Paleontologists have a similar system for keeping their fossil species straight. The first person to publish a formal description of a new species gives it a name and describes it in great detail. But the real standard is an actual physical fossil, the type specimen. Type specimens are carefully preserved in museums and university collections, where any researcher can go see them.

It may seem, I don't know, unscientific to do things that way. Imagine picking one person to be the "type human." Ideally the researcher wants many specimens of a species that show its whole range of shapes and size, so you could put together a sort of ideal picture of the species. But the first thing is to give it a name and a starting standard, to avoid confusion and establish priority.

 

Elphidium granti holotype, about 0.5 mm across, from UCMP.

When picking the type specimen, the paleontologist isn't choosing the ideal individual, or the biggest, or the most anything, just the most typical one. It has all the important features, with nothing that's unusual for the species. The type specimen might not be perfect or even complete. And what if the fossil is just a pollen grain, or a seed or an egg, or a caterpillar whose adult form is a moth? These are problems for later researchers to resolve.

The type specimen for Tyrannosaurus rex is in the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. And the University of California has put a bunch of type specimens for microfossils online. They're pretty things.

In the 200 years or so this system has been used, it's spawned its own set of words. To name a new species, a researcher writes a paper with a full description of the species, comparing it with similar species and noting the differences. The type specimen that goes with this document is the holotype. If two or more specimens are necessary, they're called syntypes, and any supplementary specimens are paratypes. If a correction or extension to the species is published, that's based on a hypotype. If a better specimen is found to replace the holotype, that's a lectotype, and if the holotype is lost or destroyed, a neotype is chosen. All of these items are permanently preserved and their whereabouts published in the literature. Oh yes, and a copy or cast of these reference materials is called a plastotype.

This system of types also works for a new genus, or group of related species. The worker who names a new genus uses a type species. And similarly, a type genus is used to define a new family, or group of related genera. (I don't think the system goes higher than that, to the orders and classes and phyla, but it might.) These definitions rest, ultimately, in the type specimens that define each species.

Geologists themselves use the type concept for the "rock species," or lithostratigraphic units, that you see on geologic maps. Every formally defined rock unit, like the Inwood Marble under Manhattan or the Pierre Shale of the Midwest, is based on a type area. That is a place where the rock unit is well exposed, particularly its bottom and top, and where the typical rocks found in the unit appear. A type locality in the type area gives the rock unit its name. At one spot in the type area, the geologist describes a type section—a sequence of rock strata that becomes the standard for that unit forever.

Where—and who—is the type specimen for the human species, Homo sapiens? Even though Linnaeus first described our species in 1758, there was no type specimen selected until 1994, when the paleontologist Robert Bakker formally declared the skull of Edward Drinker Cope as the lectotype. When Cope, himself a great paleontologist, died in 1897, he willed his remains to science, and they are held by the University of Pennsylvania.

Information provided by: http://geology.about.com