It states that the correct formal scientific name for an animal taxon, the name that is to be used, called the valid name, is the oldest available name that applies to it. There are exceptions; another name may be given precedence by any provision of the Code or by any ruling of the Commission.
It is the fundamental guiding precept that preserves the stability of zoological nomenclature. It was first formulated in 1842 by a committee appointed by the British Association to consider the rules of zoological nomenclature; the committee's report was written by Hugh Edwin Strickland.
In 1855, John Edward Gray published the name Antilocapra anteflexa for a new species of pronghorn, based on a pair of horns. However, it is now thought that his specimen belonged to an unusual individual of an existing species, Antilocapra americana, with a name published by George Ord in 1815. The older name, by Ord, takes priority; with Antilocapra anteflexa becoming a junior synonym.
In 1856, Johann Jakob Kaup published the name Leptocephalus brevirostris for a new species of eel. However, it was realized in 1893 that the organism described by Kaup was in fact the juvenile form of the European eel (see eel life history for the full story). The European eel was named Muraena anguilla by Carolus Linnaeus in 1758. So Muraena anguilla is the name to be used for the species, and Leptocephalus brevirostris must be considered as a junior synonym and not be used. Today the European eel is classified in the genus Anguilla Garsault, 1764, so its currently used name is Anguilla anguilla (Linnaeus, 1758).