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Modern scorpions, spiders and horseshoe crabs belong to the vast lineage of arthropods, which appeared on Earth nearly 540 million years ago. More precisely, they belong to a subphylum that includes organisms equipped with pincers used notably for biting, grasping prey, or injecting venom –the chelicerae, hence their name chelicerates. But what are the ancestors of this very specific group? This question has puzzled paleontologists ever since the study of ancient fossils began. It has been impossible to identify with certainty any form among early arthropods that shared enough similarities with modern species to be considered ancestors. The mystery was further compounded by the lack of fossils available for the key period between -505 and -430 million years ago, which would have facilitated genealogical investigation.

In a chapter of Lorenzo Lustri’s PhD thesis, published today in Nature Communications, we identify the synziphosurine from the ~480-million-year-old Fezouata Shale in Morocco as a pivotal fossil that fills the gap between modern chelicerates and species from the Cambrian period (505 million years ago). Fossils from the Fezouata Shale, including that synziphosurine, were discovered in the early 2000s. Many fossils have already undergone extensive analysis, yet despite being one of the most most abundant fossil in the deposit, this one had never been described before. Measuring between 5 and 10 millimeters in size, it has been named Setapedites abundantis. This animal makes it possible, for the first time, to trace the entire lineage of chelicerates, from the appearance of the earliest arthropods to modern spiders, scorpions and horseshoe crabs.

To obtain these results Lorenzo studied more than a hundred fossils, a couple of which were imaged using synchrotron X-ray computed tomography (CT) scanning at the TOMCAT beamline of the Swiss Light Source (Paul Scherrer Institut, Villigen, Switzerland) to reconstruct their anatomy in detail and in three dimensions.

By combining a detailed anatomical description of Setapedites abundantis with comparisons to numerous fossil and extant chelicerates as well as Cambrian arthropods, and using robust phylogenetic methods, we establish a new monophyletic clade at the base of Chelicerata, which also includes the Silurian synziphosurines Dibasterium and Offacolus. This new clade links Chelicerata with the Cambrian arthropod Habellia optata from the Burgess Shale, clarifying the origin of chelicerates and particularly the evolution of chelicerate gill opercula, which are shown to have originated from Habelia-like thoracic exopods.

Furthermore, we also identified potential homologies with the problematic vicissicaudatan arthropods, suggesting that Setapedites abundantis might have more secrets to reveal. This could lead to a deeper understanding of the early evolution of the chelicerate group and its relationships with other arthropods whose affinities remain highly debated.

Reference: Lustri L., Gueriau P. & Daley A.C. 2024. Lower Ordovician synziphosurine reveals early euchelicerate diversity and evolution. Nature Communications 15, 3808. Find the article (Open Access) here

On March 23rd, Allison and Pierre joined ardent amateur paleontologists Sylvie and Éric Monceret in Cabrières for the recording of an episode of the weekly science radio program Science in Action produced by the BBC World Service.

The episode, which was broadcasted on March 28th, can be listened here. The part about the Cabrières Biota starts around 16:21.

Today, the ANOM Lab, part of an international team led by ANOM Lab’s Farid Saleh and Bertrand Lefebvre from Lyon University, introduces a newly uncovered fossil site in the French department of Hérault, thanks to the perseverance of two dedicated amateur palaeontologists, Sylvie and Éric Monceret. The 470-million-year-old fossils, over 400 in number, unearthed by the couple at the so-called Cabrières site, near Pézenas, are presented in an article published in Nature Ecology & Evolution. 

This site is particularly important because it opens a unique snapshot on ecosystems that were located the closest to the Ordovician south pole hitherto discovered. Among the most bountiful and diverse fossil beds from that geologic period, the Cabrières site stands out for its remarkable degree of preservation, which enabled the rare discovery of soft-bodied organisms such as worms, a wide variety of algae and sponges, contributing to a better understanding of the pivotal role these highly decay-prone organisms played in ancient ecosystems.

Our findings challenge previous thinking that posited a decrease in biodiversity or even an extinction between the Cambrian and Ordovician Periods, approximately 485 million years ago. The high level of biodiversity observed does, however, confirm the hypothesis of species’ migration toward the Ordovician southern hemisphere, seeking refuge from the excessive temperatures of the tropical zones. 

We also conducted a first investigation of the preservation of the fossils using X-ray fluorescence major-to-trace elemental mapping and X-ray absorption spectroscopy at the PUMA beamline of SOLEIL synchrotron on a couple specimens. We established that the fossils are essentially made up of a layer of Fe(III) oxide/hydroxide crystals, varying in thickness and containing traces of metals. The distribution of other elements, notably manganese and arsenic, strongly suggests that iron oxide (hydr)oxides do not represent the original mode of preservation of these fossils, but rather result from more recent alteration processes, as demonstrated for other contemporary deposits. This highlights the biases that have affected the completeness of the newly discovered ecosystem, and enables finer comparisons with other fossil sites.

This initial study of the site inaugurates a research program that is to span several years and include large-scale excavations followed by in-depth analyses of fossils, using innovative imaging techniques. The aim is to detail the fossilized organisms’ internal and external anatomies, taxonomic relationships, and modes of life.

Reference: Saleh F., Lustri L., Gueriau P., Potin G.J.-M., Pérez-Peris F., Laibl L., Jamart V., Vite A., Antcliffe J.B., Daley A.C., Nohejlová M., Dupichaud C., Schöder S., Bérard E., Lynch S., Drage H.B., Vaucher R., Vidal M., Monceret E., Monceret S. & Lefebvre B. 2024. The Cabrières Biota (France) provides insights into Ordovician polar ecosystems. Nature Ecology and Evolution 8, 651–662. Find the article (Open Access) here

ANOM Lab members published 4 articles on the Frontiers research topic “Insights on the Rise of Animal Life from Cambro-Ordovician Lagerstätten”. 3 articles report on fossil arthropods from the Fezouata Shale in Morocco. Potin et al. describe new suspension-feeding radiodonts from the Fezouata Shale (Fig. A), and discuss their ecological and evolutionary implications. Drage et al. document the moulting behavior of the Moroccan marrellid (Fig. B) and compare it to that of other marrellomorphs and modern arthropods. Laibl et al. also focus on marrellids, but this time characterizes the morphological changes occurring during their development based a nice set of juvenile and adult specimens including tiny immature stages (Fig. C). The fourth article, by Lustri et al., explores the environmental evolution of xiphosurids (Fig. D), and compare it to other euchelicerates groups.

Additional digital ressources:

A 3D model, based on µCT data, of one of the frontal appendage of the new taxon described by Potin et al. can be seen and manipulated online on Sketchfab.

A video presenting the three-dimensional rendering of the smallest immature stage of the Fezouata marrellid can be viewed here.

Last July, Jonathan Pople successfully submitted and defended his Master’s thesis in Earth Sciences at the University of Lausanne. He carried out his thesis work over one year at the ANOM Lab, studying the ecology of giant radiodonts and other arthropods from the Early Ordovician Fezouata Shale of Morocco, and notably demonstrated that living radiodonts often carried hitchhiking brachiopods on their oversized carapaces, not unlike barnacles found on whales today. Jonathan’s work was recently awarded with 3 prizes: the 2023 Best Master Thesis Award from the Swiss Geological Society for the best MSc thesis in Earth Sciences completed at a Swiss academic institution; the Prize Auguste Lombard 2023 from the Ecole Lémanique des Sciences de la Terre (shared between the Universities of Lausanne and Geneva) rewarding both the excellence of his Master’s thesis in Geology and the quality of his university studies, and the 2023 Master’s thesis Faculty Prize from the Faculty of Geosciences and Environment at the University of Lausanne.