Mount Royal University scientist cracks century-old fossil mystery

Jan. 20, 2022

Newly discovered tiny shells in 505-million year old rocks dating from the Cambrian Period in southeastern British Columbia yield new clues to peculiar animals known as stenothecoids, which have puzzled paleontologists for more than a century.

Dr. Paul Johnston, PhD, an associate professor in the department of Earth and Environmental Sciences at Mount Royal University, made the find in Canada’s famous Burgess Shale on Mount Stephen in Yoho National Park, a collection of marine invertebrates dating from the dawn of complex life on earth, and which are considered the world’s most important animal fossils. This discovery is an important piece of the Burgess Shale puzzle that illuminates how life evolved on Earth.

Previous researchers focussed on splitting shale to expose well preserved but flattened specimens. Instead, Johnston examined nearby limestone layers and found three-dimensional shells of the mysterious stenothecoids. The problem was how to extract the delicate centimetre-sized shells from the hard encasing limestone.

“I could see with my hand lens that the stenothecoid shells had been replaced by silica during their half-billion year entombment, and so I could dissolve the limestone with acid in the lab to extract the shells,” said Johnston. “The results were excellent.”

First identified in 1884 from finds in Nevada, the stenothecoids have perplexed paleontologists because of their odd combination of shell features, which do not match any of the major groups (phyla) of modern shelled animals. Like clams, the stenothecoid shell comes in two parts, known as valves, but with one valve more inflated than the other, and therefore unlike the equally convex valves of early clams. Taken individually, the valves of stenothecoid shells are asymmetrical, and so don’t match the other major group of duo-valve shelled animals in Cambrian seas, the phylum Brachiopoda, which invariably show symmetrical valves.

This led most researchers to conclude that stenothecoids must represent an extinct group of the phylum Mollusca, which today includes creatures such as clams, snails, and squid. Some Russian researchers even proposed that stenothecoids represent their own extinct phylum. However, Johnston’s find shows that stenothecoids possessed a previously undetected small opening at the apex of the shell like that in the phylum Brachiopoda, which include living members that use the hole as an exit for a unique stalk that attaches them to the seafloor.

With that realization, Johnston teamed up with brachiopod expert, Michael Streng at Uppsala University, Sweden. Together they proposed that stenothecoids were an early evolutionary off-shoot from tubular Cambrian animals known as eccentrothecimorphs, which show multiple asymmetrical valves, and which ultimately gave rise to brachiopods that survive in modern oceans.

The newly described Burgess Shale fossils will reside in collections of the Royal Tyrrell Museum, Drumheller, and the Royal Ontario Museum, Toronto. Johnston and Streng’s study appeared online this month in the journal Acta Palaeontologica Polonica.

Their study was funded by grants from the Natural Sciences and Engineering Research Council of Canada, Mount Royal University Faculty of Science and Technology, and the Swedish Research Council.

Johnston is past editor of Palaeontographica Canadiana, Canada's national monograph series on paleontological research. An internationally recognized expert on the evolution of bivalve mollusks (clams, oysters, and scallops), his research has taken him to diverse regions of the planet including remote areas of the Rocky Mountains, Arctic Canada, Patagonia, the Gobi Desert, Australia, and the Philippines.

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