Wednesday 19 November 2014

A fossil Hibiscus leaf from the Early Eocene of Rajasthan, India.

The Bikaner District of Rajasthan is today located deep within the Thar Desert of northwest India. The area has an annual precipitation of 260-440 mm, with temperatures of 28-48.5°C in the summer and 5-23.2°C in the winter. However fossil evidence suggests this has not always been the case, with deposits from the Early Eocene suggesting that the district then had a warm humid climate, with some areas encroached by the sea.

In a paper published in the Journal of the Geological Society ofIndia in August 2014, Anumeha Shukla, RC Mehrota and JS Guleria of the BirbalSahni Institute of Palaeobotany describe a fossil Hibiscus leaf from the Early Eocene Marh Formation at the disused Prem Lata Gupta Clay Mine, which is northeast of Kolayat on Kolayat-Bikaner road.

The fossil is referred to the genus Kleinhovia, which currently contains two species, the extant Kleinhovia hospita, which is found in the tropics of Africa, Asia, Indonesia and Australia, and Kleinhovia basitruncata from the Early Eocene of Hokkaido. Of the two the Rajasthan specimen more closely resembles the modern species, but is placed in a new species of its own Kleinhovia bikanerensis, meaning ‘from Bikaner’.

Fossil leaf of Kleinhovia bikanerensis showing shape and size. Scale bar is 1 cm. Shukla et al. (2014).

The modern Kleinhovia hospita is an upright tree with a dense, rounded crown that produces sprays of pink flowers. It is an evergreen tree growing in warm, humid environments, unlike the arid terrain found in Bikanar today. In the Eocene the area is thought have been on or near the northern coast of an island continent, but since then India has collided with Eurasia, leading to uplift in the Himalayas and on the Tibetan Plateau, resulting in an arid and seasonal climate across much of northern India.

Modern leaf of Kleinhovia hospita showing similar shape and size as inthe fossil. Shukla et al. (2014).

The Hibiscus family, Malvaceae, is thought to have originated and diversified on the ancient continent of Gondwana, on or as it broke up into its component parts (South America, Africa, Madagascar, India, Australia, Antarctica and New Zealand) and the modern distribution of Kleinhovia hospitawould seem to support this. However the only fossil described from the genus Kleinhovia to date came from the Early Eocene of Japan, which is problematic as Japan was never part of Gondwana, and in the Eocene had never been particularly close to any part of it. The presence of Kleinhovia basitruncata fits better with standing palaeobiogeographical models, in that we would expect to find fossils from the genus on at least one of the Gondwanan continents, and probably on one which it is still present, though it does not help to explain the mystery of the Japanese Kleinhovia basitruncata.

See also…

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