Fossilized brain of creepy ocean creature lends new insight into evolution of insects, spiders

TORONTO — Scientists have discovered the perfectly fossilized brain of a three-eyed prawn that swam the oceans more than half a billion years ago. Its complete central nervous system is still visible, providing unprecedented insights into the ancestors of insects, spiders and crustaceans.

Named Stanleycaris hirpex, it was described as “the stuff of nightmares.”  It had two eyes “on stalks” with a bigger one in the middle, and spiked claws. It lived during the Cambrian “explosion,” a period of rapid evolution when most major animal groups emerge in the fossil record.

The creature was a member of the radiodonts — apex predators that were the “Great White sharks” of their time. Reaching more than three feet in length, the ancient prawns were a type of early arthropod that had jointed limbs. Despite Stanleycaris’ bizarre appearance, it is the contents of its head that has scientists most excited.

In 84 individuals unearthed at Burgess Shale, a prehistoric graveyard in the Canadian Rocky Mountains, the brain and nerves are still preserved. It’s a fantastic and fascinating find after 506 million years.

“We can even make out visual processing centers serving the large eyes and traces of nerves entering the appendages,” says lead author Joseph Moysiuk, a University of Toronto PhD candidate in ecology and evolutionary biology, in a statement. “The fine details are so clear it is as if we were looking at an animal that died yesterday.”

Stanleycaris hirpex
Reconstruction of a pair of Stanleycaris hirpex; upper individual has transparency of the exterior increased to show internal organs. Nervous system is shown in light beige, digestive system in dark red. (Illustration by Sabrina Cappelli © Royal Ontario Museum)

The new specimens provide a glimpse into what the ancestral nervous system looked like. Finding any fossilized soft tissue is rare, but this is unique. Most fossils we have are bone and/or hard body parts such as teeth or exoskeletons. Brains and nerves are made of fatty like substances that normally don’t survive.

The central nervous system coordinates all neural and motor functions. In vertebrates, it consists of the brain and spinal cord. In arthropods the brain is more condensed with a chain-like series of interconnected masses of nervous tissue resembling a string of beads.

Stanleycaris’ brain was composed of two segments: the protocerebrum and deutocerebrum. They connected with the eyes and frontal claws, respectively, and control vision and antenna signals in arthropods today. In modern arthropods – such as grasshoppers and other insects – the brain also has a tritocerebrum. It connects to the labrum, a moveable upper lip, and integrates sensory information from the other two brain lobes. An extra compartment has radical implications.

“We conclude a two segmented head and brain has deep roots in the arthropod lineage,” says Moysiuk, who is based at the Royal Ontario Museum. “Its evolution likely preceded the three-segmented brain that characterizes all living members of this diverse animal phylum.”

Repeated copies of many arthropod organs can be found in their segmented bodies. Working out how they line up is key to understanding diversification.

Stanleycaris hirpex
Paper summary, showing the interpretation of the nervous system from fossils of Stanleycaris and implications for understanding the evolution of the arthropod brain. The brain is represented in red and the nerve cords in purple. (Illustration by Sabrina Cappelli © Royal Ontario Museum)

“These fossils are like a Rosetta Stone, helping to link traits in radiodonts and other early fossil arthropods with their counterparts in surviving groups,” adds Moysiuk.

In addition to a pair of stalked eyes, Stanleycaris had a big central peeper at the front of its head — a feature never before seen in a radiodont.

“The presence of a huge third eye in Stanleycaris was unexpected. It emphasizes these animals were even more bizarre-looking than we thought. It also shows us the earliest arthropods had already evolved a variety of complex visual systems like many of their modern kin,” says co-author Dr. Jean-Bernard Caron, a curator of invertebrate paleontology who is Moysiuk’s supervisor. “Since most radiodonts are only known from scattered bits and pieces, this discovery is a crucial jump forward in understanding what they looked like and how they lived.”

During the Cambrian, radiodonts were among the biggest animals. Anomalocaris, dubbed the “weird wonder,” was at least 3’3″, making it a sea monster. At no more than eight inches, Stanleycaris was much smaller. But it would have been an impressive killer, and at least three times the size of most rivals.

Radiodont means “radiating teeth.” The unusual animals were named after their toothy, circular jaws. They were adapted to the dim light of deep water. Stanleycaris’ sophisticated sensory and nervous systems would have enabled it to efficiently pick out small prey in the gloom.

 

Stanleycaris hirpex
Pair of fossil specimens of Stanleycaris hirpex, specimen ROMIP 65674.1-2. (Photo by Jean-Bernard Caron © Royal Ontario Museum)

“With large compound eyes, a formidable looking circular mouth lined with teeth, frontal claws with an impressive array of spines, and a flexible, segmented body with a series of swimming flaps along its sides, Stanleycaris would have been the stuff of nightmares for any small bottom dweller unfortunate enough to cross its path,” says Moysiuk.

The study is based on an analysis of a previously unpublished collection of 268 specimens of Stanleycaris. Most were collected in the 1980s and 1990s from layers above the famous Walcott Quarry at Yoho National Park, British Columbia. They are part of the extensive collection of fossils from Burgess Shale – a World Heritage site – housed at Royal Ontario Museum.

The research paper is published in Current Biology.

Report by Mark Waghorn, South West News Service

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