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Mollusk fossils push back evolution, ROM scientists say

Two Canadian paleontologists have discovered dozens of fossils of a soft-bodied, deep-sea dweller that lived more than half a billion years ago, adding one more piece to the enigmatic puzzle that is the history of life on Earth.

The 189 well-preserved fossil specimens of Odontogriphus omalus have been interpreted as the world's oldest known soft-bodied mollusc, and were found in British Columbia's mountains in the Burgess Shale, one of the most important fossil sites in the world.

The newly discovered fossils are remarkable, one of the researchers notes, because there are perfect impressions of all of the animal's soft tissues.

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The fossils show the early mollusc had an oval body ranging in size from a few millimetres to 20 centimetres with simple gill-like structures surrounding a muscular sole or "foot" on the underside.

The stomach, intestines, outer membrane and mouth are all visible.

This discovery pushes back the history of animal evolution tens of millions of years to 560 million years ago in Precambrian time (543 million years ago and earlier), according to the Royal Ontario Museum's David Rudkin, co-author of the article published in today's issue of the journal Nature.

Very few fossil specimens have been found from that time period. The Cambrian Period (543 million to 490 million years ago) marked the sudden appearance of complex multicellular macroscopic organisms.

In the Precambrian era, before the so-called explosion, organisms were thought to be much simpler, but this study shows that was not the case.

"This is a crucial interval in evolutionary history because it seems to represent a time in which a great deal happened," he said.

" Odontogriphus seems to be a late holdover that somehow got preserved in with the creatures from the Cambrian . . . opening up new windows on evolution for us," Mr. Rudkin said.

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The specimens were collected over 15 years in the late 1980s and 1990s by the ROM and, upon closer examination, were found to have distinguishing "molluscan" features including a specialized feeding structure called a radula, made up of short rows of small, tooth-like elements that would wave and sweep food into the mouth.

The shell-less molluscs grazed on seafloor bacterial growths.

Odontogriphus, which translates to "toothed riddle" was originally discovered in 1976 from a single, poorly preserved specimen. Until now, it has been described as an "enigmatic organism," according to the study's lead author, Jean-Bernard Caron, also of the ROM.

"Our study redescribes and reinterprets previously unrecognized features that link Odontogriphus to the molluscs, one of the most diverse and important groups of animals living today," Dr. Caron said.

Odontogriphus predates modern-day molluscs -- with 200,000 living species today including snails, clams, squids and octopuses -- which began to develop hard shells during the Cambrian Period to survive.

"They were the last of their kind and they were dying out because the sea floor was changing and all these other animals started developing hard parts and new strategies for dealing with predators," Mr. Rudkin said. "The successful molluscs are those that branched off and developed shells."

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Mr. Rudkin said the fact that many molluscs have survived such a catastrophic extinction could shed light on the evolutionary path many animals may take.

"Those lessons we learn from the past -- about where groups of organisms originated, when they become extinct, how they became extinct, or if they didn't become extinct entirely, how they recovered from extinction -- we use that kind of historical background to help us predict what might happen in modern extinction circumstances. Maybe there's a lesson in there for us."

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