For decades, scientists believed that a spine with multiple segments was an exclusive feature of land-dwelling animals. But the discovery of the same anatomical feature in a 345-million-year-old eel suggests that this complex anatomy arose separately from and perhaps before the first species to walk on land.
Tarrasius problematicus was an eel-like fish that lived in shallow bodies of water in what is now Scotland, in the Carboniferous period between 359 million and 318 million years ago. Like many fish, Tarrasius was thought to have a vertebral column divided simply into body and tail segments. But in a new description of Tarrasius published in Proceedings of the Royal Society B, Lauren Sallan describes a five-segment column much more similar to the spinal anatomy of land-dwelling animals called tetrapods, including humans.
The surprising find argues against a common assumption paleontologists use to determine from fossils whether an ancient species lived on land or in water.
"It's the last trait to fall," said Sallan, a graduate student in the Program in Integrative Biology at the University of Chicago Biological Sciences. "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."
Tetrapods, which include the first species to walk on land as well as all modern mammals, reptiles, birds and humans, possess vertebrae organized into five distinct segments. From head to tail, the spinal vertebrae can be categorized into cervical, thoracic, lumbar, sacral and caudal sections, each with its own characteristic anatomy.
By contrast, fish vertebrae are typically categorized anatomically into two segments: caudal and pre-caudal. But the spinal column of Tarrasius shows a complexity more like that s
|Contact: Rob Mitchum|
University of Chicago Medical Center