More than 40 years after it was discovered in a desert in western Utah, an unassuming fossil has been re-examined and identified as a brand-new species of chelicerate - the arthropod group that includes spiders, scorpions, horseshoe crabs and sea spiders.
This species, named Megachelicerax cousteaui, has rewritten the evolutionary history of its incredibly diverse group, pushing back its origins 20 million years to the Cambrian.
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This latest discovery was made by Harvard University palaeontologist Rudy Lerosey-Aubril who, in between long days of teaching, meticulously prepared the arthropod fossil.
At first, Lerosey-Aubril considered the fossil rather unremarkable, but after uncovering a pair of frontal claws in place of expected sensory antenna his assessment suddenly changed – this was a chelicerate and based on the Cambrian age of the fossil, the oldest of its kind ever found.
“Claws are never in that location on a Cambrian arthropod. It took me a few minutes to realise the obvious, I had just exposed the oldest chelicerate ever found,” said Lerosey-Aubril, the lead author of a study published last week in Nature describing M.cousteaui. “This fossil documents the Cambrian origin of chelicerates and shows that the anatomical blueprint of spiders and horseshoe crabs was already emerging 500 million years ago.”
After more than 50 hours of careful cleaning and meticulous preparation underneath a microscope, Lerosey-Aubril revealed other, finer elements of M.cousteaui’s anatomy, including a dorsal exoskeleton consisting of a horseshoe-shaped head shield and nine separate body segments. He also found evidence of plate-like respiratory structures beneath these body segments that look remarkably similar to the book gills of modern horseshoe crabs.
Undoubtedly, the most characteristic feature of M.cousteaui is the pair of “massive, three-segmented chelicerae” projecting from its head. These grasping appendages are pincer-like and help identify M.cousteaui as an unequivocal chelicerate, rather than another type of arthropod.
The Cambrian has a rich arthropod fossil record, but until now no unambiguous chelicera-bearing arthropod from this period had been found. Prior to the discovery of M.cousteaui, the oldest confirmed chelicerates dated to the Early Ordovician of Morocco, roughly 480 million years ago.
The existence of this remarkable species 20 million years earlier places it near the base of the chelicerate family tree and informs researchers that the development of chelicerae occurred early, before several other key characters that define the group.
“Megachelicerax shows that chelicera and the division of the body into two functionally specialised regions evolved before the head appendages lost their outer branches and became like the legs of spiders today,” explained co-author Javier Ortega-Hernández. “This tells us that by the mid-Cambrian, when evolutionary rates were remarkably high, the oceans were already inhabited by arthropods with anatomical complexity rivalling modern forms.”
Today, chelicerates comprise more than 120,000 species, from spiders and scorpions to mites, horseshoe crabs and sea spiders. Behind insects, they are the second-most diverse group of arthropods, inhabiting both terrestrial and aquatic ecosystems.
However, during the Cambrian and the Ordovician, chelicerates were confined to the water and, despite the early acquisition of a relatively complex anatomy, they were ecologically overshadowed by seemingly simpler arthropod groups like trilobites. It wasn’t until the Silurian, approximately 430 million years ago, that the group successfully spread to the land.
“A similar evolutionary pattern has been documented in other animal groups,” said Lerosey-Aubril. “This shows that evolutionary success is not only about biological innovation - timing and environmental context matters.”
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