On the basis of molar cusp pattern, the family Hyrachyidae is transferred from the Rhinocerotoidea to the Tapiroidea and is classified as a subfamily of the Helaletidae. The family Rhinocerotidae is restricted to forms with the unique chisel-shaped I1 and procumbent, lanceolate I2 shearing specialization seen in Epiaceratherium and Trigonias and, with modifications, their descendants. Non-amynodontid rhinocerotoids with other anterior dental specializations are placed in the Hyracodontidae.
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Leonard Radinsky (1966) studied this question.
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