A fearsome lobster-like predator has been unearthed from the Burgess Shale, providing insight into the evolution of arthropods, a phylum of animals that includes insects and arachnids, as well as modern crustaceans. Yawunik was an apex predator of its day, equipped with three long claws, two of which had opposing rows of teeth to better grip any prey that hadn't already died of fright.

The Burgess Shale is one the richest fossil deposits in the world, providing us with unparalleled insight into the Middle Cambrian Period, 508 million years ago. The latest addition to this treasure is Yawunik kootenayi, the first new species to be identified from the Marble Canyon site. It is named after a marine creature of destruction in the creation stories of the local Ktunaxa people.

"This creature is expanding our perspective on the anatomy and predatory habits of the first arthropods, the group to which spiders and lobsters belong," says lead author and University of Toronto Ph.D. student Cedric Aria in a Paleontology paper announcing the discovery. "It has the signature features of an arthropod with its external skeleton, segmented body and jointed appendages, but lacks certain advanced traits present in groups that survived until the present day. We say that it belongs to the 'stem' of arthropods."

Like so many other fossils from the Burgess Shale, Yawunik is preserved with a clarity that belies the immense passage of time, allowing us to see an invertebrate with clarity seldom achieved even for species equipped with skeletons. Many of the Burgess species look like nothing that exists on Earth today, leading to Stephen J. Gould's argument that they represent lineages that disappered soon after. Yawunik, however, shows a clear resemblence to modern lobsters, although the paper notes that the "phylogenetic placement" of the leanchoiliida, to which it belongs, "has been much debated."

The discovery of 42 specimens at one location suggests Yawunik was common, at least at Marble Canyon, which now forms part of Kootenay National Park, British Columbia.

Credit: Robert Gaines. A Yawunik imprinted within shale.

Yawunik's most notable feature is its multipurpose appendages, which Aria says would have been opened to capture prey, but could be contracted to the body to reduce drag while swimming. "We know that the larvae of certain crustaceans can use their antennae to both swim and gather food. But a large active predator such as a mantis shrimp has its sensory and grasping functions split up between appendages. Yawunik and its relatives tell us about the condition existing before such a division of tasks among parts of the organism took place."

The authors were able to gain additional insight into their discovery's anatomy and behavior using a technique known as "elemental mapping," which reveals the atomic composition of different parts of the fossil. "The scanning electron microscope allows us to make maps of the fossils that reveal their composition. This gives us a remarkable perspective on the fossils, allowing anatomical structures to be visualized more precisely. This technique also provides insight into the unusual fossilization process that was at work here," says co-author Dr. Robert Gaines of Pomona College, California.

Credit: Lars Field / Phlesch Bubble