More than 20 years after its discovery, a 375-million-year-old fossil creature called “tiktaalik” has crawled into public view at the Canadian Museum of Nature in downtown Ottawa.
Tiktaalik roseae is a well-preserved fossil discovered in 2004 by a team of American paleontologists on Ellesmere Island in Nunavut and is a highlight in the McLeod Street museum’s new exhibit “Life Onto Land: The Devonian.” The exhibition, which runs until October, displays key archeological findings from Earth’s Devonian period — which lasted from about 420 to 359 million years ago — that lend proof to the theory that land species evolved from under water.
The museum tells the public to “Discover tiktaalik, an iconic fossil that helps us understand how fins became limbs — on view for the first time in Canada.”
Museum paleobiologist Scott Rufolo said Ottawa residents “aren’t aware how important the Canadian fossil record is to documenting this transition of life onto land.”
CMN spokesperson Dan Smythe explained that prior to the continental drift that created the present-day configuration of the planet, northern Canada was positioned significantly closer to the equator.
Because of that position, Nunavut, said Smythe, “got a mother lode of fossils from the Devonian.”
It’s not the ancestor of all life on land. It’s a cousin of the ancestor, but it’s a close cousin.
— Dr. Neil Shubin, University of Chicago
The museum houses more than 15 million specimens that “represent biodiversity and geodiversity over time,” said Smythe. He said the exhibit starring tiktaalik — one of the world’s most famous fossils because it symbolizes a watershed moment in evolution — aims to show the public how Devonian biodiversity allowed plants and animals to transition onto land.
The tiktaalik roseae was not a typical fish and not a typical four-legged creature known as a tetrapod. Its fins had strong, wrist-like bones that the animal used to crawl up from the seafloor and sit in shallow water. The 2.7-metre long, scaly animal had air sacks for respiration that were likely the precursor to lungs as we know them today.
Tiktaalik species “were really set up biologically to breathe in the water,” Rufolo said. “But they could also take advantage of oxygen in the air when they were outside of the water.”
Rufolo said evidence of life forms from 390 million years ago shows species that are fully aquatic. However, evidence shows that, by 370 million years ago, some specimens are land tetrapods.
Tiktaalik, “at 375 million years in age, falls right in that intermediate point,” said Rufolo. “They were organisms that were beginning to develop this mosaic of different anatomical features.”
Over the past two decades, tiktaalik has been the centre of many academic studies and news articles, including one that The Harvard Gazette published in 2006. It popularized tiktaalik as a “missing evolutionary link” between aquatic and land species.
University of Chicago evolutionary biologist Neil Shubin discovered tiktaalik roseae along with colleague Edward Daeschler, a paleontologist with the Philadelphia-based U.S. Academy of Natural Sciences, in 2004. Shubin said he doesn’t love the term “missing link,” because the fossil is what he called a “found link.”
“It’s not the ancestor of all life on land,” said Shubin, who is also provost of Chicago’s famed Field Museum. “It’s a cousin of the ancestor, but it’s a close cousin.”
The use of the term “missing link” was misleading to the evolutionary process, he suggested.
“Describing it as a missing link may be good for popular context, but it misses a lot of the rigor we use in science,” he told Capital Current.
Tiktaalik is now understood as one piece of the evolutionary “mosaic” that explains the transition from water to land.
Shubin says he and Daeschler have found “about 20” fossilized tiktaalik in the 22 years since its initial discovery. He said they scan each fossil to look at the structure of the bones to draw comparisons between its structure and that of other aquatic and terrestrial creatures.
“We look at the shape of the joints on the bones to understand how the bones likely moved relative to one another,” he said.
Comparing trackways — the preserved marks an animal leaves behind in the sand — is also important for scientists to predict how Devonian species traveled. In 2009, researchers from Warsaw University discovered what they believed to be tiktaalik roseae’s footsteps along the coast of Poland. These trackways potentially set tetrapods on land 18 million years prior to Shubin and Daeschler’s theory.
Shubin explained that trackways are hard to read. The Devonian was home to diverse fish species that would have left their own trackways along the bottom of shallow water.
“(Of) all those trackways that are older than this fossil, none of them have the signature anatomical structure that’s characteristic of tetrapods,” Shubin said. “And that’s digits.”
Each imprint on the trackway would have a main point of contact indicating the palm, and the track of a tetrapod would show smaller imprints surrounding the centre, indicating digits.
“What we’re seeing is a variety of creatures that exist at this time period that were walking fish, not necessarily tetrapods,” he said.
Shubin and Rufolo said that working with the Nunavut government and engaging the community’s youth in their expeditions has been key to their success.
“When we found the fossil, we knew it was going to be important,” Shubin said. “So we engaged the Nunavut community in a naming project.”
Tiktaalik is an Inuktitut word meaning “large, freshwater fish seen in the shallows.”
“We wanted a name that was meaningful to them and to us,” he said, “and that was the name they came up with.”
