It's hard to imagine a time when the mountainside Walcott quarry in Canada was submerged by the ocean. It has become one of the most famous and unique fossil sites.
Thousands of strange creatures were killed by a mudslide in the middle of the Cambrian period, creating a time capsule for scientists to look into.
The ocean waters were warm and bursting with new forms of life, and an order of animals called radiodonts flourished off their ability to swim and hunt. Radiodonts is one of the most well-known arthropods of the Cambrian period and shares a common ancestor with arthropods today.
Scientists have spent two decades documenting the most complete specimen of a radiodont ever seen from the site.
Radiodont fossils are rare, and often fragmented, which has led to scientific feuds over how to interpret them. The pieces of radiodont evolution have been sitting in the Royal Ontario Museum's collection for over two decades.
The researchers were able to see a complete view of their body plan with the preservation of some of the specimen.
The smallest of all known radiodonts, Stanleycaris, has a body length from 10 to 83 millimeter.
In 84 of the fossils, the brain of Stanleycaris was preserved with "astonishing quality".
Joseph Moysiuk said that they could make out fine details such as visual processing centers serving the large eyes and traces of nerves entering the appendages.
At the University of Toronto, Moysiuk is a PhD candidate in ecology and evolutionary biology.
The brain segments found in the fossils are linked to the compound eyes of arthropods and control nerves in the antennae. The eyes and frontal claws are connected to these segments.
The brains of arthropods give evidence for early differentiation between the head and trunk.
According to Moysiuk, the evolution of a two-segmented head and brain is related to the evolution of the three-segmented brain.
The tritocerebrum, which is a part of the arthropod brain, was not present in Stanleycaris.
Reconstruction of S. hirpex swimming above fossil specimen. (Sabrina Cappelli © Royal Ontario Museum)
It was more than made up for by the eyes.
Moysiuk's supervisor and co-author on the paper said that the presence of a huge third eye was unexpected.
The researchers looked at other panarthropods and found evidence for similar median eyes.
Their discovery supports the idea that median eyes are part of the ground plan of arthropods.
The scientists think the swimming flaps of radiodonts may have evolved to support the predator's more active lifestyle.
The earliest arthropods had already evolved a variety of complex visual systems like many of their modern kin, and these animals were even more bizarre-looking than we thought.
The scientists said that radiodont specimen are uniquely positioned to provide information about the evolution of arthropods from other relatives.
Moysiuk said that the fossils are like a "Rosetta Stone", helping to link the characteristics of radiodonts and other early arthropods with their counterparts in surviving groups.
The paper is published in a journal.
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