Located more than 1,500 feet above sea level, it’s hard to imagine a time when the Walcott Quarry on the mountain in Canada, the first excavation site within the Burgess Shale, was submerged by the ocean. And yet, this is why it has become one of the most famous and unique fossil sites in the world.
508 million years ago, thousands of strange creatures were instantly killed by a submarine mudslide that would become the Burgess Shale, creating a time capsule for scientists to look at the Middle Cambrian period.
At this time, the waters of the ocean were warm and full of new life forms, and an order of animals called radiodonts prospered thanks to their ability to swim and hunt prey. Radiodonts share a common ancestor with modern arthropod animals such as insects, arachnids, and crustaceans, and is one of the iconic Cambrian creatures discovered in the Burgess shale.
And yet the amount of fossils in the site is so enormous that it has taken scientists more than two decades to properly document the most complete specimens of a radiodont ever seen.
Radiodont fossils are rare, and often fragmented, which has led to scientific disputes over how they are interpreted. But new research, in which scientists analyzed a cache of 268 specimens of Stanleycaris hirpex that has been in the collection of the Royal Ontario Museum for more than two decades, has begun to gather pieces of the evolution of the radiodont.
Some of the study specimens were preserved intact, which gave researchers a complete view of their body plan.
Stanleycaris is the smallest of all known radiodonts: its bodies in the fossil record range from 10 to 83 millimeters in length.
And yet these specimens offered a never-before-seen look at Stanleycaris’s brain, which in 84 of the fossils was preserved with “amazing quality.”
“We can even distinguish details such as visual processing centers that serve the large eyes and nerve traces that enter the appendages,” said lead author Joseph Moysiuk.
Moysiuk, a doctoral student in ecology and evolutionary biology at the University of Toronto, said the details were so clear it was “like we were looking at an animal that died yesterday.”
The fossils revealed two brain segments: a protocerebrum (a section linked to the compound eyes of modern arthropods) and a deutocerebrum (which, in living arthropods, controls the nerves of the antennae and plays a role in their “olfactory” version. “). In Stanleycaris, these segments are connected to the eyes and frontal claws, respectively.
Scientists believe that the fossilized brain of Stanleycaris provides evidence of early differentiation between the head and trunk segments in arthropods.
“We conclude that a head and a two-segment brain have deep roots in the lineage of arthropods and that their evolution probably preceded the three-segment brain that characterizes all living members of this diverse animal phylum,” Moysiuk said.
This third segment of the arthropod brain, the tritocerebrum, which could usually be associated with arthropod mouthparts and the front end of the animal’s digestive system, was absent in Stanleycaris.
Reconstruction of S. hirpex swimming over a fossil specimen. (Sabrina Cappelli © Royal Ontario Museum)
But what Stanleycaris might have lacked in his brain made up for it with his eyes.
“The presence of a huge third eye in Stanleycaris was unexpected,” said Jean-Bernard Caron, Moysiuk’s doctoral supervisor and co-author of the paper.
This feature, never before noticed in a radiodont, led researchers to reevaluate other Cambrian panarthropods, where they found similar middle-eyed evidence.
His discovery supports the theory that middle eyes are part of the “ground plane” of arthropods, along with the most familiar pair of lateral eyes we see in creatures like crabs.
Scientists also noted that radiodont swimming fins emerged at the same time as these more complex eye features, which they believe could have evolved to support the predator’s more active lifestyle.
“These animals looked even weirder than we thought, but it also shows us that early arthropods had already developed a variety of complex visual systems like many of their modern relatives,” Caron said.
The scientists also said the radiodont specimens are in a unique position to provide information about the evolution of arthropods from other nearby living relatives, such as velvet worms and tardigrades.
“These fossils are like a Rosetta stone, helping to link features in radiodonts and other early fossil arthropods with their counterparts in surviving groups,” Moysiuk said.
This article is published in Current Biology.