Evolution of the equid single hoof: Hipparion tracks

Hipparion tracks and horses’ toes: the evolution of the equid single hoof
R Soc Open Sci. 2023 Jun 21;10(6):230358. doi: 10.1098/rsos.230358. eCollection 2023 Jun.
Authors
Alan R Vincelette 1 , Elise Renders 2 , Kathleen M Scott 3 , Peter L Falkingham 4 , Christine M Janis 5 6
Affiliations

1 Department of Pathology, St. John’s Seminary, Camarillo, 93012, CA, USA.
2 Department of Functional Morphology, Faculty of Veterinary Medicine, Utrecht University (Ret.), Utrecht, 3584 CM, The Netherlands.
3 Department of Cell Biology and Neuroscience, Rutgers University, New Brunswick, 08854, NJ, USA.
4 School of Biological and Environmental Sciences, Liverpool John Moores University, Liverpool, L3 3AF, UK.
5 Bristol Palaeobiology Group, School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK.
6 Department of Ecology and Evolutionary Biology, Brown University, Providence, 02912, RI, USA.

PMID: 37351494
PMCID: PMC10282582
DOI: 10.1098/rsos.230358

Abstract

The traditional story of the evolution of the horse (family Equidae) has been in large part about the evolution of their feet. How did modern horses come to have a single toe (digit III), with the hoof bearing a characteristic V-shaped keratinous frog on the sole, and what happened to the other digits? While it has long been known that the proximal portions of digits II and IV are retained as the splint bones, a recent hypothesis suggested that the distal portion of these digits have also been retained as part of the frog, drawing upon the famous Laetoli footprints of the tridactyl (three-toed) equid Hipparion as part of the evidence. We show here that, while there is good anatomical and embryological evidence for the proximal portions of all the accessory digits (i.e. I and V, as well as II and IV) being retained in the feet of modern horses, evidence is lacking for the retention of any distal portions of these digits. There is also good ichnological evidence that many tridactyl equids possessed a frog, and that the frog has been part of the equid foot for much of equid evolutionary history.

Keywords: Equus; digit reduction; frog; hoof; horse; limb evolution.