What is considered the earliest fossil form in the phylogeny of a horse?

The earliest fossil form in the phylogeny of a horse is Eohippus. This genus, also known as the "dawn horse," existed around 50 million years ago and showcases some of the primitive characteristics that mark the early evolutionary stages of equines. Eohippus was a small, fox-sized animal that lived in the forests of North America and Europe, exhibiting features such as a more elongated body, multiple toes, and a diet that consisted of soft leaves and plants. Its anatomical structure was significantly different from modern horses, which reflects the changes in the evolutionary path that horses took over millions of years.

Eohippus is pivotal for understanding horse evolution as it represents the transition from small, multi-toed ancestors to the larger, single-toed forms we see today, like Equus, the genus that includes all modern horses. The lineage leading to modern horses demonstrates how environmental changes and shifts in habitat required adaptations that Eohippus and its descendants would gradually undergo. This makes Eohippus a crucial part of understanding the evolutionary history of horses.