By Will Dunham
WASHINGTON (Reuters) - Researchers on Wednesday described fossilized remains unearthed in China showing in fine detail the brain structures of a bizarre group of sea creatures that were the top predators more than half a billion years ago.
The fossils show an animal called Lyrarapax unguispinus that lived during the Cambrian Period, a pivotal juncture in the history of life on Earth when many major animal groups first appeared. It was a member of a group known as anomalocaridids - primitive relatives of arthropods, which include crustaceans, insects and spiders - that hunted prey with a pair of claw-like grasping appendages in front of the eyes.
Even though anomalocaridids do not have any direct descendants alive today, the brain structures of Lyrarapax closely resemble those of worm-like animals called velvet worms that crawl along the ground in tropical and semitropical forests in the Southern Hemisphere.
The researchers said the similarities suggest that velvet worms may be very distant cousins of the anomalocaridids, whose best-known example is Anomalocaris, known from a Canadian fossil site called the Burgess Shale.
Velvet worms, land animals also known as onychophorans, grow to a few inches in length, have two long feelers extending from the head and have numerous pairs of stubby, unjointed tubular legs that each end in a pair of small claws.
Lyrarapax, whose scientific name means spiny-clawed, lyre-shaped predator, lived 520 million years ago. Its neuroanatomy resembles that of velvet worms in multiple ways, with a simple brain and a pair of ganglia – a cluster of nerve cells – placed in the front of the optic nerve and the base of the grasping appendages.
The soft parts of any animal's body typically decay after death, meaning that fossils usually preserve only hard parts like bones, teeth and shells. But under exceptional circumstances, soft tissue and anatomical organs can be preserved in fossils.
Lyrarapax was much smaller than some other anomalocaridids. It measured about 6 inches (15 cm) long, roughly the size of a large shrimp. Peiyun Cong, a paleontologist at Yunnan University in China, said the three specimens of Lyrarapax that were found "may represent immature stages of the animal, so it might be larger."
"Anomalocaridids preserved with the whole body are very rare. None of them have been reported with the brain," Cong said.
The fossils show that anomalocaridids possessed brains that were less complex than those of animals it may have hunted.
University of Arizona neuroscientist Nicholas Strausfeld, another of the researchers, said the threat posed by predators like these creatures may have helped drive brain complexity among animals in the ancient seas.
"Predation may have in part contributed to the evolution of more elaborate brains that could process more complex ecological cues that might have offered camouflage or other protection," Strausfeld said.
The study was published in the journal Nature.
(Reporting by Will Dunham)