New York, Mar 22 (IANS): Many of the species now perishing due to the ongoing sixth mass-extinction event -- especially the smaller ones -- may vanish without a permanent trace, says researchers.

The fossil record is much more durable than any human record, said one of the researchers Roy Plotnick, professor of earth and environmental sciences at University of Illinois at Chicago, US.

"There are species going extinct today that have never been described," Plotnick said.

"Others are going extinct that are known only because someone wrote it down," Plotnick noted.

All such species would thus be unknown in the far future, he said, if the written historical record is lost -- as it might well be.

Animals least likely to be found as fossils are "the small, cute and fuzzy ones, like rodents and bats", Plotnick said.

"Body size is an obvious factor -- bigger things tend to leave a fossil record, as do things with larger geographical ranges," he pointed out.

For the study, the researchers compared the "Red List" of endangered species with several ecological databases of living species and three paleontological databases of catalogued fossils.

"Comparing the current biodiversity crisis, often called the 'sixth extinction,' with those of the geological past requires equivalent data," Plotnick pointed out.

They ran a statistical analysis to indicate which threatened species were most likely to disappear with no mark of their existence.

The researchers were shocked to find that more than 85 percent of the mammal species at high risk of extinction lack a fossil record.

And those at highest risk have about half the probability of being incorporated into the fossil record compared to those at lower risk, the researchers said.