Fossil brains force to rethink the evolution of insects and spiders

July 8. () –

New fossils with traces of the brain and nervous system of a 500 million year old marine predator determine the need to rethink the evolution of insects and spiders.

The findings, obtained in the formation of the Burgess Shale (Canada), correspond to ‘Stanleycaris’, which belonged to an ancient and extinct branch of the evolutionary tree of arthropods called Radiodonta, related to modern insects and spiders. Posted in ‘Current Biology’shed light on the evolution of the arthropod brain, vision, and head structure.

What researchers are most excited about is what’s inside Stanleycaris’s head. In 84 of the fossils, the remains of the brain are still preserved and nerves after 506 million years.

New research from @CambroJoe & Jean-Bernard Caron sheds light on the evolution of the arthropod brain, vision & head structure.

Marine predator Stanleycaris of the Burgess Shale belonged to an ancient, extinct offshoot of the arthropod evolutionary tree called Radiodonta. pic.twitter.com/NatOUvwP4p

—Royal Ontario Museum (@ROMtoronto) July 8, 2022

“Although the fossilized brains of the Cambrian are not new, this discovery stands out for the amazing quality of its conservation and the large number of specimens“says Joseph Moysiuk, lead author of the research and a doctoral candidate in Ecology and Evolutionary Biology at the University of Toronto (U of T), based at the Royal Ontario Museum.

“We can even make out fine details, such as the visual processing centers that serve the large eyes and the traces of nerves that enter the appendages,” he continues. “The details are so clear that it is as if we are seeing an animal that Died yesterday”.

The new fossils show that the Stanleycaris brain was made up of two segments, the protobrain and the deutobrain. connected with the eyes and the front claws, respectively.

“We conclude that the two-segmented head and brain have deep roots in the arthropod lineage and that their evolution likely preceded the three-segmented brain that characterizes all living members of this diverse animal phylum,” Moysiuk added.

In modern arthropods, such as insects, the brain consists of a protobrain, deutobrain, and tritobrain. Although the one-segment difference may not seem like a game changer, it actually has radical scientific implications.

Since repeated copies of many organs can be found in the segmented bodies of arthropods, figuring out how the segments line up between different species is key to understanding how these structures diversified within the group. “These fossils are like a Rosetta stone, helping to relate the traits of radiodonts and other early fossil arthropods to their counterparts in surviving groups,” he notes.

In addition to its pair of stalked eyes, Stanleycaris had a large central eye at the front of its head, a feature never before observed in a radiodont.

“The presence of a huge third eye in ‘Stanleycaris’ was unexpected. It highlights that these animals were even stranger looking than we thought, but it also shows us that early arthropods they had already developed a variety of complex visual systems like many of their modern relativesexplains Dr Jean-Bernard Caron, Richard Ivey Curator of Invertebrate Paleontology at ROM, and supervisor of Moysiuk’s PhD.

Discovery of a 2-segmented head & brain in this early arthropod lineage has radical scientific implications.

“These fossils are like a Rosetta Stone, helping to link traits in radiodonts & other early fossil arthropods with their counterparts in surviving groups.” #FossilFriday pic.twitter.com/p8izZoz52P

—Royal Ontario Museum (@ROMtoronto) July 8, 2022

“Since most radiodonts are only known from scattered fragments, this discovery is a crucial leap in understanding what they were like and how they lived,” adds Caron, who is also a U of T Associate Professor in Ecology and Evolution. and Earth Sciences.

In the Cambrian period, radiodonts were some of the largest animals, such as the famous ‘Anomalocaris’, which reached at least a meter in length. At no more than 20cm in length, Stanleycaris was small for its group, but in an age when most animals grew no larger than a human finger, it would have been an impressive predator. The sophisticated sensory and nervous systems of ‘Stanleycaris’ they would have enabled it to effectively detect small prey in the dark.

With large compound eyes, a formidable circular mouth full of teeth, front claws with an impressive array of spines, and a flexible, segmented body with a series of swim fins along its sides, ‘Stanleycaris’ would have been the nightmare of any little bottom dweller unlucky enough to cross his pathsay the researchers.