A recent study led by researchers from the Canadian Museum of Nature and the University of Chicago has reconstructed the anatomy of Norselaspis glacialis, a jawless fish that lived over 400 million years ago. Using advanced imaging techniques, the team examined a fossil preserved in sandstone collected during a 1969 French expedition to Spitsbergen, Norway. The analysis revealed features previously thought unique to jawed vertebrates.
“These are the opening acts for a key episode in our own deep evolutionary history,” said Tetsuto Miyashita, research scientist at the Canadian Museum of Nature and lead author of the study published in Nature on August 6.
According to Michael Coates, professor and chair of organismal biology and anatomy at the University of Chicago and senior author of the study, “But there is a large data gap beneath this transformation. We’ve been missing snapshots from the fossil record that would help us order the key events to reconstruct the pattern and direction of change.”
The fossil’s preservation allowed scientists to scan and digitally reconstruct impressions of its heart, blood vessels, brain, nerves, inner ears, and muscles. Kristen Tietjen, digital imaging specialist at UChicago, collaborated with Miyashita and Coates to build a detailed digital atlas.
“With this exquisite digital atlas, we now know Norselaspis in greater anatomical detail than many living fishes,” Miyashita said.
The findings suggest that acute senses and enhanced cardiovascular systems evolved before jaws and teeth. “We found features in a jawless fish, Norselaspis, that we thought were unique to jawed forms,” Miyashita explained. “This fossil from the Devonian Period more than 400 million years ago shows that acute senses and a powerful heart evolved well before jaws and teeth.”
Norselaspis had seven muscles controlling its eyeballs—one more than humans—and proportionally large inner ears and heart. “If Norselaspis was to our scale, its inner ears would be each the size of an avocado, and its heart would be as large as a cantaloupe melon,” Miyashita said.
The fish also had paddle-like fins positioned behind its gills. Coates noted these would have aided sudden movements: “One might even say Norselaspis had the heart of a shark under the skin of a lamprey,” Miyashita added.
Researchers believe these traits helped Norselaspis evade predators rather than pursue prey. As Coates stated: “When jaws evolved against this background, it brought about a pivotal combination of sensory, swimming and feeding systems, eventually leading to the extraordinary variety and abundance of Devonian fishes.”
Miyashita emphasized that early jaws were likely used for suction feeding rather than predation: “It wasn’t as simple as marching straight from a bottom feeder to an apex predator.”
The study also disputes previous theories about how shoulders evolved in tetrapods. The nerve leading to Norselaspis’ shoulder was found separate from those going to its gills—a sign that shoulders did not originate from modified gill structures but emerged independently along with new body regions like necks.
“A lot of these evolutionary changes have to do with how the head is attached to the trunk,” Miyashita said.
Paleontologists continue investigating what drove such anatomical changes. Some attribute them to shifts during periods when marine life became more active swimmers—a time known as the Nekton Revolution.
“For a historical event, we often emphasize one or two symbolic moments to the point of becoming a cliché. In this sense, the evolution of jaws is like a gunshot in Sarajevo starting World War I in 1914,” Miyashita said. “But it is imperative we understand the context. With Norselaspis, we can really find it in its heart.”