Indian fossils support new hypotheses about the origin of ungulate mammals

New research published today on Journal of Vertebrate Paleontology describes a fossil family that sheds light on the origin of the perissodactyls, the group of mammals that includes horses, rhinos and tapirs. It provides insights into the controversial question of where these ungulates evolved, concluding that they arose in or near present-day India.

With more than 350 new fossils, the 15-year study brings together a nearly complete picture of the skeletal anatomy of Cambaytherium, an extinct cousin of perissodactyls that lived on the Indian subcontinent nearly 55 million years ago.

The findings included a sheep-sized animal with moderate running ability and intermediate characteristics between specialized perissodactyls and their more generalized mammalian predecessors. Comparing its bones with many other living and extinct mammals, it emerged that Cambaytherium represents a more primitive evolutionary stage than any known perissodactyl, supporting the group’s origin in or near India – before they dispersed to other continents when they became it formed the land connection with Asia.

This landmark new article was selected for publication as part of the Society of Vertebrate Paleontology’s prestigious series of memoirs, a special annual publication that provides a deeper analysis of the most significant vertebrate fossils.

Cambaytherium, first described in 2005, is the most primitive member of an extinct group that branched out shortly before the evolution of the perissodactyls, providing scientists with unique clues to the group’s ancient origins and evolution.

“The modern orders Artiodactyla (even-toed ungulates), Perissodactyla and Primates suddenly appeared in the early Eocene around 56 million years ago in the Northern Hemisphere, but their geographic origin remained a mystery,” he explained. Ken Rose, professor emeritus at Johns Hopkins University and lead author of the study.

Prof. Rose was fascinated by a new hypothesis which suggests that perissodactyls may have evolved in isolation in India. India was then an island continent drifting north, but it later collided with the Asian continent to form a continuous continental mass.

“In 1990, Krause and Maas proposed that these orders may have evolved in India during its northward drift from Madagascar, dispersing across the northern continents when India collided with Asia.”

Armed with this new hypothesis, Rose and colleagues secured funding from the National Geographic Society to explore India for rare age-corrected fossil rocks that could provide critical evidence for the origin of perissodactyls and other mammal groups.

The first trip to Rajasthan in 2001 had little success, “Although we only found a few fish bones on that trip, the following year our Indian colleague, Rajendra Rana, continued to explore the lignite mines to the south and went on stumbled upon the Vastan mine in Gujarat “.

This new mine turned out to be much more promising. Rose added: “In 2004 our team was able to return to the mine, where our Belgian collaborator Thierry Smith found the first mammalian fossils, including Cambaytherium.”

Encouraged, the team returned to the mines and collected fossil bones from Cambaytherium and many other vertebrates, despite the harsh conditions.

“The heat, constant noise and coal dust in the lignite mines were harsh, basically trying to work hundreds of feet near the bottom of the open cast lignite mines which are actively mined 24/7.” he said.

Through the accumulation of many years of demanding field work, the team can finally shed light on a mammalian mystery. Despite the abundance of perissodactyls in the Northern Hemisphere, Cambaytherium suggests that the group probably evolved in isolation in or near India during the Paleocene (66-56 million years ago), before dispersing to other continents when the terrestrial connection with Asia was formed.

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The Memoir series of the Society of Vertebrate Paleontology represents one of the few paper publication platforms for monographic treatments such as that completed for Cambaytherium by Rose and colleagues. Particularly noteworthy is that this work uses a broad digital modeling approach (CT / μCT), with the data accessible to researchers via Morphosource (http: // www.morphosource.org); the phylogenetic information used in the full study is accessible via Morphobank (http: // www.morphobank.org).

Funding used to support field and laboratory research was provided by the National Geographic Society, the LSB Leakey Foundation and the US National Science Foundation.