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Fossilized brains of 500-million-year-old creatures reveal clues to the origin of insects

The exceptionally preserved fossils of the Stanleycaris Hirpex revealed something scientists didn't know the arthropod had: a third eye.

(CN) — Cambrian-era fossils of ancient bug-like creatures will probably never receive the same fanfare as their dinosaur counterparts, but they can contribute significant evolutionary understanding of one of our most diverse modern animal groups.

Published Friday in the journal Current Biology, researchers with the Royal Ontario Museum in Canada revealed the results of their study of the Stanleycaris Hirpex and its role in the evolutionary tree. By studying the fossils of the ancient arthropod, paleontologists gained a deeper understanding of the connection between arthropod brain and body segments and their connection to modern arthropods like insects and spiders.

“Since arthropods themselves are by far the most diverse animal group today, making up some 80% of all species, this is pretty important for understanding how modern biodiversity came to be,” explains Joseph Moysiuk, lead author of the study and a University of Toronto doctoral student in ecology and evolutionary biology.

The Stanleycaris Hirpex was a type of radiodontia, an early arthropod that lived in the waters of the Cambrian period, a time known for its particularly intense burst of evolution. Leading up to this period, complex multicellular organisms began to develop in earnest, with this stage of rapid and widespread biodiversity accounting for is sometimes called the Cambrian explosion. Some of our earliest fossils can be traced back to this time roughly 500 billion years ago. Although many of the creatures that emerged during this time period are long extinct, they continue to influence our evolutionary understanding.

Radiodontia were characterized by the presence of frontal appendages that paleontologists theorize were used like claws, along with mouths full of sharp tooth plates. Radiodontia were some of the larger organisms of the time, with some reaching three feet in length. Stanleycaris is theorized to have been able to reach up to six inches in length, much smaller than some of its radiodontia fellows, but still larger than many of the other Cambrian organisms. Stanleycaris would have been considered an early predator, evidenced by its size and comparatively complex sensory and nervous systems.

The fossilized Stanleycaris the researchers studied had a segmented brain, with each segment connected to either the eyes or the frontal claws of the creature. This discovery allowed researchers to compare the segmented Stanleycaris nervous system to similar structures present in insects and spiders, which can help pinpoint where the creatures may have diverged during evolution.

The preserved brains and nerves of the Stanleycaris provided Moysiuk and his co-author Dr. Jean-Bernard Caron a way to link traits in radiodonts and other fossilized arthropods with their counterparts in surviving groups.

“The key finding here is that understanding how many segments composed Stanleycaris’ brain helps us line up the segments in its nervous system and body with those in modern arthropods,” Moysiuk said in an email interview. “This also helps us figure out what’s going on in some related fossil forms for which we haven’t yet found fossilized nervous tissues. The nervous system is crucial for understanding this as it’s often more conserved and easier to align between different groups than external body segments.”

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. Photo by Jean-Bernard Caron © Royal Ontario Museum

Stanleycaris Hirpex was first documented nearly 20 years ago from other fossils found at the Burgess Shale in British Columbia, but specimens were incomplete and scattered. The fossils examined in this study were wholly preserved and with exceptional detail — enough to get a clear look not only at the brain and nervous system of the creature like never before, but also to document an eye not seen in prior research.

“The presence of a huge third eye in Stanleycaris was unexpected. It emphasizes that these animals were even more bizarre-looking than we thought, but also shows us that the earliest arthropods had already evolved a variety of complex visual systems like many of their modern kin,” Caron said in a statement accompanying the study.  

The clarity of these fossils can be attributed to the preservation qualities of the Burgess Shale. A UNESCO World Heritage Site located in Canada’s Yoho National Park near Calgary, the Burgess Shale has been the source of many significant ecological and biological discoveries. Where other fossil sites may have been able to preserve animals over time as sediment slowly covered them, the Burgess Shale displays evidence of having been muddy phyllopod beds that could have begun preserving the organisms the instant they were killed by sudden mudslides.

Consequently, the fossils of the Burgess Shale are especially numerous and detailed, even for typically hard-to-preserve soft-bodied animals. This kind of detail in the well-preserved brains and nervous systems in the studied Stanleycaris fossils was crucial to this new discovery.

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