Fossil helps show how spider relatives’ earlier appendages may have evolved
An incredibly preserved fossil shows that the first relatives of spiders and scorpions were already armed with their characteristic front claws around half a billion years ago.
Newly described animal reserves the oldest clear example ever found of these specialized appendagespaleontologist Rudy Lerosey-Aubril and his colleagues report April 1 in Nature. The discovery helps settle a long-standing debate about the evolution of claws and shows that chelicerates – the group that today includes horseshoe crabs, ticks and daddy long legs – had already adopted a surprisingly modern body plan.
“This creature has ultramodern anatomy for a 500 million-year-old animal,” says Lerosey-Aubril of Harvard University.
The fossil’s obvious claws rest on a pair of appendages next to its mouth. In modern chelicerae, these appendages, called chelicerae, have evolved to perform different tasks. In spiders, they became fangs, sometimes used inject venom. In scorpions, these are small mouthparts used for feeding.
Younger fossils clearly show clawed chelicerae, but previous candidates had not preserved them. This left open the possibility that the structures evolved from the sensory antennae seen on insects, a related animal group. Instead, the new fossil’s well-developed claws suggest that chelicerae evolved from the striking “large appendages” seen on some earlier arthropods, says Lerosey-Aubril.
