Silhouettes show Mesohippus primigenium, an early ancestor of the modern horse that lived 40 million years ago and was previously believed to have three toes, and the modern horse. Photographs of both animals’ hand bones appear alongside renderings of the researchers’ proposed digit identities. The researchers argue that missing digits one and five are partially expressed on the surfaces of the side toes (shown in red/blue). While the horse is described as being monodactyl, with only one complete digit, the researchers demonstrate that digits two and four are expressed as the splint bones and frog (padding of the foot), as shown in yellow/green. Missing digits one and five are expressed as ridges on the splint bones and as the hoof cartilages, as shown in the lower red/blue areas.