Platychoerops richardsoni was one of the largest known plesiadapids, a group of Palaeogene euarchontan mammals close to the broader primate radiation but not equivalent to modern primates. Its dentition had moved well beyond a simple insect-eating pattern: the premolars became strongly molar-like and the cheek teeth developed tall relief, numerous crests and crenulated enamel, greatly increasing the area available for breaking down plant food.

That dental architecture points to a predominantly herbivorous animal. The strongly crested molars are consistent with substantial leaf processing, while fruit and other plant tissues may also have formed part of the diet; these are functional inferences from the teeth. The enlarged anterior incisor would have been important in acquiring or cropping food, giving the animal a feeding apparatus unlike that of any living primate.

The London Clay type is an incomplete palate preserving upper P4 and molars, and the species is known chiefly through cranial and dental material. Its postcranial anatomy is too poorly documented to state confidently whether P. richardsoni itself was arboreal, scansorial or mainly terrestrial, even though arboreal habits occur elsewhere among plesiadapiforms.