Researchers have discovered a new species of fossil horse that lived 4.4 million years ago in Ethiopia, a discovery that fills a gap in the evolutionary history of horses.

About the size of a small zebra, Eurygnathohippus woldegabrieli had three-toed hooves and grazed the grasslands and shrubby woods in the Afar Region, scientists said.

The horse fills a gap in the evolutionary history of horses but is also important for documenting how old a fossil locality is and in reconstructing habitats of human forebears of the time, said Scott Simpson, professor of anatomy at Case Western Reserve’s School of Medicine, and co-author of the research.

The researchers found the first E woldegabrieli teeth and bones in 2001, in the Gona area of the Afar Region. This fossil horse was among the diverse array of animals that lived in the same areas as the ancient human ancestor Ardipithecus ramidus, commonly called Ardi.

“The fossil search team spreads out to survey for fossils in the now arid badlands of the Ethiopian desert,” Dr Simpson said. “Among the many fossils we found are the two ends of the foreleg bone — the canon — brilliant white and well preserved in the red-tinted earth,” he added.

A year later, they returned and found part of the connecting shaft, which was split lengthwise but provided the crucial full length of the bone.

The long slender bone indicates this ancient species was an adept runner, similar to modern zebras, and analyses of their teeth indicated they relied heavily on eating grasses in the grassy woodland environment. The horse had longer legs than ancestral horses that lived and fed in forests about 6 million to 10 million years ago, Dr Simpson said.

The change helped the more recent horses cover long distances as they grazed and flee lions, sabre-tooth cats and hunting hyenas that would run down their prey. The other fossils they found included teeth, which are taller than their ancestors’ and with crowns worn flatter — more signs the horses had adapted to a grazing life.

Analyses of the isotopic composition of the enamel confirmed that E woldegabrieli subsisted on grass. Horse expert Raymond L Bernor, from the Laboratory of Evolutionary Biology at the Howard University College of Medicine in Washington DC, led the fossil analysis.

The bones, which remain at the National Museum of Ethiopia in Addis Ababa, Ethiopia, showed this was a significantly different animal than the horses more than 5 million years old, and those 3.5 million years old and younger. The findings are published in the Journal of Vertebrate Paleontology.