Change of form of the odontoid process of the second or axis vertebrae from a cone to a hollow half-cylinder.
The primitive Artiodactyla thus probably had the typical number (44) of incisor, canine and molar teeth, brachyodont molars, conical odontoid process, four distinct toes on each foot, with metacarpal, metatarsal and all the tarsal bones distinct, and no frontal appendages.
The odontoid process of the second vertebra is pig-like: and the tibia and fibula and radius and ulna are severally distinct.
Sometimes the pad is reduced to a ventral semi-ring or meniscus; it retains its largest almost original shape and size in the second vertebra, the axis or epistropheus, where it forms a separately ossifying piece which connects, and coossifies with, the odontoid process (the centrum of the atlas) and the centrum of the second vertebra.
The odontoid process of the axis is wide, flat, and hollowed above, as in the ruminants.