Spiders, scorpions and ticks have a characteristic that is typical of all jawed claws: their first head appendage is not an antenna, but a claw – the chelicere. Now paleontologists have discovered the oldest evidence of such a jaw claw in a 500-million-year-old fossil from the Utah desert. This animal proves for the first time that the basic body plan of the chelicerates and in particular their jaw claw already existed in the Cambrian era – around 20 million years earlier than previously assumed. At the same time, the prehistoric chelicerate, named Megachelicerax cousteaui, also clarifies the question of which body shape and number of segments was the original one for the group of jawed claw bearers and arachnids.
Chelicerates, insects and crustaceans have a common origin – they all belong to the arthropods, the arthropods. However, fossil finds suggest that these groups diverged as early as the Cambrian period, more than 500 million years ago. Around this time, the ancestors of all spiders, scorpions, horseshoe crabs and mites must have developed their namesake feature – the jaw claw. This three-part head appendage is used by horseshoe crabs, mites and scorpions as a gripper when catching prey. In spiders, the chelicerae usually contain poison glands, and some also use their claws to spin their silk or for body care. “This remarkable functional versatility was likely crucial to the evolutionary success of these species-rich, ecologically ubiquitous and globally distributed arthropods,” explain Rudy Lerosey-Aubril and Javier Ortega-Hernández from Harvard University.
Fossil with jaw claw instead of antenna
But it was still unclear when this core feature of the chelicerate blueprint arose. Some of the oldest fossils of potential precursors of this group of animals date from around 520 million years ago. However, their chelicerae were not yet developed. The oldest fossil clearly assigned to jaw-claw bearers is 480 million years old and comes from Morocco. Now the two paleontologists have identified a fossil that represents the oldest clear evidence of a jaw claw in a prehistoric arthropod. The approximately 500 million year old find from the desert of the US state of Utah lay largely unnoticed in a museum collection at Harvard before Lerosey-Aubril took care of this fossil and painstakingly prepared it and examined it under the microscope. He discovered that this supposed primordial arthropod had a claw as its first head appendage instead of the expected antenna.
“You never find a claw in this position in Cambrian arthropods,” reports Lerosey-Aubril. “It took me a few minutes to come up with the obvious explanation: I had just uncovered the oldest chelicerae ever found.” The fossil, which is a good eight centimeters tall, has a clearly recognizable movable claw on each of its first two head legs, which, like modern chelicerates, only have one branch and are three-parted. Like today’s spiders, this jaw claw emerges from the animal’s front body, which is covered by a head shield made up of several segments. This prosoma is typical of modern spiders, but also scorpions and horseshoe crabs. Because of the particularly large jawed claw and in honor of the French oceanographer Jacques-Yves Cousteau, the paleontologists named their find Megachelicerax cousteaui.
Link of chelicerate evolution
“This fossil documents the Cambrian origin of the chelicerates,” says Lerosey-Aubril. “It shows that the anatomical blueprint of spiders and horseshoe crabs developed around 500 million years ago.” Megachelicerax cousteaui represents an important link between cheliceral-less ancestral arthropods and later precursors of horseshoe crabs. “The astonishingly modern anatomy of Megachelicerax is also reflected in its body structure: it already had a prosoma made up of seven segments and an articulated opistosoma (rear body) with appendages that were used for breathing and swimming,” explain the researchers. The fossil thus postpones the formation of the core features of the chelicerates by at least 20 million years into the past. Accordingly, the first forms of this group of animals arose during the Cambrian explosion – the phase in which the diversity of the prehistoric animal world increased drastically in a short period of time.
The fossil also provides the first clues as to how and where Megachelicerax cousteaui once lived. Accordingly, it probably used its widened rear pair of legs to swim just above the seabed and search for prey or carrion. With its head appendages and, above all, its large jaw claws, it then grabbed its prey and consumed it. Despite its modern blueprint and the reinvention of the jaw claw, Megachelicerax and other early chelicerates did not initially prevail: for millions of years, the more primitive and simpler trilobites initially dominated the prehistoric seas. Only when the first arthropods conquered the land were the chelicerates able to exploit their advantages. “We also see a similar pattern in some other animal groups,” explains Lerosey-Aubril. “This shows that evolutionary success does not only depend on biological innovation – the timing and environmental conditions also have to be right.” Today, chelicerates number at least 120,000 species, which have conquered almost all regions of the world and most ecosystems.
Source: Rudy Lerosey-Aubril and Javier Ortega-Hernández (Harvard University, Cambridge, USA), Nature, doi: 10.1038/s41586-026-10284-2