3 eyes for radiodonts, at the bottom of the arthropod group

Burgess Shale, located and Yoho National Park, British Columbia in Canada, is home to the most amazing fossilized fauna ever discovered on Earth. It testifies to a formidable period of evolutionary innovations that took place over a period between 540 million and 500 million years, that is, the Cambrian explosion. This era saw the emergence of innumerable life forms, some of which formed the basis of most major groups of modern animals. The Burgess fauna also contains a wealth of enigmatic and exotic cellular configurations that have proven to be so many failed attempts.

Cambrian apex predators

Among these animals that have been extinct for ages, the order of radiodonts is one of the most notable. For two main reasons. The first is their strange appearance: they vaguely evoke the morphology of crustaceans, and especially shrimp for some, but their size can vary, depending on the species, from a few centimeters to almost two meters for anomalocarididae, which are considered to be the top predators in this species, time and probably as the first top predators that Earth has known. The second reason that arouses interest among researchers for radiodonts is that they are considered the first specimens of the genus of euarthropods (or higher arthropods), which today include all animals with an exoskeleton, d a segmented body and limbs. That is, insects, spiders, crustaceans and myria pods! To understand them is to understand the origin and development of the group.

And new discoveries from scientists know The Royal Ontario Museum (ROM) will no doubt contribute to this. They relate to Stanleycaris hirpex, a radiodont about twenty inches long. Small therefore for the standards of this order. Nevertheless, most animals in the Cambrian did not exceed the size of the little finger of the hand. S. hirpex was therefore a predator impressive with its compound eyes, circular mouth surrounded by teeth, frontal claws and spines that dot much of its body.

Fossils of Stanleycaris hirpex. The black markings on the head are remnants of nerve tissue. Credits: Jean-Bernard Caron, © Royal Ontario Museum.

third eye

For this study, ROM researchers studied an unpublished collection of 268 Stanleycaris specimens. The fossils were mostly collected in the 1980s and 90s from the rock layers above the famous Walcott Quarry site. In 84 of the fossils, the remains of brains and nerves are still preserved after 506 million years. “Although fossilized brains from the Cambrian period are not new, this discovery stands out for its astonishing quality of preservation and the large number of specimens.“, Joseph Moysiuk, lead author of this work, said in a press release, published in the journal Current biology. The investigations even revealed fine details such as visual treatment centers that serve the eyes and traces of nerves entering the appendages.

Above all, it has now been proven that Stanleycari’s brain was composed of two segments, the protocerebrum and the deutocerebrum, connected by the eyes and the frontal claws, respectively. In modern arthropods such as insects, the brain is divided into 3 and consists of protocerebrum, deutocerebrum and tritocerebrum. Radiodonts therefore constitute an intermediate and very old stage in the development of the articular brain. “These fossils are like a Rosetta stone, helping to connect features of radiodonts and other early fossil arthropods with their surviving counterparts.“, Joseph Moysiuk rejoices.

reconstruction of Stanleycaris with his three eyes. Credit: Sabrina Cappelli © Royal Ontario Museum.

In addition to his pair of stalked eyes, Stanleycaris also had a large central eye on the front of his head, a trait never seen before in a radiodont. Only the very good preservation of the fossils has made it possible to notice this peculiarity. It proves it if necessary these animals looked even stranger than expected and that radiodonts already possessed a number of complex visual systems, such as many of their modern relatives.

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