The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families, the Leporidae (hares and rabbits), and the Ochotonidae (pikas). The name of the order is derived from the Greek lagos (λαγος, "hare") and morphē (μορφή, "form").
Though these mammals can resemble rodents (order Rodentia) and were classified as a superfamily in that order until the early twentieth century, they have since been considered a separate order. For a time it was common to consider the lagomorphs only distant relatives of the rodents, to whom they merely bore a superficial resemblance.
The earliest fossil lagomorphs, such as Eurymylus, come from eastern Asia and date back to the late Paleocene or early Eocene. The leporids first appeared in the late Eocene and rapidly spread throughout the northern hemisphere; they show a trend towards increasingly long hind limbs as the modern leaping gait developed. The pikas appeared somewhat later in the Oligocene of eastern Asia.
Lagomorphs differ from rodents in that:
- they have four incisors in the upper jaw (not two, as in the Rodentia);
- they are almost wholly herbivorous (unlike rodents, many of which will eat both meat and vegetation; the few recorded exceptions within the Lagomorpha occur among members of both Lepus and Ochotona, and involve the occasional foraging for carrion as a supplementary winter food source);
- the male's scrotum is in front of the penis (unlike rodents', which is behind); and
- the penis contains no bone (baculum), unlike in rodents.
However, they resemble rodents in that their teeth grow throughout their life, thus necessitating constant chewing to keep them from growing too long.