taxonomic units at species level, rather than as (for example) subtaxa of other species. According to the Principle of Coordination, the same epithets can also be applied at subspecies level, i.e. as the third name in a trinomial name, should the taxonomic treatment being followed prefer such an arrangement. Taxonomy Taxonomy classifies organisms together which possess common characteristics. Nomenclature does not determine the rank to be accorded to any assemblage of animals but only whether or not a particular name is "available" for use, once a particular taxonomic decision has been made. Therefore, zoologists are free to propose which group of animals with similar characteristics that a taxon might belong to. In 1978, a review to reduce the number species listed under genus Canis proposed that "Canis dingo is now generally regarded as a distinctive feral domestic dog. Canis familiaris is used for domestic dogs, although taxonomically it should probably be synonymous with Canis lupus." In 1982, the first edition of Mammal Species of the World included a note under Canis lupus with the comment: "Probably ancestor of and conspecific with the domestic dog, familiaris. Canis familiaris has page priority over Canis lupus" [in fact this is irrelevant: page priority is not a concept that exists any more in zoological nomenclature] "but both were published simultaneously in Linnaeus (1758), and Canis lupus has been universally used for this species". In 1999, a study of the maternal lineage through the use of mitochondrial DNA (mtDNA) as a genetic marker indicated that the domestic dog may have originated from the grey wolf, with the dingo and New Guinea singing dog breeds having developed at a time when human populations were more isolated from each other. In 2003, the ICZN ruled in its Opinion 2027 that the "name of a wild species ... is not invalid by virtue of being predated by the name based on a domestic form". Additionally, the ICZN placed the taxon lupus as a conserved name on the official list under this opinion, meaning that both this epithet and familiaris are available names (one does not override the other) in the event that a taxonomist wishes to differentiate between the two taxa at species level. In the third edition of Mammal Species of the World published in 2005, the mammalogist W. Christopher Wozencraft listed under the wolf Canis lupus its wild subspecies, and proposed two additional subspecies: "familiaris Linnaeus,