A 500-million-year-old clawed beast rewrites the origins of spiders and crabs.

It had been a long day teaching Rudy Lerosey-Aubril. As a reward, he returned to clean the Cambrian arthropod fossils he had just discovered for analysis. At first, the model showed all the expected characteristics of its time – however, something was missing. In place of the antenna, claws appeared.

“Claws are out of place in a Cambrian arthropod,” said Lerosey-Aubril, “It took me a few minutes to realize that I had just uncovered the oldest chelicera ever found.”

In a study published in NatureResearcher Rudy Lerosey-Aubril and Assistant Professor Javier Ortega-Hernández, Curator of Invertebrate Paleontology at the Museum of Comparative Zoology – both in the Department of Organismic and Evolutionary Biology at Harvard – explain Megachelicerax cousteaui500-million-year-old sea monster discovered in Utah’s Western Desert. It is the oldest known chelicerate, an arthropod group that includes spiders, scorpions, horseshoe crabs, and sea spiders. The discovery pushes back the evolutionary history of chelicerates by 20 million years.

Lerosey-Aubril noted: “These fossils provide evidence for the origin of Cambrian chelicerates, and show that the blueprint for spiders and crabs was already emerging by 500 million years ago.”

Lerosey-Aubril spent more than 50 hours carefully cleaning the fossil under a microscope using a fine needle to reveal its terrifying shape. To a little over 8 inches tall, M. cousteaui it retains a backbone with a head shield and nine body segments. These two species have unique appendages: six pairs of specialized limbs for feeding and sensing in the head shield, and plate-like breathing parts on the underside of the body that resemble the gills of modern horse crabs.

However, its unusual feature is its chelicera – the pincer-like feeding appendages that define the subphylum Chelicerata and distinguish spiders from insects. While insects have sensory antennae as their main appendages, chelicerates have grasping apparatus, usually venomous. Despite the rich fossil record of the Cambrian, no chelicera-bearing arthropod from that time has been found – until now.

Before this discovery, the oldest known chelicerate belonged to the Fezouata Biota of the Early Ordovician of Morocco, about 480 million years ago. The presence of M. cousteaui 20 million years earlier it was an offshoot of the chelicerate family, an important evolutionary line that combines Cambrian arthropods that appear to lack chelicera with a much smaller crab-like chelicerate known as synziphosurines.

“Megachelicerax it shows that the chelicera and the division of the body into two specialized functional parts existed before the head parts lost their external branches and resembled the legs of spiders today,” explains Ortega-Hernández, “in agreement with several competing theories; in a way, everyone was right.”

The fossils take an important step in the assembly of the chelicerate body plan, revealing that key elements were already present after the Cambrian explosion – a period of rapid evolutionary activity.

“This tells us that in the middle of the Cambrian, when the rates of evolution were incredibly high, the seas already had arthropods with a structure that rivals the modern species,” Ortega-Hernández added.

Interestingly, the first discovery of this complex structure did not immediately lead to environmental control or diversification. Instead, chelicerates remained invisible for millions of years, covered by seemingly simple groups like trilobites, before successfully colonizing the soil.

Lerosey-Aubril said: “Similar evolutionary patterns have been reported in other animal groups. This shows that the success of evolution is not only about biological creativity – time and environmental conditions.”

M. cousteaui was collected in the middle Cambrian Wheeler Formation of Utah’s House Range. The fossil was discovered by noted fossil collector Lloyd Gunther and given to the Kansas University Biodiversity Institute and Natural History Museum in 1981 for further study. It was among a collection of seemingly unremarkable fossils from Utah that Lerosey-Aubril volunteered to investigate as part of her research on early arthropods.

Megachelicerax cousteaui named after the French explorer Jacques-Yves Cousteau. Lerosey-Aubril – who is also French – and Ortega-Hernández chose Cousteau for his work to raise awareness of the beauty and vulnerability of the ocean.

Lerosey-Aubril said: “Cousteau and his crew inspired generations to look beneath the surface, it seemed appropriate to name this ancient marine animal after someone who changed the way we look at marine life.” As if Megachelicerax cousteaui changed the way we look at the chelicerate.

Today, chelicerates include more than 120,000 living species – from spiders and scorpions to mites, horse crabs, and sea spiders – that live in terrestrial and aquatic environments.

“For thousands of years, these animals have quietly existed among us, deeply influencing our lives from pop-culture to medical and agricultural contributions,” Ortega-Hernández concluded. “This discovery of fossils sheds new light on their origins.”

Congratulations

The authors acknowledge the important role of scientific collections, such as those of the University of Kansas Biodiversity Institute and the Natural History Museum, and the dedication of the curators – especially B. Lieberman and J. Kimmig. – preserving examples for decades until new questions, and new eyes, reveal their full value.

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