The Cambrian Period, which lasted about 53 million years, is considered as one of the most important periods in the history of life on earth as it is marked by dramatic evolutionary changes that produced the first representatives of modern animal groups, a phenomenon known as the "Cambrian explosion".

It is during this period that a group of creatures known as anomalocaridids emerged and became the top marine predators in the ancient seas. The creatures, which could grow up to six feet long, are characterized by bladed body armor and cone-shaped mouth that hunted with a pair of claw-like frontal appendages.

Anomalocaridid was believed to be closely related to arthropods, which include spiders, insects and crustaceans but its affinity has remained unclear because most of the specimens that paleontologists found are not preserved well and it was difficult to match the features of the creature's appendages with those of the living arthropods.

A team of paleontologists, however, was able to examine a well-preserved and nearly-complete fossil of Lyrarapax unguispinus, a member of the anomalocaridids group, which was found in near Kunming in the Yunnan province in Southwest China in 2013. The fossil included traces of muscles, digestive tract and a brain that the researchers described as simple and even less complex when compared with some of the fossils of the creature's possible prey.

"The traces of brain provide the first direct evidence for the segmental composition of the anomalocaridid head and its appendicular organization," Nicholas Strausfeld, from the University of Arizona, and colleagues described their findings in the journal Nature on July 16.

Strausfeld said that the brain structure found in the Lyrarapax fossil suggests that the creature is similar to a modern group of worm-like animals which are called onychophorans, also known as velvet worms. The researchers said that the anomalocaridid fossil has a number of similarities with today's onychophorans. Onychophorans, for instance, have a simple brain in front of their mouth and these similarities in brains and other features suggest that the ancient predator nomalocaridids could be distantly related to the modern-day velvet worms.

"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," Strausfeld 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."