• The top two images are of living plankton in the swimming stages of their life cycle. The bottom right image is of a 50 million-year-old fossil cyst of the plankton Dapsilidinium pastielsii. The bottom left image is the empty cyst of the modern ancestor thought to have gone extinct a million years ago. (Photo: Mertens et al., 2014)

It looks like a lone twig at the end of a once bushy branch on a family tree.

That’s how Brock University geologist Martin Head describes his latest find of a species of plankton thought to have been extinct for one million years.

“We had assumed that this particular species had gone extinct a million years ago because we know from the fossil records it would’ve preferred warm conditions,” Head says. “About a million years ago, the global climate got colder and it was assumed this species was a victim of global cooling.”

The plankton was found living at the bottom of the Pacific Ocean in an area off Japan called the “Indo-Pacific Warm Pool,” where Head says it remained warm during the last global cold snap. “If you’re a warm-water species wanting to sit out a time when the global conditions are cooling, then the central western Pacific is the place to be.”

As part of their life cycle, some plankton produce a cyst with a durable outer layer. This cyst stage allows the plankton to sit out adverse conditions for months, possibly even a few years. When conditions improve, they hatch out of the cysts and the life cycle continues. The cyst stage is crucial for researchers because the shell is hard enough to be preserved in the geological record, even if the animal inside dies. Because each species has a distinct shell, scientists can map how a species has changed over millions of years.

With the cyst stage being the only part of the life cycle that remains in the geological record, Head says researchers can only learn so much about the plankton. But this living fossil gives scientists an unprecedented window into the lives of creatures that died out millions of years ago.

Head says the latest find was an accident. His colleague had been looking at plankton cysts in ocean floor sediment when he stumbled on one he couldn’t identify. It turns out the closest match was from a million years ago. Even more surprising is it was the last remaining member of a lineage that was abundant 50 million years ago.

“What it means is that the western Pacific is acting as a refuge for these warm-water species that have died out everywhere else. This is the first time a living fossil has been found in the plankton of the western Pacific Ocean. It may be time for other biologists to see what else might be there.”