"It's as if the roles persist, but the players change: the cast is transformed dramatically. Something happened that almost wiped the slate clean, and, of the few stragglers that made it through, a handful then re-radiate spectacularly."
Scientists have long theorized that the Late Devonian Kellwasser event--considered to be one of the "Big Five" extinctions in Earth's history--was responsible for a marine invertebrate species shake-up.
But an analysis of the vertebrate fossil record by Sallan and Coates pinpointed a critical shift in diversity to the Hangenberg extinction event 15 million years later.
Prior to the extinction, lobe-finned forms such as Tiktaalik and the earliest limbed tetrapods such as Ichthyostega had made the first tentative "steps" toward a land-dwelling existence.
But after the extinction, a long stretch of the fossil record known as "Romer's Gap," is almost barren of tetrapods, a puzzle that had confused paleontologists for many years.
Sallan and Coates' data suggest that the 15-million-year gap was the hangover after the traumatic Hangenberg event.
"Something that's seen after an extinction event is a gap in the records of survivors," Sallan said. "You have a very low diversity fauna, because most things have been killed off."
When tetrapods finally recovered, those survivors were likely the great-great-grandfathers to the vast majority of land vertebrates present today.
Modern vertebrate traits--such as the motif of five-digit limbs that is shared by all mammals, birds and reptiles in utero--may have been set by this early common ancestor, the authors propose.
"Extinction events remove a huge amount of biodiversity," Coates said. "That shapes in a very significant way the patchiness of biodiversity that persists to the present day."
The analysis benefitted from recent advances in filling in the vertebrate
|Contact: Cheryl Dybas|
National Science Foundation