Extinction at a snail’s pace

After more than half a decade of research and editing, USF professor Gregory Herbert has a feeling of relief. Herbert, a visiting assistant professor at USF, has finally finished work on the effects of mass extinction, which was published in the Dec. 24 issue of Science.

With the help of colleagues from Yale University and the University of California-Davis, seeing the culmination of his research has been worth it for Herbert.

“We’ve been working on the project for almost four years and we had been editing and revising the paper for two years,” said Herbert. “(Getting published is) an unbelievable relief. To see it in print means we finally did something right.”

The research deals with the long-term effects of mass extinction on the population of muricid snails. They studied the feeding habits of the snails to gauge the effects of mass extinction.

“Most muricids bore a hole in their clam prey,” said Herbert. “Because the drill holes preserve so well, we can study how predatory behaviors … have evolved over time by looking at the fossils,” said Herbert.

At the end of the Pliocene era there was a mass extinction that eliminated nearly 70 percent of all species. That severely affected the number of predators hunting muricid snails. Herbert and his colleagues examined fossilized clams from before and after the mass extinction.

The clams from before the mass extinction had drill holes at the edge of the clam where the shell is the thinnest, which would take a snail about two days to bore. The snails target that area of the shell to reduce the risk of being attacked by predators while they are eating.

The clams from after the mass extinction — including those from the present — have holes bored in the thickest part of the clam, which takes about a week for the snails to drill through. This leaves them vulnerable for a much longer time.

That the eating habits of the snails haven’t returned to their pre-extinction form suggests that the ecosystem has not fully recovered from the Pliocene mass extinction, Herbert said.

Recovery from this type of mass extinction event was previously thought to be rapid in relation to other natural cycles, which take place over millions of years.

“Our work redefines recovery,” Herbert said. “Even after biodiversity rebounded from the extinction we studied, muricids were leaving us hints that they were perceiving a world that was still profoundly disturbed. The hint they left was an absence of edge-drilling attacks on clams, which the muricids only do when there are lots of predators around. So it calls into question how paleontologists should define the recovery interval as well as how long the process actually takes.”

The research also sheds light on some issues of evolution.

“The extinctions culled predators from the ecosystem — not numbers of species, just numbers of individual predators,” Herbert said.

“Without abundant predators, however, a primary impetus for evolution has been removed — predation of the weak, lazy, slow or otherwise unfit is an important agent of natural selection. This should give us concern for present-day biodiversity losses, which are heavily biased toward top predators. How will this affect future evolutions?”