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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.

The Fezouata Shale has yielded abundant remains of ancient marine organisms, including hundreds of radiodont specimens. Frontal appendages are the most commonly preserved body parts of radiodonts, and their well-preserved anatomical characters are crucial for describing taxonomic diversity at the species level, while also providing essential data on mode of life, paleoecology, and feeding behaviour. In a study led by PhD student Gaëtan Potin and published today in Frontiers in Ecology and Evolution, we examined more than a hundred radiodont frontal appendages specimens from Fezouata curated in the collections of the Musée Cantonal de Géologie de Lausanne and the Yale Peabody Museum, allowing us to revise Aegirocassis Van Roy et al., 2015, the first and only named radiodont yet described from the Fezouata Shale, and to identify several new taxa, most belonging to family Hurdiidae.

We document new details of differentiated endites in Aegirocassis benmoulai Van Roy et al., 2015 and propose an emended diagnosis for this taxon. Additionally, using X-ray computed tomography (CT-scanning) we were able to reconstruct a specimen showing several spinose fragments of the enigmatic fossil Pseudoangustidontus Van Roy & Tetlie 2006, articulated together as endites attached to podomeres of an appendage with an arrangement typical for hurdiids. We also identify a new second species of this genus, Pseudoangustidontus izdigua.

A 3D model of the CT-scanned Pseudoangustidontus izdigua specimen that can be manipulated at will!

Finally, we investigated the ecology of these organisms. The thin, densely-packed auxiliary spines of elongated endites indicate that they were suspension feeders. Among all the examined specimens, we identify suspension-feeding as the feeding strategy for 112 specimens, and sediment-sifting for 9 specimens that possess endites bearing auxiliary spines that are shorter and more robust with a wider interspace between them, forming a coarse, resistant mesh for sifting through sediment for infaunal prey. 

Altogether, our study shows that radiodonts in the Fezouata Shale were highly diverse and employed a variety of feeding strategies. The abundance of suspension feeders may be linked to the “Ordovician Plankton Revolution”, which saw a huge radiation in plankton diversity during the Great Ordovician Biodiversification Event. It also highlights the decline of active raptorial predation in radiodonts, as suggested by their absence, so far, in the Fezouata Shale Formation.

Reference: Potin G.J.-M., Gueriau P. & Daley A.C. 2023. Radiodont frontal appendages from the Fezouata Biota (Morocco) reveal high diversity and ecological adaptations to suspension-feeding during the Early Ordovician. Frontiers in Ecology and Evolution 11, 1214109. Find the article (Open Access) here

We are pleased to welcome Nora Corthésy to our lab. Nora will be doing decay experiments on various animal models to constrain preservation biases in the rock record, particularly during the Cambrian Explosion.

Gaëtan Potin and Allison Daley published today in Frontiers in Earth Science an exhaustive review paper focusing on radiodonts, an iconic Cambro-Ordovician arthropod group. This open-access article offers an in-depth summary of all research conducted on Anomalocaris, the earliest known apex predator, and its affiliated taxa. Many aspects are discussed, such as diversity, evolutionary implication, paleobiogeography and stratigraphic repartition… If you want to know more about these amazing animals, check out the article following this link: https://www.frontiersin.org/articles/10.3389/feart.2023.1160285/full.