345 Million Year Old Eel Had Human-like Spine

Posted on May 22, 2012

Tarrasius problematicus, a 345-million-year-old eel-like fish, had a surprisingly human-like spine. The eel-like fish lived in shallow bodies of water in what is now Scotland, in the Carboniferous period between 359 million and 318 million years ago.

Tarrasius was originally thought to have a vertebral column divided simply into body and tail segments. However, Lauren Sallan, a graduate student in the Program in Integrative Biology at the University of Chicago Biological Sciences, describes Tarrasius with a five-segment column much more similar to the spinal anatomy of land-dwelling animals called tetrapods, including humans. The find argues against a common assumption paleontologists use to determine from fossils whether an ancient species lived on land or in water. The report was published in Proceedings of the Royal Society B.

Sallan says, "It's the last trait to fall. First, limbs were thought to show that a species was on land and walking, and now the vertebral morphology doesn't mean that they're on land either. So a lot of the things we associate with tetrapods actually arose first in fishes, and this is another example of that."

Sallan says the appearance of tetrapod-like spinal organization in a ray-finned fish shatters the presumed relationship between complex vertebral anatomy and both walking and terrestriality. The eel-like Tarrasius possessed no hind fins and a long dorsal fin, indicating it used its intricate spinal column for swimming, not walking. Sallan speculates that the bony vertebrae may have been useful in propelling the fish's body during fast swimming, similar to the stiff vertebrae of modern marlins.

Tarrasius lived several million years after the first tetrapods with hands and feet, but Sallan says the discovery of these spinal features in a fish species confirms that this anatomy can evolve separate from the evolution of walking behavior.

Sallan says, "You can't use this trait to say that something was definitely on land or to identify a tetrapod, which is the way it is used in the field now."

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