Final answer:
Horses that graze on rocky soils appear older due to wear on their teeth.
Step-by-step explanation:
Horses that graze on rocky soils appear relatively older because of wear on the teeth. The oldest horse fossils show that horses changed as the climate became drier and grasslands replaced marshes. With rocky soils, horses have to chew their food more thoroughly, causing more wear on their teeth and making them appear older.