Four hundred fifty-five million years ago, Earth was a hot place with high sea levels that had flooded many of the landmasses, including what would later become North America. This spreading ocean was exploding with new life, such as the bony-plated fish that still stand as some of the oldest-known vertebrates.
When they died, some became buried in the sediment that covered the ocean floor and fossilized, to be discovered later by paleontologists living in a drier time. One such find, from ancient deposits in Colorado, contains parts of a jawless fish that a new study has scanned with modern X-ray tomography.
A Jawless Fish With Unusual Cranial Bones
This 455-million-year-old specimen of Eriptychius americanus contains the earliest ever seen neurocranium — the cartilage protecting the fish’s brain — and one of the strangest. According to the study, the fish’s cartilage fits neatly yet loosely around the brain in a design with no known analogue.
As the earliest vertebrate neurocranium ever studied, it fills a 100-year gap between the earliest such fossils ever recovered previously (from about 400 million years ago) and the origin of vertebrate fish some 500 million years ago.
What Is a Neurocranium?
One of the greatest innovations of the vertebrate body plan, the neurocranium both protects the brain and helps to connect it to sensory organs, the mouth and more. In humans, the neurocranium is the portion of the skull that contains the brain, and the skull’s seam-like sutures allow it to expand all the way until early adulthood to allow for growth.
While the human skull is composed of 22 different bony pieces, E. americanus’ neurocranium was composed of 10 long pieces of cartilage that fit together without being fused, according to the X-ray scans. The imaging also revealed that canals snaked through the cartilage to deliver either blood supply or connections to sensory organs. The fish’s skin wrapped tightly around the whole structure, though the scientists noted a clear anatomical distinction between the two.
This novel neurocranium falls somewhere between the loose, open cartilage style seen with lampreys and the more closed-off designs present in gnathosomes, a group that includes humans.
The reconstructed neurocranium of Eriptychius americanus, as seen from above. (Credit: Field Museum of Natural History/Richard Dearden)
Read More: The Minds of Ancient Fish May Explain the Evolution of Tetrapods
The Early Days of Cranial Evolution
Vertebrates exist with both structures, and scientists have tried to determine how they evolved. The new paper speculates that the largely locked-up cranium we enjoy evolved later down the line from E. americanus.
Ultimately, that layout became a central part of gnathosomes as we know them (and us) today.
“These are tremendously exciting results that may reveal the early evolutionary history of how primitive vertebrates protected their brains,” said Ivan Sansom, a paleobiologist and one of the study authors, in a statement.
Read More: The Fish That Ate Our Ancestors
