Evolution and extinction risks of sharks, rays, and skates

Sharks, skates, and rays (Elasmobranchii) form together with chimaeras (Holocephali) the last extant members of cartilaginous fish (class Chondrichthyes), an ancient group of jawed vertebrates that has been around for more than 400 million years and survived all five mass extinctions. During their evolution, they developed a diverse set of morphological and biological traits allowing them to cope with changes in their environment and to become important predators in a variety of different ecological occuppying top positions in food-webs. Today, the biodiversity on our planet is experiencing an anthropogenically driven extinction event, with extinction levels very similar to past mass extinctions. Despite being well-adapted predators, this new mass extinction event doesnt spare sharks, rays, and skates, with an estimated one third of all species already being threatened according to the IUCN Red List. Extinction of only a few species of this evolutionary distinctive clade can cause significant pruning of the tree of life with subsequent loss of millions of years of evolutionary history. Although being a versatile group, the mechanisms how elasmobranchs have coped with past mass extinctions, the drivers for the evolution of morphological traits and the correlated body plans, the tempo and timing of morphological trait evolution, whether appearance changes occurred regularly over time or were increased during times of extinction events, and how biological traits like body size have developed during their evolution, are not fully understood yet. To answer these questions, multidisciplinary approaches are necessary, in which various phylogenetic methods, molecular clock and other approaches for dating origins and evolutionary events as well as morphological data comprising skeletal and dental traits are combined with morphospace methods to estimate the relationships of taxa, duration of individual evolutionary lineages, lineage specific rates, divergence rates, evolutionary diversifications, fossilization and life history traits within a single coherent phylogenetic time tree. This time tree will be crucial for our understanding of the evolutionary history of elasmobranchs and the evolution of biological traits, which have helped elasmobranch fishes to become top predators of todays seas. Furthermore, the newfound understanding of the role of different biological traits on the survival and longevity of elasmobranchs in deep time can serve as basis, on which extinction risks and conservation priorities of distinct elasmobranch groups can be formulated within a strict evolutionary framework towards maintaining their morphological diversity and evolutionary history, and therefore ensuring the continued existence of this group in the future. Univ.-Prof Dr. J. Kriwet, J. Türtscher, MSc and P.L. Jambura, MSc of the University of Vienna with support of Dr. G. Naylor (Univ. Florida, Gainesville, U.S.A.) und Dr. G. Marramà (Univ. Turin, Italien) will conduct this project.

Elasmobranchii (sharks, rays, skates) are the last extant members of a previously highly diverse vertebrate group representing one of the two clades of jawed vertebrates. Living lineages originated ca. 200 million years ago, either in the latest Triassic or earliest Jurassic according to this project. They developed diverse morphological and biological traits resulting in their evolutionary success and enabeling them to occupy a variety of different ecological niches as meso- or apex predators. Recently, it has been demonstrated that extinction of only few species can cause significant loss of evolutionary history and environmental perturbations. As the project goals for better understanding the past as well as the current and future fate of these ecological important predators turned out to be very ambitious covering many different aspects, the project was reorganized into four work packages, i.e., Taxonomy & Systematics, Diversity Patterns, Conservation Aspects, and Morphology & Trait Evolution. In WP1 (14 contributions), many controversial aspects related to the taxonomic identity and systematic position of almost 50 extinct taxa, but also many extant rays could be solved. Data from this WP contributed largely to WP2 and WP4. Major conclusions from WP2 (10 contributions) and WP3 (six contributions) are that both abiotic (climate, plate tectonics, and varying atmospheric CO2 concentrations) as well as biotic factors (demographic patterns, physiology, occurrence of nursery areas) were key factors driving their diversity patterns in deep time, but not any competition patterns. This information will help to identify areas of major conservation concerns. WP4 (15 contributions) focused on specific morphological traits and their bearing on phylogenies (e.g., labial cartilages, neurocranium), shape analyses for identifying morphological characters useful for taxonomy and systematics (thus also contributing to WP1), and the evolution of traits (e.g., teeth, lower jaw, pectoral fins, body size) for better understanding macroevolutionary dynamics through time. All information from WPs 1, 2, 4 is necessary to establish the evolutionary distinctiveness (ED), which is a phylogenetic metric to ascribe conservation values by measuring a species' relative contribution to the evolutionary diversity and current extinction risk of taxa to identify species that are both evolutionarily distinct and globally endangered ("EDGE" species). With this approach, it is possible to identify taxonomic groups that require conservation priorities. The only available study up to now is inclusive as the systematic position and exact age of single taxon used for calibration was ambiguous (solved in this project). Three final studies are being finalised: 1) relationships of extinct freshwater rays, 2) macroevolutionary dynamics of morphological traits, and 3) a comprehensive EDGE analysis including all major extinct groups whose systematic position and exact stratigraphic age were determined in this project. These final studies will provide important insights for future species conservation.