Introduction to Taxonomic Keys


Taxonomic keys are methods to identify unknown biological specimens. In the form used here, each number has a pair of statements; starting at couplet 1, one (and only one) of the two statements should apply to the specimen in question. At the end of the applicable statement is either a boldface identification or number (or both, if the identification is to a taxon above the species level). If a number is present, this guides you to the next applicable couplet, and the process continues until an identification is made. Ideally, the identification should then be checked against a source that gives a more complete description, because errors are relatively easy to make during the learning process.

In the form used here, one member of the couplet is identified with a number and the alternate member is given the same number follow by the prime sign ('). For example:

1 Background green....2
1' Background red....3

2 With white speckles....Xus albus
2' With yellow speckles....Yus montanus

3 With white speckles.....Onus virginianus
3' With yellow speckles.....Tuus marylandicus

The interpretation is that if the background is green, go to number 2 and then pick between the couplets (e.g., are the speckles white or yellow); if the background is red, go to number 3 and choose between the couplets.

The example is highly simplified, and keys may be scores of couplets long. If the identifier comes to a couplet where neither member fits, it is likely that a mistake was made in an earlier choice; work backwards to find the error. Whenever possible, check out your identification against a description of the taxon (field guides, professional literature, websites, etc.) to see if it seems to match; if it doesn't, then likely an error was made during the keying.

Mammal Identification

Every taxonomic group has its own set of terminology that makes using a key difficult for the beginner. A number of the terms used here are defined in the glossary or examples are shown within the keys themselves. In some cases, there is a direct link to a definition; such links are shown in bold green, as for this link to the glossary. Standard skull measurements are shown on their own measurements page.

Unlike most birds, many mammals are not easily identifiable solely by external characteristics; often the skull is required; likewise, in some groups, the skull alone is insufficient for sure identification.

When skull are available, tooth formulas often are useful to restrict the number of possibilities. Mammalian teeth are classified (from front to back) as incisors, canines, premolars, and molars (in humans, canines often are called eyeteeth and premolars often are called bicuspids). Primitively, placental mammals are considered to have 44 teeth: on each side of the upper and lower jaws, 3 incisors, 1 canine, four premolars, and three molars. Most mammals have evolutionarily lost some of these; for example, the human tooth formula is 3/3 1/1 2/2 and 3/3 (three upper and three lower incisors on each side of the jaw, 1 upper and one lower canine on each side of the jaw, etc.). The tooth formulas for North American mammals north of Mexico are given.

To use the keys, go the order key. Links in the order key lead to keys to other groups, although the armadillo, opossum, and horse are represented by single taxa and directly key out in the key to orders).

The keys given here are primarily to mammals found in New Mexico. The El Paso Region falls naturally into the sphere of New Mexican mammals, with no mammals occurring around El Paso that do not also occur in New Mexico. Also, many mammals not occurring in El Paso do occur in northern Chihuahuan Desert Region, which includes the mountain islands such as the Sacramento Mountains.


Last Update: 25 Dec 2006