Using advanced imaging techniques, an international research team has reconstructed an ancient extinct fish’s heart, brain, and fins from an intricately detailed, fingernail-sized fossil fragment. The results: a googly-eyed mixture of a tadpole, horseshoe crab, and Gary the Snail from Spongebob Squarepants. But cartoon lookalikes aside, the creature may help rewrite one of the earliest chapters in animal evolution. Its details are described in a study published on August 6 in Nature.
A fishy timeline
Earth’s first fish arrived about half a billion years ago, but not anywhere near the ocean’s surface. Instead, they started their evolutionary lives much closer to the seafloor, where they could suck up food before they evolved jaws and teeth. The widely accepted theory is that these anatomical additions literally arose with the fish as they ascended higher into the water column. By around 400 million years ago, jawed fish dominated the oceans. From there, evolution slowly introduced limbed vertebrates, land-dwelling creatures, and eventually humans. But while the “jaws first, then limbs” hypothesis has legs (so to speak), there are still issues.
“There is a large data gap beneath this transformation,” UChicago biologist and study senior author Michael Coates explained in a statement. “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.”
For decades, a potentially pivotal snapshot has remained buried in a paleontological archive. A 1969 expedition to Norway’s Arctic Spitsbergen archipelago yielded thousands of fossil-embedded sandstone rocks, but it took another 40 years before researchers could dedicate enough time to sort through them. Amid all the sandstone splitting, the study co-authors were surprised to find a perfectly preserved, half-inch-long Norselaspis cranium dating back 410–407 million years.
The team soon sent their discovery to Switzerland’s Paul Scherrer Institute, where experts used synchrotron-based X-ray microtomography to scan the specimen layer-by-layer. The results stunned them. After thousands of hours of digital reconstruction, the 3D scans clearly depicted tissue-thin bones encasing organs and musculature preserved in unprecedented detail.
“With this exquisite digital atlas, we now know Norselaspis in greater anatomical detail than many living fishes,” said lead author and paleobiologist Tetsuto Miyashita.
Although jawless, Norselaspis included anatomical features that biologists previously believed only existed in jawed species. In particular, the ancient fish had already evolved an extremely powerful heart and widened vessels facilitating greater blood flow.
“One might even say Norselaspis had the heart of a shark under the skin of a lamprey,” explained Miyashita.
Avocado ears and cantaloupe hearts
Norselaspis’ sensory organs were also impressive. Seven small muscles controlled its eyeballs—one more than humans possess—while its inner ears were comparatively gigantic.
“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 added.
The evolutionary advantages weren’t all internal. Norselaspis also swam using angled, paddle-like fins located behind its gills that allowed it to quickly turn, stop, and speed up as needed. None of this was likely used to catch prey, however. Given its lack of jaws and teeth, the lumpy fish potentially used its unexpected anatomy to evade the predators who were evolving chompers. This interplay between species led to an explosion in oceanic diversity.
“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,” said Coates.
Head and shoulders
Norselaspis isn’t only challenging the timeline of jawed fish. Researchers also noted that a nerve linked to its shoulder was separate from the nerves reaching the gills. Because of this, they now theorize the shoulders seen in tetrapods (four-limbed vertebrates) evolved as a new structure connected to the neck which separated the torso from the head. Most early jawless fishes had torsos leading continuously to the head, whereas jawed vertebrates evolved a neck and throat. Norselapsis straddles both anatomies, with researchers likening it to a human whose arms extended from behind their cheeks.
Although it’s still unclear what instigated jaw formations in the first place, Norselaspis reveals the evolutionary journey of vertebrates didn’t follow a singular path.
“It wasn’t as simple as marching straight from a bottom feeder to an apex predator,” Miyashita said.
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