Canadian paleontologists uncover unusual Cambrian fossil from Burgess Shale

Canadian paleontologists have uncovered fossil remains of a large sea creature from the Cambrian Period 506 million years ago. The fossils were found in the rocks of the Burgess Shale in the Canadian Rockies, an area known for the preservation of its fossils, including soft parts.

The newly discovered fossils are of a hurdiid radiodont, an extinct group of arthropods, and have been named Titanokorys gainesi. 

A report of the find appears in Royal Society Open Science, Sept. 8. The authors are J.B. Caron and Joseph Moysiuk, from the Royal Ontario Museum's Department of Natural History and the University of Toronto.

At about a half-meter long, Titanokorys is considered giant compared to the other tiny sea creatures of the Cambrian. It has a "large ovoid-shaped central carapace," the authors write, with an anterior spine and multiple rake-like limbs that could sift through the sea bottom in search of prey. It also has a series of flaps that help it swim.

The head is so long compared to the body that study co-author Moysiuk characterized Titanokorys' appearance as little more than a "swimming head."

An unusual find

Moysiuk, who is a Ph.D. student, said the reason why the new find is so unusual is "The size of the animal is remarkable as it’s one of the largest known animals from the Cambrian Period."

"Also surprising was the discovery the Titanokorys lived in the same environment as a smaller relative called Cambroraster," he said. "The two probably fed in a similar way, grubbing through the mud with their rake-like claws in search of buried prey."

Moysiuk emphasized the unusual quality of the size of the fossil. 

"Radiodonts are known for being relatively large in the Cambrian, when most other animals were no more than a few centimeters long," he said. "Even so we were awestruck when we uncovered the largest carapace of Titanokorys, as it was the largest single body part we’ve ever uncovered at the Burgess Shale."

Comparing the morphological characteristics of Titanokorys to other Cambrian fossils, Moysiuk said, "Like other radiodonts, Titanokorys had a pair of jointed claws, a circular, tooth-ringed mouth, compound eyes and a series of swimming flaps."

Moysiuk said "the most remarkable attribute" is the set of three large carapaces that covered the animal’s head. 

"Two of these were shaped like paddles and situated on the underside," he added. "The third covered the upper side of the head and looked something like a rocket ship. The small notches at the rear of this carapace would have accommodated the eyes, nearly halfway back along the length of the animal."

Titanokorys has a mixture of characters seen in other radiodonts, the authors state. But its three-part carapace is substantially larger than the largest other specimens of Cambrian hurdiids.

They suggest two possibilities for the evolution of the singular features of the Titanokorys carapace, related to its sea floor (benthic) activities. 

"...[I]t is possible that the initial enlargement of the carapaces constituted an adaptation to an increasingly specialized, typically benthic niche, serving for example for sediment ploughing or as a filtration chamber," they wrote. "Alternatively, this enlargement could be a side consequence of other factors, such as the reduced requirement for agile motility in specialized benthic forms which could have enabled the evolution of bulky defensive armament." 

Expanding our view of the Cambrian

The new finding is important in filling out knowledge of the Cambrian, a period when an explosion of new and more complex and diverse species appeared in the fossil record.  

"Titanokorys expands our knowledge of the size and diversity of large predators in the Cambrian," Moysiuk said. The Cambrian is the time when animals first started to burrow deeply into the seafloor, and this was probably stimulated by an arms race with newly-evolved, specialized predators like Titanokorys. The finding that it coexisted with the smaller Cambroraster emphasizes the high complexity of the whole food chain that sustained them," he told Current Science Daily.

The smaller hurdiid Cambroraster is more abundant, the authors note, and they suggest that there might have been "selective resource exploitation," with the Titanokorys eating the larger prey. They also raise the possibility that the smaller number of Titanokorys in the area may be the result of the location of the fossil being at the edge of its range.

Titanokorys and the Burgess Shale

The Titanokorys fossils were found in an area of the Burgess Shale called Marble Canyon. The Burgess Shale was designated a UNESCO World Heritage Site and is part of the larger Canadian Rocky Mountain Parks World Heritage Site.

How did the species get its name? 

The authors say the genus name Titanokorys gainesi comes from the giant Titans of Greek mythology and the Greek word for helmet, korys, which refers to the large carapace. Gainesi is for Robert Gaines, the geologist who co-discovered the area of the fossil deposit in 2012.

Specimens of the Titanokorys fossils went on display in the Royal Ontario Museum in Toronto in December. 

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J.-B. Caron & J. Moysiuk, A giant nektobenthic radiodont from the Burgess Shale and the significance of hurdiid carapace diversity, Royal Society Open Science, Sept. 8, 2021. DOI: https://doi.org/10.1098/rsos.210664