The Protoceratidae apparently resembled deer, although they were not closely related to them. Their body length ranged from 1 to 2 m, approximately from the size of roe deer to Vapiti . Their teeth resembled the teeth of modern deer and bovid . It is assumed that they fed on hard herbs and had complex stomachs. It is believed that at least some species lived in herds [2] .
The horns are the most original feature of the Protoceratidae. In addition to the usual pair of ruminant horns, they had additional horns on the nose. These horns are either paired, as in Syndyoceras , or they are one horn, splitting closer to the end, as in Synthetoceras . The horns were probably covered in leather, much like the ossicons of giraffes . The females were either hornless, or had much smaller horns than the males. This means that the horns, apparently, served to attract females or demonstration behavior of males. In the later forms, the horns were large enough for the males to butt them, like modern deer [3] .
The family of Protoceratidae was singled out by Charles Marsh in 1891 based on the type genus Protoceras and placed in a detachment of unmarried ones [4] . The position of the family in the phylogenetic tree of the order is controversial: various taxonomists placed it in the Pecora infraorder ( Cook in 1934); the suborder of ruminants (Ruminantia) ( Thurmond and Jones in 1981); as a suborder of the Mozolodene (Tylopoda) ( Carroll in 1988 and Webb with colleagues in 2003); in the detachment of artiodactyls ( Hulbert and Whitmore in 2006, as well as Prothero and Ludtke in 2007) [1] , until in 2009 by Spaulding , O'Leary and Gatesy together with ruminants it was not allocated to the Ruminantiamorpha clade [5] .
According to the Fossilworks site, as of November 2016, the family includes the following extinct taxa up to and including genus [6] :
- Childbirth incertae sedis
- Heteromeryx
- Leptoreodon
- Leptotragulus
- Poabromylus
- Toromeryx
- Trigenicus
- Subfamily Protoceratinae
- Subfamily Synthetoceratinae
- Triba Kyptoceratini
- Tribe Synthetoceratini
- Lambdoceras
- Prosynthetoceras
- Synthetoceras
