Unravelling early chelicerate evolution and the origin of the sea spiders combining high quality paleontological and genomic data

Project Information

PhyloPycno

Grant agreement ID: 101023218

DOI

10.3030/101023218

Project closed

EC signature date 19 March 2021

Start date 1 October 2021

End date 30 September 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 212 933,76

EU contribution

€ 212 933,76

Coordinated by

UNIVERSITY OF BRISTOL
United Kingdom

Periodic Reporting for period 1 - PhyloPycno (Unravelling early chelicerate evolution and the origin of the sea spiders combining high quality paleontological and genomic data)

Reporting period: 2021-10-01 to 2023-09-30

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

I examined 58 fossil plates from five collections in Berlin, Bonn, Mainz, Munich, and Paris. I used X-ray microtomography to investigate the internal parts hidden by the surrounding sediment, and I used RTI to analyse in detail the structure on the surface of fossils. I also had the occasion to collaborate with Derek Siveter (University of Oxford), Derek Briggs (University of Yale), David Siveter (University of Leicester) and Mark Sutton (Imperial College), to examinate two new fossils of the Silurian species Haliestes dasos (~425 million years old). These data provided many new information about the eight species investigated, allowing for new formal descriptions and often, new taxonomic assignments.
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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

During the project I have achieved a complete review of eight of the 11 pycnogonid fossils, presenting new interpretations of the morphological features of these species which help to understand both their biology and their phylogenetic relationships. This review provides lot of information for future research in evolution and palaeontology of chelicerates. In addition, I reconstructed the first robust phylogeny of Pycnogonida including fossils. With the dating of the phylogeny of pantopods, this research provides the first comprehensive insight of the evolutionary history of this poorly studied group.

The fossil record of pycnogonids

Palaeoisopus problematicus from the Devonian. Scale bar: 2 cm. Picture Romain Sabroux, ©NHMMZ, Mainz

Palaeopycnogonides gracilis from the Jurassic. Scale bar: 2 cm. Picture Noël Podevigne, ©MNHN, Paris