Fossils from a previously-unknown horse species that lived about 4.4 million years ago have been uncovered in Ethiopia.

Researchers say remains of the newfound horse, which was the size of a small zebra, were located in Ethiopia's arid but fossil-rich Middle Awash valley, which has produced the world's longest and most continuous record of human evolution.

The extinct horse, known by the scientific name Eurygnathohippus woldegabrieli, likely co-existed with the 4.4 million-year-old human ancestor, Ardipithecus ramidus.

The find is detailed online in the November edition of the Journal of Vertebrate Paleontology.

Researchers say back in 2001 they had found bone and teeth fragments that belonged to the ancient horse, among a great variety of animals that migrated across the planet over the Earth together, along the early human ancestors.

The horse fossils have helped to explain a wide gap in the equine evolutionary narrative.

After that first discovery, research teams fanned out across the Ethiopian landscape, in order to better study the myriad of fossils they had found in the country's badlands. During the renewed archeological search, the teams ended finding two ends of a leg bone belonging to the ancient horse, shiny white and very well-preserved in the dry desert sand.

Almost a year later, scientists came upon another bone, which demonstrated the ancient beast was a strong runner and that its legs longer than those of horses living between 5 and 9 million years ago.

The physical changes, researches deduced, permitted the Eurygnathohippus woldegabrieli to run far distances and be able to easily get away from predators.

Tooth fossils suggest the horse ate mostly grass and had adjusted to grazing as time passed, with teeth longer than younger horse species. Their teeth had become longer than horses that were older than they were.

When scientists examined the enamel composition, they were able to confidently reconstruct what the horse's diet way back when generally was.

"Grasses are like sandpaper," said study researcher Scott Simpson of Case Western Reserve's School of Medicine. "They wear the teeth down and leave a characteristic signature of pits and scratches on the teeth so we can reliably reconstruct their ancient diets."

The new horse species was named in honor of Giday WoldeGabriel, a geologist at Los Alamos National Laboratory who had studied human fossils at the Ethiopian Rift system and provided essential understanding of the fossil environment in Ethiopia.

The ancient animal belonged to a group of ancient horses called Hipparionines, which had three-toed hooves and flourished in North America about 16 million years ago, before spreading into Eurasia, presumably over the land bridge that scientists believe once existed between Alaska and Siberia.