Helohyidae were a group of

artiodactyl The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s. They were most prominent in the mid-to-upper

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

(~50 to 39 million years ago).

Description

Helohyidae share vague similarities to present-day

pig The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of ''Sus ...

s, though were slimmer in build. They possessed prominent canines and molars with

bunodont The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

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 A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...

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

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

s 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 ''Achaenodon'' is an extinct artiodactyl mammal, possibly belonging to the eloiids. It lived in the Mid-To-Upper Eocene (about 43-39 million years ago) and its fossil remains have been found in North America. Description This animal was large ...

''), it was very short. Compared to other primitive artiodactyls such as dichobunids, 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, ''Pakkokuhyus'' of Burma and ''Progenitohyus'' of Thailand. 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 A peccary (also javelina or skunk pig) is a medium-sized, pig-like hoofed mammal of the family Tayassuidae (New World pigs). They are found throughout Central and South America, Trinidad in the Caribbean, and in the southwestern area of North ...

. Helohyids have been variously classified as relatives of archaic dichobunids or as close to the origin of anthracotheres. The current opinion is to classify them as relatives of the dichobunids.McKenna and Bell, 1997; Stucky, 1998; Rose, 2006

References

{{Taxonbar, from=Q10294574 Prehistoric mammal families