One valid inference that can be made regarding the evolution of modern horses is that : Pliohippus had teeth adapted for grazing
From the information provided in the question The modern day horse resembling the pliohippus can be inferred to have teeth which is adapted for grazing just like the pliohippus.
The pliohippus evolved from the merychippus which is classified as a grazer. because of its longer/taller teeth the pliohippus was more adapted to grazing than been a browser.
Hence we can conclude that One valid inference that can be made regarding the evolution of modern horses is that : Pliohippus had teeth adapted for grazing
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