A Definition of Living Fossils

Definition

By definition, fossils are the remains of dead organisms; however, living fossils are living species that exhibit little or no distinctive differences from their counterparts from early geological periods. Simply stated, the organisms of today are the same as the organisms of the past.

Some living fossils are common, such as the cockroach or opossum. Other less common fossils include the ginkgo tree and the horseshoe crab. Fossil evidence shows that the horseshoe crab developed around 300 million years ago during the Carboniferous Period and survived unaffected by various environmental changes, including fluctuations in water salinity and changing food sources (the horseshoe crab will eat anything), and also because of its ability to tolerate pollutants.

Many of today's living fossils include the rediscovery of organisms thought to be extinct. The most famous living fossil is the coelacanth--a fish thought to be extinct since the Cretaceous Period (65 to 144 million years ago). Comparing the living coelacanth with 400 million-year-old fossils showed the two appeared identical. Since 1938, more than 200 coelacanths have been caught. Perhaps the continuing tales of creatures such as the American "Bigfoot" or the Loch Ness monster hold some truth.

The term "Lazarus taxon," used interchangeably with "living fossil," describes a species that suddenly reappears in the fossil record or in nature after a period of assumed extinction. The term Lazarus taxon, from the resurrection of Lazurus in the Bible, is flawed, since no species suddenly resurrects but instead shares a family heritage, living or extinct. In December 2008, the BBC News reported on the discovery of the Laotian rock rat, the sole survivor of the Diatomyidae family understood to have died out 11 million years ago.

As generations pass, bodies and behavior also change, and these changes pass on to offspring. Some changes increase the likelihood of survival, while others do not. Although not officially listed as a living fossil, the peppered moth is a superb example of natural selection--using its light-colored coating as camouflage on lichen-covered trees. When pollution covered the trees with soot, the offspring naturally born darker in color survived while the lighter peppered moths became easier for predators to see. Natural selection offers one possible reason for the survival of living fossils.

Darwin stated in the Origin of Species that "innumerable transitional forms must have existed." The coelacanth was once portrayed as the intermediate or missing link for the first amphibians. Its rediscovery changed that philosophy. The German naturalist Hans Fricke observed the ancient fish in its natural habitat, discovering that the creature swam forward, backward and sometimes with its head tilted down but never moved along the ocean floor using its lobed fins. Although the coelacanth is no longer classified as a missing link, it is a living fossil.