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 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.
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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)
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