Were Anomalocaridids Really Velvet Worm Relatives, not Arthropods?

An international team of paleontologists has identified the exquisitely preserved brain in the fossil of one of the world's first known predators that lived in the Lower Cambrian, about 520 million years ago. The discovery revealed a brain that is surprisingly simple and less complex than those known from fossils of some of the animal's prey.

The find for the first time identifies the fossilized brain of what are considered the top predators of their time, a group of animals known as anomalocaridids, which translates to "abnormal shrimp." Long extinct, these fierce-looking arthropods were first discovered as fossils in the late 19th century but not properly identified until the early 1980s. They still have scientists arguing over where they belong in the tree of life.

"Our discovery helps to clarify this debate," said Nicholas Strausfeld, director of the University of Arizona's Center for Insect Science. "It turns out the top predator of the Cambrian had a brain that was much less complex than that of some of its possible prey and that looked surprisingly similar to a modern group of rather modest worm-like animals."

The brain in the fossil, a new species given the name Lyrarapax unguispinus – Latin for "spiny-clawed lyre-shaped predator" – suggests its relationship to a branch of animals whose living descendants are known as onychophorans or velvet worms. These wormlike animals are equipped with stubby unjointed legs that end in a pair of tiny claws.

Onychophorans, which are also exclusively predators, grow to no more than a few inches in length and are mostly found in the Southern Hemisphere, where they roam the undergrowth and leaf litter in search of beetles and other small insects, their preferred prey. Two long feelers extend from the head, attached in front of a pair of small eyes.

The anomalocaridid fossil resembles the neuroanatomy of today's onychophorans in several ways, according to Strausfeld and his collaborators. Onychophorans have a simple brain located in front of the mouth and a pair of ganglia – a collection of nerve cells – located in the front of the optic nerve and at the base of their long feelers.

"And – surprise, surprise – that is what we also found in our fossil," Strausfeld said, pointing out that anomalocaridids had a pair of clawlike grasping appendages in front of the eyes.

"These top predators in the Cambrian are defined by just their single pair of appendages, wicked-looking graspers, extending out from the front of their head," he said. "These are totally different from the antennae of insects and crustaceans. Such frontally disposed appendages are not found in any other living animals with the exception of velvet worms."

The similarities of their brains and other attributes suggest that the anomalocaridid predators could have been very distant relatives of today's velvet worms, Strausfeld said.

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