The preserved stomach of a Gogo fish fossil under the microscope. Pictured at the WA Museum. Credit: Yasmine Phillips, Curtin University
The analysis of the oldest—and best preserved—fish heart fossil ever found clarifies an evolutionary hole on the way from jawed stem vertebrates to living jawed vertebrates, which includes mammals and humans.
Researchers discovered the 380-million-year-old heart alongside a separate fossilized stomach, intestine and liver in an ancient jawed fish in the Gogo Formation, formerly a large reef in Western Australia that is now a treasure trove for anthropologists.
According to the study, published in Science, the position of the organs in the body of arthrodires—an extinct class of armored fishes that flourished through the Devonian period from 419.2 million years ago to 358.9 million years ago—is similar to modern shark anatomy, offering new evolutionary clues.
What's really exceptional about the discovery of the fishes is that their soft tissues are preserved in three dimensions. Most cases of soft-tissue preservation are found in flattened fossils, where the soft anatomy is little more than a stain on the rock.
“As a paleontologist who has studied fossils for more than 20 years, I was truly amazed to find a 3D and beautifully preserved heart in a 380-million-year-old ancestor,” said lead researcher Kate Trinajstic, distinguished professor of paleontology at Curtin University (Australia).
The researchers used neutron beams and synchrotron X-rays to scan the specimens, still embedded in the limestone concretions, and constructed three-dimensional images of the soft tissues inside them based on the different densities of minerals deposited by the bacteria and the surrounding rock matrix.
The findings show—for the first time—a 3D model of a complex s-shaped heart in an arthrodire that is made up of two chambers, with the smaller chamber sitting on top. These features are considered advanced for such early vertebrates, offering a unique window into how the head and neck region began to change to accommodate jaws, a critical stage in the evolution of human bodies.
“For the first time, we can see all the organs together in a primitive jawed fish, and we were especially surprised to learn that they were not so different from us,” said Trinajstic.
The liver was large and enabled the fish to remain buoyant, just like today’s sharks. However, Trinajstic points out one critical difference. Some of today’s bony fish, such as lungfish and birchers, have lungs that evolved from swim bladders, but the researchers found no evidence of lungs in any of the extinct armored fishes. This suggests today’s lungs evolved independently in bony fishes at a later date.
“Evolution is often thought of as a series of small steps, but these ancient fossils suggest there was a larger leap between jawless and jawed vertebrates,” she explains. “These fish literally have their hearts in their mouths and under their gills—just like sharks today.”