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. The surprising find argues against a common assumption paleontologists use to determine from fossils whether an ancient species lived on land or in water.
Lauren Sallan, a graduate from the University of Chicago Biological Sciences Division, described a new interpretation of the spinal structure of a 345 million year old eel fossil, Tarrasius, which resembles a modern-era wolf-eel (top photo), that is very similar to land dwelling animal spines.
When Sallan began her research on Tarrasius, she wasn't looking for an unusual spine, but rather how the species fit evolutionarily among other early ray-finned fishes. While examining undescribed fossils at the National Museums Scotland in Edinburgh, Sallan found some unexpected features.
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 seen in tetrapods, with five segments separated by abrupt transitions.
The appearance of tetrapod-like spinal organization in a ray-finned fish shatters the presumed relationship between complex vertebral anatomy and both walking and land-dwelling. Tarrasius possessed no hind fins and a long dorsal fin, indicating it used its spinal column solely for swimming, not walking. And while Tarrasius lived several million years after the first tetrapods with hands and feet, the discovery of these spinal features in a fish species confirms that this anatomy can evolve separate from the evolution of walking behavior.
"It's the last trait to fall," said Sallan, "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."
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