Sea spiders, also known as pycnogonids, belong to a group of marine arthropods, like insects, millipedes, centipedes, spiders, and scorpions. Despite their name, they are not spiders. Presently, there are about 1,400 known species of pycnogonids distributed across 11 families. They all share a similar body plan, placing them under the same Order, Pantopoda. They are now widely accepted as chelicerates (such as spiders, scorpions, and horseshoe crabs) thanks to molecular phylogeny, but unambiguous morphological evidence is still missing. Sea spiders are regarded as an early offshoot among arthropods. Therefore, understanding the early evolution of pycnogonids means unravelling the most diversified phylum on Earth. Finding morphological evidence to support the position of Pycnogonida is also paramount to understand the phylogenetic position of many ambiguous arthropod fossils. But extant pycnogonids are morphologically very divergent from other arthropods, making this task difficult. The fossil record can however provide further insight.
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.
