500 million-year-old clawed predator rewrites the origins of spiders and horseshoe crabs

It was a long day of instruction for Rudy LeRochy-Aubril. As a reward, he returned to cleaning an interesting Cambrian arthropod fossil he had recently received for review. Initially, the specimen showed all the characteristics expected at the time, but still something was different. It looked like it had claws instead of antennae.

“Cambrian arthropods never had claws in such a place,” Lillocy-Aubril said. “It took me a few minutes to realize the obvious fact that I had just exposed the oldest chelicerae ever discovered.”

In a study published in natureresearch scientist Rudy Lerosy Aubril and associate professor Javier Ortega Hernández, curator of invertebrate paleontology at the Museum of Comparative Zoology in the Department of Biological and Evolutionary Biology at Harvard University. Megachelyserax custoia 500-million-year-old sea predator discovered in the Western Desert of Utah. It is the oldest known chelicera, a group of arthropods that includes spiders, scorpions, horseshoe crabs, and sea spiders. This discovery pushes the evolutionary history of chelicerates back 20 million years.

“This fossil records the Cambrian origin of chelicerates and shows that the anatomical blueprint for spiders and horseshoe crabs appeared already 500 million years ago,” Lerosy-Aubril said.

LeRochy Aubril spent more than 50 hours carefully cleaning the fossil under a microscope with a fine needle to reveal its shocking anatomy. in The body length is over 8 cm, M. Kustoy The head shield and dorsal exoskeleton consisting of nine segments are preserved. These two regions are characterized by different appendages. The head shield has six pairs of limbs specialized for feeding and sensing, and the underbody has plate-like respiratory structures similar to the gills of modern horseshoe crabs.

But its most extraordinary feature is the unmistakable chelicerae. It is the pincer-like feeding appendage that defines the chelicerae and distinguishes spiders from insects. Insects have sensory antennae as their anterior-most appendages, while chelicerates have grasping, often poisonous tools. Despite the rich Cambrian fossil record, no distinct chelicerae-bearing arthropods from that period have been discovered to date.

Prior to this discovery, the oldest known chelidae had been dated to the Early Ordovician Fezuata biota of Morocco, approximately 480 million years ago. existence of M. Kustoy It was placed as an early branch of the chelicerate family tree 20 million years ago, and is thought to be an important transitional species between Cambrian arthropods, which appear to lack chelicerae, and the much younger horseshoe crab-like chelicerates known as syndiphosphrines.

“Mega Cherry Serax “This shows that the chelicerae and the division of the body into two functionally specialized regions evolved before the head appendages lost their lateral branches and became like today’s spider legs,” Ortega-Hernández explained. In a sense, everyone was partially right. ”

This fossil captures a key step in the assembly of the scissor-like body plan and reveals that key elements had already evolved in the immediate aftermath of the Cambrian explosion, a period of very rapid evolutionary innovation.

“This shows that by the mid-Cambrian, when the rate of evolution was significantly higher, arthropods with an anatomical complexity comparable to modern forms were already living in the ocean,” Ortega-Hernández added.

Interestingly, the early acquisition of this complex anatomy did not immediately lead to ecological advantage or diversification. Instead, chelodonts remained relatively inconspicuous for millions of years, overshadowed by seemingly simpler groups such as trilobites, until they successfully colonized land.

“Similar evolutionary patterns have been documented in other animal groups,” Lerosy-Aubril says. “This shows that biological innovation alone is not important for evolutionary success; timing and environmental conditions are also important.”

M. Kustoy Collected in the Central Cambrian Wheeler Formation of the Howth Mountains of Utah. The fossil was discovered by noted hobbyist fossil collector Lloyd Gunther and donated to the University of Kansas Biodiversity Institute and Museum of Natural History in 1981 for further study. It was one of several seemingly unremarkable fossil collections discovered in Utah that Lillocy-Aubril offered to examine as part of his research into early arthropods.

Megachelyserax custoi It is named after the French explorer Jacques-Yves Cousteau. Lourosie-Aubril – he is also French – and Ortega-Hernández chose Cousteau for his work raising awareness of the beauty and fragility of the ocean floor.

“Cousteau and his crew inspired generations to look beneath the surface, and we thought it fitting to name this ancient marine animal after the man who changed the way we look at marine life,” says LeRochy-Aubril. just the same Megachelyserax custoi My perspective on chelicerae has changed.

Today, chelicerae include more than 120,000 species that live in both terrestrial and aquatic ecosystems, from spiders and scorpions to ticks, horseshoe crabs, and sea spiders.

“For thousands of years, these animals have quietly existed among us, deeply influencing our lives, from popular culture to contributions to medicine and agriculture,” concluded Ortega Hernández. “The discovery of this fossil sheds new light on their origin.”

Acknowledgment

The authors acknowledge the important role of scientific collections such as the University of Kansas Biodiversity Institute and the Museum of Natural History and the dedicated efforts of the experts who maintain them, especially B. Lieberman and J. Kimmig. – Preserve specimens for decades until new questions and new eyes can fully reveal their significance.

####