Author: Pierre Gueriau

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

Today’s arthropods often show very complex development with their juveniles and larvae living and feeding in a different way than the adults. A classical example is a flying butterfly with an edacious caterpillar or a sessile barnacle with a floating larva. But was such a complex kind of development present in the early members of Arthropoda? In a study led by former ANOM Lab member Lukas Laibl, now at the Institute of Geology of the Czech Academy of Sciences, we tackled the question by studying a few millimeters-long larvae of 480 million years old arthropod species from Morocco belonging to an extinct group called marrellids, which thrived in early Paleozoic seas. The results are presented in a paper published today in Frontiers in Ecology and Evolution.

To better understand the anatomy of this Paleozoic animal, we imaged the tiniest larvae using synchrotron computed tomography (CT) scanning at the Paul Scherrer Institut in Switzerland. Thanks to the high-resolution data obtained and meticulous segmentation, we have been able to virtually extract and render in three dimensions a tiny, about 2 mm long, specimen on a micrometer scale, allowing us to recognize morphological details, from tiny claws to delicate hairs on the animal legs.

Detailed examination of the appendages of this early arthropod larvae tells a lot about its mode of life. On the head, it had sensory antennae, a pair of robust legs used to orientate its body or to anchor itself on the sediment, and a pair of walking limbs. Two additional pairs of legs on the trunk were also used for walking, while the rear limbs of the trunk bore spines and delicate hairs and were used for capturing small organic particles that the animal was eating. All trunk limbs also had a gill branch used for respiration.

Interestingly the adult stages of various marrellids seem to have a very similar mode of life as these tiny babies, contrary to what is seen in many recent arthropods. We also show that the tiny larvae even lived in the same locations and environment as the adults of the same species, on the seafloor at the margin of an ancient continent called Gondwana, just below the storm wave base.

Marrellids are considered to be very early arthropods, showing many ancestral features. We therefore think that such development, with no major change in feeding and ecology between larvae and adults, was ancestral for arthropods as a whole. Other ancient arthropod groups present a similar development, thus supporting our conclusions. This indicates that complex life cycles must have evolved independently later in several arthropod groups.

Reference: Laibl L., Gueriau P., Saleh F, Pérez-Peris F., Lustri L., Drage H.B., Bath Enright O.G., Potin G. & Daley A.C. 2023. Early developmental stages of a Lower Ordovician marrellid from Morocco suggest simple ontogenetic niche differentiation in early euarthropods. Frontiers in Ecology and Evolution 11, 1232612. Find the article (Open Access) here

In an article published today in Biology Letters, AnomLab postdoc Pierre Gueriau and his collaborators have resolved an 80-year-old mystery by deciphering the true nature of a 400-million-year-old marine creature from Germany. Since its initial description in 1943, this fossil, named Gilsonicaris rhenanus, has perplexed scientists who have alternatively interpreted it as an anostracan crustacean (‘fairy or brine shrimp’), a myriapod, or even a part of a starfish arm. Using a 3D X-ray scanner, the team reveals that Gilsonicaris is, in fact, a polychaete annelid (‘bristle worm’). This discovery unequivocally dismisses the existence of marine anostracans 400 million years ago, while also offering a wealth of new information regarding the early evolutionary history of bristle worms and their soft tissues.

A 3D model that can be manipulated at will is available on Sketchfab.

Reference: Gueriau P., Parry L.A. & Rabet N. 2023. Gilsonicaris from the Lower Devonian Hunsrück slate is a eunicidan annelid and not the oldest crown anostracan crustacean. Biology Letters 19: 20230312. Find the article (Open Access) here.