Evolutionary dynamics of Eocene Antarctic cartilaginous fishes

The project applied for here aims at documenting and analysing the biotic effects of both short-term and long-term climate and palaeogeographic changes in Antarctica. We focus on the taxonomic composition and diversity dynamics of Eocene Antarctic chondrichthyan fishes (Holocephali, Elasmobranchii), which will serve as model organisms for evolutionary patterns in high-latitudes. An Argentine-Swedish field party (SWEDARP) assembled the material that forms the basis of this project (> 3.500 specimens comprising isolated dental plates, teeth, placoid scales, etc.) in the years 2011 2013. All material comes from the Eocene La Meseta Fm. (Ypresian Priabonian) on Seymour Island (Antarctic Peninsula) and is the most complete collection of chondrichthyans from the Palaeogene of the Southern Hemisphere in terms of number of specimens, taxonomic covering and stratigraphic distribution. Thus, it enables us to employ rigorous methods for investigating various aspects of the evolutionary history of these important Eocene chondrichthyan assemblages for the first time. Studying originations, extinctions, diversity and diversification patterns and the palaeoecology of chondrichthyans in combination with extrinsic factors, which might influence evolutionary processes (e.g., climatic changes, palaeogeographic constellations) throughout the Eocene until the thermal and geographic isolation of Antarctica will not only provide deeper insights into adaptive and evolutionary patterns of high-latitude cartilaginous fishes but also into the development and probably the origin of the conspicuous modern-day Antarctic fish fauna (absent holocephalans and sharks). It also will enable us testing previous hypotheses stating, for instance, that there is a continuous diversity increase until the middle Eocene and that the absence of chondrichthyans in the uppermost Eocene of Antarctica is the result of the onset of the thermal isolation of the Antarctic continent. The first part of the project is devoted to analysing the taxonomic composition, the palaeoecology, and faunal relationships of the chondrichthyan assemblages on regional and global scales. Diversity and diversification patterns of chondrichthyans from all stratigraphic levels of the La Meseta Fm. are established during the second part of the project. Rigorous and state- of-the-art analytical methods will be employed in both parts. Diversity dynamics will be analysed by ascertaining various standard measures (e.g., standing diversity, species richness, eveness, numbers of Lazarus, holdover and carryover taxa, diversification, origination, extinction, and turnover rates), indices (e.g., rarefied diversity, Simpson`s D, Shannon-Wiener H, Margalef) and new shareholder quorum methods (estimates diversity based on a homogenous coverage of the rank-abundance distribution). Results then will be correlated with abiotic factors (climatic fluctuations, environmental changes, plate-tectonic constellations) to provide generalized evolutionary models and to identify extrinsic factors for speciation / extinctions events for marine vertebrates in high-latitudes.

It is widely accepted that abiotic factors such as global climate change, sea levels, and geographic changes affect marine fish diversity patterns. The extent and severity of these environmental shifts, however, still are ambiguous and not well established. The few available studies for extant marine fishes suggest that global climate change results in latitudinal and/or depth range shifts. These shifts subsequently lead to local extinctions but also invasions. Speciation effects related to climate changes, however, remain unknown. To test the effects of abiotic factors (climate change, sea-level variations, palaeogeographic constellations) on diversity and diversification patterns of marine fishes we analysed new and abundant fossil remains of cartilaginous fishes (sharks, rays, skates, chimeroids) from Antarctica. Antarctica, located today in the Southern Ocean, is one of the most remote and coldest places in the world. Antarctica plays a key-element in understanding both Earth processes and global climate changes, because this continent maintained its palaeogeographic position more or less unchanged over long periods of Earth history and moreover preserves the highest number of marine fossils from one of the most important time periods world-wide, the Eocene (56 33 million years ago) in the Southern Hemisphere. This period witnessed major transitions from greenhouse to icehouse climates and was marked by short-term climate maxima followed by a significant drop in temperature resulting in extensive glaciations of Antarctica at the end of the Eocene. New material from this important time interval of Antarctica enabled us to identify diverse cartilaginous fish assemblages throughout the Eocene covering all major climatic and palaeogeographic episodes and to create a verified database of almost 3,500 detailed stratigraphic and geographic occurrences. We employed robust statistic approaches to better understand diversity, extinction, origination, and diversification patterns during the Eocene on local (e.g., Antarctica) and regional (e.g., Southern Hemisphere) scales and correlated the results with abiotic factors to identify drivers of diversity fluctuations. The detailed analyses of age distributions of cartilaginous fishes throughout the Eocene in Antarctica reveals that each time-interval comprises different faunal compositions that relates to global climatic changes rather than to palaeogeographic or habitat settings. Global climate also is the main diversity driver in cartilaginous faunas on the regional scale. Sea level also influences their diversity dynamics to some extent, but sea levels also are dependant of global climates. It also is evident that highest diversity and diversification (origination) rates in the Southern Hemisphere as well as in Antarctica occurred in the Eocene during times of significantly elevated global temperatures resulting in extensive greenhouse conditions. Increase of global temperatures, however, also triggered subordinate local extinctions, which mainly affected cold-water adapted taxa. The main conclusions from this study thus indicate that increasing sea-surface temperatures result in long-term elevated local and regional origination rates at species level and high endemisms, while declining temperatures cause local and regional extinctions and latitudinal shifts. The evidence obtained from this study also helps to make future predictions about cartilaginous fish diversity patterns related to increasing global sea- surface temperatures and consequently might assist in informing policy-making.