Question

Which one among the following is the oldest horse genus?

• A. Orohippus
• B. Mesohippus
• C. Merychippus
• D. Pliohippus

Hint:

Remember horse evolution in order of geologic time:
Eohippus (Hyracotherium) $\Rightarrow$ Orohippus $\Rightarrow$ Mesohippus $\Rightarrow$ Merychippus $\Rightarrow$ Pliohippus $\Rightarrow$ Equus (modern horse).

Answer 1

The Correct Option is A

Step 1: Understanding horse evolution.
The evolutionary history of the horse shows a clear progression from small, multi-toed forest dwellers to larger, single-toed grazers adapted to open grasslands. Each genus represents a stage in this evolutionary transition. Step 2: Geological time sequence of the genera.
- Orohippus (Eocene, about 50 million years ago): One of the earliest horse genera, evolved soon after \textit{Eohippus} (Hyracotherium). It had four toes on the front feet and three on the hind.
- Mesohippus (Oligocene, about 37–32 million years ago): Slightly larger, three-toed horse, adapted to more open woodland.
- Merychippus (Miocene, about 17 million years ago): A grassland horse, three-toed but with the central toe dominant, an ancestor to modern horses.
- Pliohippus (Miocene–Pliocene, about 12–5 million years ago): First true one-toed horse, close to the ancestor of modern \textit{Equus}. Step 3: Determining the oldest genus.
Among the given options, Orohippus belongs to the \textit{Eocene epoch}, making it the oldest genus listed. \[ \boxed{\text{Orohippus is the oldest horse genus.}} \]