Final answer:
The oldest horse fossil among Equus, Pliohippus, Merychippis, and Mesohippus is Mesohippus, placing the correct answer as D) Mesohippus.
Step-by-step explanation:
Based on the fossil record, we can determine the oldest horse fossil among Equus, Pliohippus, Merychippis, and Mesohippus.
The evolutionary history of horses shows that the genus Eohippus evolved into more horse-like species over time.
The fossil record includes Mesohippus, which lived from about 40 to 30 million years ago and is known for its adaptation to a gradually drying climate which turned forests into prairies.
Looking at the line of evolution, Merychippis is recognized as an intermediate stage that showed further adaptation to grazing habits.
Then, we have Pliohippus, believed to be a direct ancestor of today's horses, evolving before Equus. Finally, Equus is the genus of modern horses, representing the latest stage of horse evolution in this lineage.
Given this sequence from the fossil record, the correct answer to which fossil is the oldest among the four mentioned is D) Mesohippus.