Helohyidae were a group of artiodactyl .
Description
Helohyidae share vague similarities to present-day
, though were slimmer in build. They possessed prominent
canine tooth and molars with bunodont cusps, bulging dental wreaths, and wrinkled enamel. Their upper molars were usually squared, due to the enlargement and displacement of the
metaconule, but there was also a small
hypocone and
hypoconule. The
paraconule was reduced and there was no
mesostyle. Their lower molars increased in size as they proceeded to the bottom of the jaw, and the
paraconid was small or absent. Some forms (e.g.
Gobiohyus) possessed small
that separated the premolars from each other. The snout was usually elongated (e.g. in
Helohyus), but in some forms ascribed to this family (
Achaenodon), it was very short. Compared to other primitive artiodactyls such as
Dichobunidae, they possessed higher sagittal ridges; the genus
Achaenodon, in particular, possessed a large sagittal crest and its size was much larger than those of other helohyids.
Classification
The family Helohyidae was established by Marshall in 1877 to accommodate some forms of early artiodactyl mammals of the American Eocene. In addition to the genus
Helohyus, the North American
Parahyus and
Achaenodon were later ascribed to this family. Other forms come from the Upper-Middle-East Eocene of Asia:
Gobiohyus of Inner Mongolia, and
Pakkokuhyus of Myanmar. The latter form may be close to the origin of the family of hippo-like anthracotheres. The artiodactyl
Simojovelhyus was once thought to be an unusually late-surviving genus of helohyid from the Upper Oligocene (extending the families temporal range by around 10 million years),
however recent studies consider it a
peccary.
Helohyids have been variously classified as relatives of archaic dichobunids or as close to the origin of anthracotheres. Some authors consider them close relatives of Dichobunidae,
