Pachydermata

Pachydermata (from two Greek words παχύς pachys, "thick" and δερμα derma, "skin", meaning 'thick skin') is an obsolete order of mammals described by Gottlieb Storr, Georges Cuvier and others, at one time recognized by many systematists. Because it is polyphyletic, the order is no longer in use, but it is important in the history of systematics. Outside strict biological classification, the term "pachyderm" remains commonly used to describe elephants, rhinoceroses, and hippopotamuses.

Cuvier's Pachydermata included the three families of mammals he called Proboscidiana, Pachydermata Ordinaria, and Solipedes, all herbivorous. They are now divided into the Proboscidea, represented among living species only by three species of elephants; the Perissodactyla, or odd-toed ungulates, including horses, tapirs and rhinoceroses; the Suina, or pigs and peccaries; the Hippopotamidae; and the Hyracoidea, or hyraxes.

Cuvier himself defined Pachydermata as "animals with hoofs, nonruminants", whereas Storr had described it as "mammals with hoofs with more than two toes". Cuvier added horses to the order. One naturalist has speculated that -

Baron Cuvier, it is probable, was led to arrange the horse genus among the Pachydermata, less on account of the thickness and tenacity of the skin, than on the slight departure from a true monodactylous character, which every member of this family exhibits in having vestiges of two additional toes under the skin.

Although the former order of Pachydermata is often described as an artificial grouping of unrelated mammals, it was recognised by notable zoologists, including Charles Darwin, as a grade of hoofed mammals to the exception of other ungulates; and anatomical characters support the affinities of "pachyderm" mammals to each other and to other ungulates.