Periodic Reporting for period 1 - DEADSharks (Divergence times, Evolution, and Anatomy Deciphered in early Sharks)
Reporting period: 2022-12-01 to 2024-11-30
Elasmobranchs (sharks & rays) are of great interest both to evolutionary biologists, as one of the most evolutionarily distinct groups of jawed vertebrates, and to conservationists, being important in food-webs but vulnerable to extinction. However, the timing of the evolution of living elasmobranch biodiversity remains poorly understood, in part due to a heavy reliance on the fossil record of isolated teeth to reconstruct the timing of the elasmobranch tree of life.
In this project I aimed: (1) to use computed tomographic (CT) methods to reconstruct and redescribe the internal skeletons of key fossil elasmobranchs from the Mesozoic three-dimensionally; (2) incorporate these data into morphological phylogenetic analyses to give insight into the relationships of these extinct elasmobranchs; (3) combine this with genomic data in a first total-evidence tip-dating approach to the elasmobranch tree of life; and (4) combine shape data on the lower jaws of these taxa with data from extant elasmobranchs to understand how and when elasmobranch feeding diversity evolved.
In this project I aimed: (1) to use computed tomographic (CT) methods to reconstruct and redescribe the internal skeletons of key fossil elasmobranchs from the Mesozoic three-dimensionally; (2) incorporate these data into morphological phylogenetic analyses to give insight into the relationships of these extinct elasmobranchs; (3) combine this with genomic data in a first total-evidence tip-dating approach to the elasmobranch tree of life; and (4) combine shape data on the lower jaws of these taxa with data from extant elasmobranchs to understand how and when elasmobranch feeding diversity evolved.
The first stage of data collection in this project was the collection of CT data targeting Mesozoic elasmobranch fossil taxa, especially from the Cretaceous Chalk of the UK. To do this I visited several museum collections in London, Oxford, Cambridge, and Brighton, identified candidate taxa for CT scanning, and then CT scanned them at the Natural History Museum, London, and the University of Warwick. Using these datasets I visualised and described the endoskeletons of two elasmobranch taxa: Synechodus (a now extinct taxon with mysterious relationships) and Pararhincodon (related to living carpet sharks). These new data allow these taxa to be incorporated into phylogenies, and bolstering the value of the Cretaceous elasmobranch tooth record by allowing 3D skeletal morphologies to be linked directly to teeth.
I built morphological phylogenetic datasets using the literature and notes from museum specimens, and incorporating morphological information from these taxa. These suggest that Synechodus is a stem-group galeomorph, and that Pararhincodon is a stem-group collared carpet shark (parascylliid). These placements have implications for elasmobranch evolutionary history, for the relationships of Synechodus and relatives (Synechodontiformes) and for the evolution of modern carpet shark anatomy.
I assembled genomic data for a range of elasmobranch taxa in the morphological dataset. These datasets were gathered from GenBank and spanned 15 mitochondrial genes and 1 nuclear gene which I aligned and concatenated. I analysed these data using a total-evidence tip-dating approach with the aim of establishing the age of major divergences in early elasmobranch evolution independently from teeth. The results of this suggest that these divergences occurred more recently than predicted from previously published estimates based on teeth.
Using the data I had collected I was able to create 3D models of the lower jaws of several Cretaceous shark taxa. I compared these to the jaw shapes of a range of living sharks, with the aim of understanding the evolution of feeding ecology in sharks. Those Cretaceous sharks with close relationships to living forms show extremely similar jaw shapes to them. However, the lower jaws of the extinct shark Synechodus appear similar to those of carcharhiniforms despite of no close relationship, suggesting to be the result of convergent evolution and probably filling a similar ecological niche.
I built morphological phylogenetic datasets using the literature and notes from museum specimens, and incorporating morphological information from these taxa. These suggest that Synechodus is a stem-group galeomorph, and that Pararhincodon is a stem-group collared carpet shark (parascylliid). These placements have implications for elasmobranch evolutionary history, for the relationships of Synechodus and relatives (Synechodontiformes) and for the evolution of modern carpet shark anatomy.
I assembled genomic data for a range of elasmobranch taxa in the morphological dataset. These datasets were gathered from GenBank and spanned 15 mitochondrial genes and 1 nuclear gene which I aligned and concatenated. I analysed these data using a total-evidence tip-dating approach with the aim of establishing the age of major divergences in early elasmobranch evolution independently from teeth. The results of this suggest that these divergences occurred more recently than predicted from previously published estimates based on teeth.
Using the data I had collected I was able to create 3D models of the lower jaws of several Cretaceous shark taxa. I compared these to the jaw shapes of a range of living sharks, with the aim of understanding the evolution of feeding ecology in sharks. Those Cretaceous sharks with close relationships to living forms show extremely similar jaw shapes to them. However, the lower jaws of the extinct shark Synechodus appear similar to those of carcharhiniforms despite of no close relationship, suggesting to be the result of convergent evolution and probably filling a similar ecological niche.
The key impacts of this project are scientific and feed into our understanding of the evolution of elasmobranchs, a major group of vertebrates. This project has generated a wealth of 3D data on the skeletons of Mesozoic elasmobranchs, many of which will require further research. As these are published these will have a major impact on our understanding of shark endoskeletal anatomy in the Mesozoic. A second key impact is the revised understanding of timings in elasmobranch evolution, which I expect to have a major impact on our understanding of elasmobranch evolutionary history and how it relates to abiotic events in the early Mesozoic. Finally, these data are the starting point for opening up questions about the origins of shark feeding ecology based on skeletons rather than just the tooth record.
In the longer term revised estimates of timing in the early elasmobranch tree of life resulting from this project may impact conservation prioritisation metrics that incorporate evolutionary distinctiveness.
In the longer term revised estimates of timing in the early elasmobranch tree of life resulting from this project may impact conservation prioritisation metrics that incorporate evolutionary distinctiveness.