Evolutionary pathways toward gigantism in sharks and rays

Through elasmobranch (sharks and rays) evolutionary history, gigantism evolved multiple times in phylogenetically distant species, some of which are now extinct. Interestingly, the world's largest elasmobranchs display two specializations found never to overlap: filter feeding and mesothermy. The contrasting lifestyles of elasmobranch giants provide an ideal case study to elucidate the evolutionary pathways leading to gigantism in the oceans. Here, we applied a phylogenetic approach to a global dataset of 459 taxa to study the evolution of elasmobranch gigantism. We found that filter feeders and mesotherms deviate from general relationships between trophic level and body size, and exhibit significantly larger sizes than ectothermic‐macropredators. We confirm that filter feeding arose multiple times during the Paleogene, and suggest the possibility of a single origin of mesothermy in the Cretaceous. Together, our results elucidate two main evolutionary pathways that enable gigantism: mesothermic and filter feeding. These pathways were followed by ancestrally large clades and facilitated extreme sizes through specializations for enhancing prey intake. Although a negligible percentage of ectothermic‐macropredators reach gigantic sizes, these species lack such specializations and are correspondingly constrained to the lower limits of gigantism. Importantly, the very adaptive strategies that enabled the evolution of the largest sharks can also confer high extinction susceptibility

Diversification of the Neoselachii (Chondrichthyes) during the Jurassic and Cretaceous

The Neoselachii are a monophyletic group including all of the extant sharks and rays. They underwent rapid diversification throughout the Jurassic and Cretaceous, going from low-diversity assemblages of members of extinct orders in the Late Triassic to diverse assemblages containing representatives of most extant clades by the end of the Cretaceous. The known fossil record of Mesozoic neoselachians is composed largely of isolated teeth, with articulated skeletal remains being known from a limited number of sites. The small tooth size of a large proportion of neoselachians, including almost all taxa in existence prior to the mid Cretaceous, led to very poor representation in older publications. Their state of knowledge has improved dramatically since 1970 with the increased use of bulk sampling for isolated dental remains. Despite this, the high proportion of Lazarus taxa from some stages suggests that the state of knowledge is still intermittent. Increase in assemblage diversity throughout the Jurassic and Cretaceous suggests that radiation events resulted in real and dramatic increases in diversity, and that the perceived diversification is not an artefact of poor knowledge. Cladogenesis inferred from the fossil record typically compares more favourably with divergence predicted from molecular analysis, where Batoidea form a discrete basal clade, than with divergence predicted from morphological analysis, where Batoidea are considered a derived crown group within the Squalea. The timing of diversification events is discussed in light of the known fossil record, cladistically generated divergence times, and the paleoenvironmental distribution of faunas

Functional diversity of sharks and rays is highly vulnerable and supported by unique species and locations worldwide

Elasmobranchs (sharks, rays and skates) are among the most threatened marine vertebrates, yet their global functional diversity remains largely unknown. Here, we use a trait dataset of >1000 species to assess elasmobranch functional diversity and compare it against other previously studied biodiversity facets (taxonomic and phylogenetic), to identify species- and spatial- conservation priorities. We show that threatened species encompass the full extent of functional space and disproportionately include functionally distinct species. Applying the conservation metric FUSE (Functionally Unique, Specialised, and Endangered) reveals that most top-ranking species differ from the top Evolutionarily Distinct and Globally Endangered (EDGE) list. Spatial analyses further show that elasmobranch functional richness is concentrated along continental shelves and around oceanic islands, with 18 distinguishable hotspots. These hotspots only marginally overlap with those of other biodiversity facets, reflecting a distinct spatial fingerprint of functional diversity. Elasmobranch biodiversity facets converge with fishing pressure along the coast of China, which emerges as a critical frontier in conservation. Meanwhile, several components of elasmobranch functional diversity fall in high seas and/or outside the global network of marine protected areas. Overall, our results highlight acute vulnerability of the world’s elasmobranchs’ functional diversity and reveal global priorities for elasmobranch functional biodiversity previously overlooked

Trends in Chondrichthyan Research: An Analysis of Three Decades of Conference Abstracts

Given the conservation status and ecological, cultural, and commercial importance of chondrichthyan fishes, it is valuable to evaluate the extent to which research attention is spread across taxa and geographic locations and to assess the degree to which scientific research is appropriately addressing the challenges they face. Here we review trends in research effort over three decades (1985–2016) through content analysis of every abstract (n = 2,701) presented at the annual conference of the American Elasmobranch Society (AES), the oldest and largest professional society focused on the scientific study and management of these fishes. The most common research areas of AES abstracts were reproductive biology, movement/telemetry, age and growth, population genetics, and diet/feeding ecology, with different areas of focus for different study species or families. The most commonly studied species were large and charismatic (e.g., White Shark, Carcharodon carcharias), easily accessible to long-term established field research programs (e.g., Lemon Shark, Negaprion brevirostris, and Sandbar Shark, Carcharhinus plumbeus), or easily kept in aquaria for lab-based research (e.g., Bonnethead Shark, Sphyrna tiburo). Nearly 90% of all described chondrichthyan species have never been mentioned in an AES abstract, including some of the most threatened species in the Americas. The proportion of female* first authors has increased over time, though many current female* Society members are graduate students. Nearly half of all research presented at AES occurred in the waters of the United States rather than in the waters of developing nations where there are more threatened species and few resources for research or management. Presentations based on research areas such as paleontology and aquarium-based research have declined in frequency over time, and identified research priorities such as social science and interdisciplinary research are poorly represented. Possible research gaps and future research priorities for the study of chondrichthyan fishes are also discussed

The first three dimensional fossils of Cretaceous sclerorhynchid sawfish Asflapristis cristadentis gen et sp nov and implications for the phylogenetic position of the Sclerorhynchoidei (Chondrichthyes)

A new fossil batoid (ray) Asflapristis cristadentis gen. et sp. nov. is described from six exceptionally well-preserved, three-dimensional skeletal remains from the Turonian (Late Cretaceous) of Morocco. Mechanical and acid preparation and CT scanning of these specimens reveal details of much of the proximal skeleton, especially the skull, synarcual and pectoral skeleton, with only the more distal parts of the skeleton missing. These fossils represent a relatively large animal (62 cm preserved length, estimated total length to approximately 2 meters) possessing a robust rostrum that lacks enlarged rostral denticles. It has a narrow and small chondrocranium with jaws that are relatively large compared to the rest of the skull and robust with highly ornamented teeth that lack cusps. The branchial skeleton shows a large second hypobranchial without anterior process which was probably fused to the basibranchial as in other sclerorhynchoids. The synarcual is large and lacks centra through its entire length, and with no direct connection to the pectoral girdle was observed. Pectoral fins probably possessed enlarged proximal elements (propterygium, mesopterygium and metapterygium), the reduced articulation facet between the coracoid with the pectoral elements was reduced. A phylogenetic analysis using both parsimony and bayesian methods was performed incorporating this new taxon. Both analyses recovered a phylogenetic topology that places the sclerorhynchoids in a close relation to rajoids and clearly separated from the morphologically similar Pristidae within the Rhinopristiformes. In respect to the extant taxa, the phylogenies generated are similar to that obtained in molecular analysis of modern batoids. The palaeoecological implication of this discovery suggests that the Asfla assemblage was not from a ‘normal’ open carbonate shelf but rather a restricted environment favouring a low diversity chondrichthyan fauna

Paleogene origin of Planktivory In The Batoidea

The planktivorous mobulid rays are a sister group to, and descended from, rhinopterid and myliobatid rays which possess a dentition showing adaptations consistent with a specialized durophageous diet. Within the Paleocene and Eocene there are several taxa which display dentitions apparently transitional between these extreme trophic modality, in particular the genus Burnhamia. The holotype of Burnhamia daviesi was studied through X-ray computed tomography (CT) scanning. Digital renderings of this incomplete but articulated jaw and dentition revealed previously unrecognized characters regarding the jaw cartilages and teeth. In addition, the genus Sulcidens gen. nov. is erected for articulated dentitions from the Paleocene previously assigned to Myliobatis. Phylogenetic analyses confirm Burnhamia as a sister taxon to the mobulids, and the Mobulidae as a sister group to Rhinoptera. Shared dental characters between Burnhamia and Sulcidens likely represent independent origins of planktivory within the rhinopterid – myliobatid clade. The transition from highly-specialized durophagous feeding morphologies to the morphology of planktivores is perplexing, but was facilitated by a pelagic swimming mode in these rays and we propose through subsequent transition from either meiofauna-feeding or pelagic fish-feeding to pelagic planktivory

Determining how risk effects predator-prey interactions of marine communities in the nearshore environment of South Bimini, The Bahamas

Predators often have strong top-down effects on ecosystems and are considered a priority for conservation and management. Predator activity can influence prey distribution, abundance, and foraging behaviors and are likely to influence habitat by impacting ecological and environmental characteristics as well as presence of competitor species. There are knowledge gaps of the functional diversity of fish assemblages, non-consumptive predator effects, and environmental effects on fish assemblages. With this study, effects of top marine predators, such as sharks and great barracuda, on diversity and abundance of prey communities were examined in putative low (north side of South Bimini = lagoon) and high-risk (south side of South Bimini = flat) areas around South Bimini, The Bahamas. Baited remote underwater video surveys (BRUVs) deployed in the nearshore habitat captured abundance and potential predator-prey interactions. Predator and prey abundances at each site were compared to determine potential risk affect within high and low risk environments. A general baseline of predator and prey species was established throughout six months of observation (January- June 2018). Results showed a difference in prey communities between high and low risk habitats. Teleost abundance was highest on the south side of South Bimini. There were no differences in flight behavior of prey from predator (sharks vs barracuda). Longitude, depth, temperature, salinity, and dissolved oxygen were significantly linked to biotic assemblages. The identification of significant factors influencing predator-prey interaction is important in understanding community composition and for future implementation of conservation and management practices pertaining to nearby mangrove and seagrass habitats

Micro computed tomography imaging reveals the development of a unique tooth mineralization pattern in mackerel sharks (Chondrichthyes; Lamniformes) in deep time

The cartilaginous fishes (Chondrichthyes) have a rich fossil record which consists mostly of isolated teeth and, therefore, phylogenetic relationships of extinct taxa are mainly resolved based on dental characters. One character, the tooth histology, has been examined since the 19th century, but its implications on the phylogeny of Chondrichthyes is still in debate. We used high resolution micro-CT images and tooth sections of 11 recent and seven extinct lamniform sharks to examine the tooth mineralization processes in this group. Our data showed similarities between lamniform sharks and other taxa (a dentinal core of osteodentine instead of a hollow pulp cavity), but also one feature that has not been known from any other elasmobranch fish: the absence of orthodentine. Our results suggest that this character resembles a synapomorphic condition for lamniform sharks, with the basking shark, Cetorhinus maximus, representing the only exception and reverted to the plesiomorphic tooth histotype. Additionally, †Palaeocarcharias stromeri, whose affiliation still is debated, shares the same tooth histology only known from lamniform sharks. This suggests that †Palaeocarcharias stromeri is member of the order Lamniformes, contradicting recent interpretations and thus, dating the origin of this group back at least into the Middle Jurassic

Increasing morphological disparity and decreasing optimality for jaw speed and strength during the radiation of jawed vertebrates

The Siluro-Devonian adaptive radiation of jawed vertebrates, which underpins almost all living vertebrate biodiversity, is characterized by the evolutionary innovation of the lower jaw. Multiple lines of evidence have suggested that the jaw evolved from a rostral gill arch, but when the jaw took on a feeding function remains unclear. We quantified the variety of form in the earliest jaws in the fossil record from which we generated a theoretical morphospace that we then tested for functional optimality. By drawing comparisons with the real jaw data and reconstructed jaw morphologies from phylogenetically inferred ancestors, our results show that the earliest jaw shapes were optimized for fast closure and stress resistance, inferring a predatory feeding function. Jaw shapes became less optimal for these functions during the later radiation of jawed vertebrates. Thus, the evolution of jaw morphology has continually explored previously unoccupied morphospace and accumulated disparity through time, laying the foundation for diverse feeding strategies and the success of jawed vertebrates