Paleontologists have long known that the group of animals known as Perissodactyla, which includes horses and rhinos, existed as early as 56 million years ago at the beginning of the Eocene epoch, but much about their earlier evolution has been a mystery.
Now, researchers have found fossils of an animal in India that hints of a common ancestor of the modern-day horses, rhinos and tapirs, indicating that the animal group may have likely originated from this subcontinent when it was still an isolated island and had not yet collided with Asia.
For their study published in the journal Nature Communications on Nov. 20, Ken Rose from the Johns Hopkins University School of Medicine and colleagues unearthed 54.5 million-year-old fossils of an animal called Cambaytherium thewissi from an open-pit coal mine in India's Gujarat state.
The bones, which included the teeth, foot and vertebrae of the animal, have allowed researchers to come up with a picture of what the Cambaytherium possibly looked like and found this ancient creature to have characteristics that closely matched those of the earliest perissodactyls.
"Here we report new dental, cranial and postcranial fossils of Cambaytherium, from the Cambay Shale Formation, Gujarat, India (~54.5 Myr)," the researchers wrote. "These fossils demonstrate that cambaytheres occupy a pivotal position as the sister taxon of Perissodactyla, thereby providing insight on the phylogenetic and biogeographic origin of Perissodactyla."
The researchers said that the animal, which was about the size of a wild pig and may have weighed between 45 and 75 pounds, is the closest thing that they have found to the common ancestor of the modern-day horses and rhinos, indicating the possibility that it could be the missing link in the evolution of modern-day perissodactyls.
By analyzing the bones, the researchers also believe that the Cambaytherium likely had five finger- or toe-like bones, albeit the number was likely reduced as they developed their modern hooves.
The findings also give credence to the theory that several groups of animals from the early Eocene possibly evolved while the Indian subcontinent was still an isolated island.
"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," Rose said. "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."