"The morphology is just completely different in each series of vertebrae," Sallan said. "Like a tetrapod, you can tell which segment you're looking at from the basic morphology."
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. Instead of the flexible notochord characteristic of most ancient fish species, Tarrasius possessed heavy vertebral bones organized into five anatomically distinct sections.
Armed with this new anatomical information, Sallan re-examined many other fossils of the species and detected evidence for spinal complexity that researchers previously had missed.
"This was in a different museum from where most of the specimens are, and previous workers had just been looking at the same fossils over and over," Sallan said. "It's basically an issue of finding what you expect to be there instead of what's actually there."
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 surprisingly intricate spinal column 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.
"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," Sallan said.
Instead, the commonalities suggest that similar en
|Contact: Rob Mitchum|
University of Chicago Medical Center