The ongoing sixth mass extinction are making many species die without leaving their mark. Animals that are now going extinct may vanish without leaving a fossil record, and previous extinctions may have been underestimated as well, scientists say.

In a study published in the journal Ecology Letters, paleontologists compared the extent of the ongoing biodiversity crisis, which has been called the "sixth extinction," with five previous mass-extinction events in prehistory.

To compare the extinction events, the paleontologists needed equivalent data. For this, they researched and correlated databases of living species in the IUCN Red List of Threatened Species with three paleontological databases of catalogued fossils as well as with a number of ecological databases of living species.

By comparing the known fossil records of modern animal species, Roy Plotnick, Felisa A. Smith and S. Kathleen Lyons found that animals that are likely to go extinct may disappear without a fossil trace. For instance, the researchers found that no less than 85 percent of mammal species with a high risk extinction are lacking fossil records.

According to the researchers, "small, cute and fuzzy" animals such as bats and rodents are the most likely ones to vanish without fossil records because of the size of their bodies. Larger animals, and those with wider geographical ranges, are more likely to be found as fossils.

Because animals are starting to vanish without a permanent trace, the extent of the current extinction event seems noticeably reduced. This scenario may be even worse for vertebrates dwelling on land.

"We found that only 3.3 percent of threatened bird species (26 out of 777) and 1.6 percent of threatened reptile species (9 out of 551) have a recorded fossil record," the paleontologists note.

The researchers added that fossils are the only reliable record of life on Earth. Books may also perish compared to clay tablets, and even electronic media may not last millions of years compared to a fossil. Fossils are far more durable compared to human-made recorders.

"As humanity has evolved, our methods of recording information have become ever more ephemeral," says Plotnick, who is from the Department of Earth & Environmental Sciences at the University of Illinois in Chicago. "If we put everything on electronic media, will those records exist in a million years? The fossils will."

Photo: Robert Payne | Flickr