further unravel this complex relationship, no single factor can be considered the sole satisfactory explanation for observed deep time diversification trends in Elasmobranchii to the exclusion of the other.

Ecological erosion and expanding extinction risk of sharks and rays

The true state of ocean biodiversity is difficult to assess, and there are few global indicators to track the primary threat of overfishing. We calculated a 50-year Red List Index of extinction risk and ecological function for 1199 sharks and rays and found that since 1970, overfishing has halved their populations and their Red List Index has worsened by 19%. Overfishing the largest species in nearshore and pelagic habitats risks loss of ecomorphotypes and a 5 to 22% erosion of functional diversity. Extinction risk is higher in countries with large human coastal populations but lower in nations with stronger governance, larger economies, and greater beneficial fisheries subsidies. Restricting fishing (including incidental catch) and trade to sustainable levels combined with prohibiting retention of highly threatened species can avert further depletion, widespread loss of population connectivity, and top-down predator control.

Ecological drivers of jaw morphological evolution in lepidosaurs

Ecology is a key driver of morphological evolution during adaptive radiations, but alternative factors like phylogeny and allometry can have a strong influence on morphology. Lepidosaurs, the most diverse clade of tetrapods, including lizards and snakes, have evolved a remarkable variety of forms and adapted to disparate ecological niches, representing an ideal case study to understand drivers of morphological evolution. Here, we quantify morphological variation in the lower jaw using three-dimensional geometric morphometrics on a broad sample of 153 lepidosaur species. Our results suggest that phylogeny has significantly influenced mandibular shape evolution, and snakes have diverged from a lizard-like jaw morphology during their evolution. Allometry and ecological factors like diet, foraging mode and substrate also appear to drive the diversification of mandibular forms. Ecological groups differ in patterns of disparity, convergence and rates of evolution, indicating that divergent evolutionary mechanisms are responsible for the acquisition of different diets and habitats. Our analyses support that lepidosaurs ancestrally use their jaws to capture prey, contrary to the traditional view favouring lingual prehension as ancestral. Specialized or ecologically diverse lineages show high rates of jaw shape evolution, suggesting that morphological innovation in the mandible has contributed to the spectacular ecomorphological diversification of lepidosaurs.

Evolutionary trends in the elasmobranch neurocranium

The neurocranium (braincase) is one of the defining vertebrate characters. Housing the brain and other key sensory organs, articulating with the jaws and contributing to the shape of the anteriormost portion of the body, the braincase is undoubtedly of great functional importance. Through studying relationships between braincase shape and ecology we can gain an improved understanding of form-function relationships in extant and fossil taxa. Elasmobranchii (sharks and rays) represent an important case study of vertebrate braincase diversity as their neurocranium is simplified and somewhat decoupled from other components of the cranium relative to other vertebrates. Little is known about the associations between ecology and braincase shape in this clade. In this study we report patterns of mosaic cranial evolution in Elasmobranchii that differ significantly from those present in other clades. The degree of evolutionary modularity also differs between Selachii and Batoidea. In both cases innovation in the jaw suspension appears to have driven shifts in patterns of integration and modularity, subsequently facilitating ecological diversification. Our results confirm the importance of water depth and biogeography as drivers of elasmobranch cranial diversity and indicate that skeletal articulation between the neurocranium and jaws represents a major constraint upon the evolution of braincase shape in vertebrates.

The origins and drivers of sexual size dimorphism in sharks

While sexual size dimorphism (SSD) is abundant in nature, there is huge variation in both the intensity and direction of SSD. SSD results from a combination of sexual selection for large male size, fecundity selection for large female size and ecological selection for either. In most vertebrates, it is variation in the intensity of male-male competition that primarily underlies variation in SSD. In this study, we test four hypotheses regarding the adaptive value of SSD in sharks-considering the potential for each of fecundity, sexual, ecological selection and reproductive mode as the primary driver of variation in SSD between species. We also estimate past macroevolutionary shifts in SSD direction/intensity through shark phylogeny. We were unable to find evidence of significant SSD in early sharks and hypothesise that SSD is a derived state in this clade, that has evolved independently of SSD observed in other vertebrates. Moreover, there is no significant relationship between SSD and fecundity, testes mass or oceanic depth in sharks. However, there is evidence to support previous speculation that reproductive mode is an important determinant of interspecific variation in SSD in sharks. This is significant as in most vertebrates sexual selection is thought to be the primary driver of SSD trends, with evidence for the role of fecundity selection in other clades being inconsistent at best. While the phylogenetic distribution of SSD among sharks is superficially similar to that observed in other vertebrate clades, the relative importance of selective pressures underlying its evolution appears to differ.

Drivers of morphological evolution in the toothed whale jaw

Toothed whales (odontocetes) emit high-frequency underwater sounds (echolocate)-an extreme and unique innovation allowing them to sense their prey and environment. Their highly specialized mandible (lower jaw) allows high-frequency sounds to be transmitted back to the inner ear. Echolocation is evident in the earliest toothed whales, but little research has focused on the evolution of mandibular form regarding this unique adaptation. Here, we use a high-density, three-dimensional geometric morphometric analysis of 100 living and extinct cetacean species spanning their ∼50-million-year evolutionary history. Our analyses demonstrate that most shape variation is found in the relative length of the jaw and the mandibular symphysis. The greatest morphological diversity was obtained during two periods of rapid evolution: the initial evolution of archaeocetes (stem whales) in the early to mid-Eocene as they adapted to an aquatic lifestyle, representing one of the most extreme adaptive transitions known, and later on in the mid-Oligocene odontocetes as they became increasingly specialized for a range of diets facilitated by increasingly refined echolocation. Low disparity in the posterior mandible suggests the shape of the acoustic window, which receives sound, has remained conservative since the advent of directional hearing in the aquatic archaeocetes, even as the earliest odontocetes began to receive sounds from echolocation. Diet, echolocation, feeding method, and dentition type strongly influence mandible shape. Unlike in the toothed whale cranium, we found no significant asymmetry in the mandible. We suggest that a combination of refined echolocation and associated dietary specializations have driven morphology and disparity in the toothed whale mandible.