Periodic Reporting for period 1 - PhyloPycno (Unravelling early chelicerate evolution and the origin of the sea spiders combining high quality paleontological and genomic data)
Période du rapport: 2021-10-01 au 2023-09-30
The fossil record of sea spiders comprises only 11 species from Silurian to Jurassic. These fossils are only poorly understood, due to the limited means of previous studies. The PhyloPycno project aims to reexamine two major sea spider fossil sites, La Voulte sur Rhône (Jurassic, ~160 million years old) and the Hunsrück slate (Devonian, ~400 million years old). By using cutting-edge palaeontology tools such as X-ray microtomography and Reflectance Transformation Imaging (RTI), we aim to deepen our understanding of their morphoanatomy. Additionally, we intend to position these fossils in the pycnogonid Tree of Life and use this information, along with phylogenetics and molecular clock tools, to date critical evolutionary steps in Pycnogonida's history.
Conclusions of the action are as follows:
- Jurassic pycnogonids of La Voulte sur Rhône all belong to Pantopoda; Colossopantopodus boissinensis belongs to Colossendeidae, Palaeoendeis elmii to Endeidae, while Palaeopycnogonides gracilis belongs to a newly described family, Palaeopycnogonididae.
- Devonian pycnogonids of the Hunsrück Slate do not belong to Pantopoda. They can be divided into two clades, one including the swimming pycnogonids (Palaeoisopus problematicus and Pentapantopus vogteli) and the one with a divided femur (Palaeopantopus maucheri and Flagellopantopus blocki).
- (Haliestes dasos( belong the swimming pycnogonids clade.
- Pantopoda diversified around 485-353 million years ago, i.e. between early Ordovician and early Carboniferous.
Based on these new descriptions, I produced a standardised morphological dataset coding for the morphological characters of each fossil as well as modern pycnogonids and other past and present arthropods. I used this dataset in conjunction with available molecular datasets to reconstruct the phylogeny of pycnogonids and replace them within the Tree of Life of chelicerates. I also used the fossils as calibration points for a molecular clock, allowing to investigate the timing of the evolution of pycnogonids.
These results show that the lineage that gave birth to all the extant pycnogonid fauna (referred to as pantopods) is a sister group to the Devonian and Silurian pycnogonids. The Jurassic pycnogonids derive from the same lineage as the extant fauna and belong therefore to pantopods. Two of the Jurassic fossils I studied belong to two extant pycnogonid families, while a third one belongs to a family that went extinct since. Molecular clock analyses show that pantopods diversified at some point between the Late Cambrian and the Early Devonian. These results were the focus on two publications, two conference talks and one poster. More will come.