A missing link of horses and rhinos has been discovered, shedding light on a mystery tracing back 55 million years.

Fossils from the Early Eocene suggest ancestors of the Perissodactyla order of mammals may have originated on the subcontinent of India. These long-extinct animals lived in the area 54.5 million years ago, then modern-day India was a lone island, swiftly heading, through the power of continental drift, for a collision with Asia.

Fossils of Cambaytherium thewissi, helped reveal likely body features of theoretical predecessors to Perissodactyla. The oldest Perissodactyla fossils are 55 million years old, older than the Cambaytherium artifact, but little was known about possible ancestors of the species until now.

This discovery in paleontology could also help answer a question in geology.

"Around Cambaytherium's time, we think India was an island, but it also had primates and a rodent similar to those living in Europe at the time. One possible explanation is that India passed close by the Arabian Peninsula or the Horn of Africa, and there was a land bridge that allowed the animals to migrate. But Cambaytherium is unique and suggests that India was indeed isolated for a while," Ken Rose of the Johns Hopkins University School of Medicine, said.

Perissodactyla are commonly known as odd-toed ungulates, due to the uneven number of toes on their hind feet. This order of mammals, which possess simpler digestive systems than even-toed ungulates, also includes tapirs, zebras, and donkeys. Many species of Perissodactyla went extinct at the end of the last major ice age.

"Only seventeen species of perissodactyls remain on the Earth today, a shadow of the group's former glory. Perissodactyls were once much more diverse, including the enormous horned brontotheres, the bizarre browsing, clawed chalicotheres, and the largest land mammal of all time, the Eocene Indricotherium... It stood five meters (over sixteen feet) tall at the shoulder," Berkeley researchers wrote on a Web page decribing Perissodactyla.

The fossils were discovered in a coal mine, among many being worked by an energy company. Operators of the business permitted Rose and his team to excavate, even as they harvested coal from surrounding mines.

"It was frustrating to knowing that countless fossils were being chewed up by heavy mining equipment," Rose told the press.

Workers in the mines filled caves with debris after the shafts had been harvested clean of coal. Investigators have moved on to other caves for further research and exploration.

Investigation of the ancient ancestor of rhinoceroses and horses was detailed in the journal Nature Communications.