Scientists in India have made a significant fossil discovery in the Himalayan floodplains in North India. The fossil, named Golunda mohandensis, belongs to an ancient ancestor of the Indian bush rat (Golunda ellioti), which is the only surviving species of its genus.
This discovery, published last week could indicate that genus Golunda, which was once widely-distributed and much more diverse, originally evolved in India before radiating out to West Asia, South Asia and East Africa.
The fossil could reveal clues about ancient environments
Small mammals, including rodents such as Golunda, tend to show a good abundance, diversity and evolutionary rate in the fossil record. This makes them very useful in reconstructing paleoenvironments.
In the region that Golunda was found, i.e. the lower Himalayas of North India, and in the time period to which it belongs, the fossil record of rodents is well documented. This completeness of the record has helped reconstruct the biostratigraphy of many localities where geochronological dating methods are not available.
3D surface models of Golunda mohandensis sp. nov., (a–c) WIMF/A MR1, left maxilla with M1 and M2; (d,e) WIMF/A MR2, left M3; (f–h) WIMF/A MR3, left M1; (i) WIMF/A MR4, left M3; (j) WIMF/A MR5, left M3. Images in a, e, f, i, and j in occlusal view. Images in c, d, and h in buccal view. Images in b and g in lingual view. Scale 1 mm. Image Credit: Dr. Singh et al
“Rodents are sensitive to environmental conditions such as diet, climate, and vegetation cover, making their presence or absence reliable bioindicators of local paleoecology.” Dr. Ningthoujam Premjit Singh from the Wadia Institute of Himalayan Geology, Dehradun explained in an email interview with the Triassic Times.
“For instance, discovering rodent species adapted to arid conditions may suggest a past desert environment, while species that thrive in forests would indicate a different habitat”, he explained. “Changes in the size, shape, and structure of their teeth and jaws directly reflect adaptations to available flora and shifts in climate.”
“Today, the Indian Bush Rat (Golunda ellioti) is commonly found in areas covered by tropical thorn scrub or clumps of grass. During the Pliocene in the Siwaliks (less than 5 million years ago), grasslands expanded, suggesting that the newly discovered fossil species, Golunda mohandensis, could have survived in dry, arid climates with scrublands and grassland areas.”
The teeth were diagnostic
Dr. Singh and team were able to differentiate Golunda mohandensis (meaning the Golunda of Mohand) on the basis of tts teeth. The team found a total of six teeth – “Two fragments of upper jaws attached with first, second and third molars, one isolated upper molar and two isolated lower third molars”.
3D digital surface models were first created from high-resolution μCT scans. This helped in the study of the small dental specimens. After this a comparative analysis with presently-known Golunda species was done. “To compare Mohand specimens with other fossil and extant Golunda, we obtained published dental length and width measurements from the literature and created bivariate plots of shape vs. size.”, the scientists wrote in their paper.
Comparison of M1 cusp orientation between (a) Golunda tatroticus, (b) Golunda kelleri, (c) Golunda dulamensis, and (d) Golunda mohandensis
Scientists noted that G. mohandensis’ upper molars are small in size and less elongated compared to other known Golunda species, although there is some variation in shape.
A possible Indian origin story
The origin of Golunda as a genus has been debated since the late 1970s. It was previously thought that Golunda originated in Africa and moved eastwards around 3.5-2.5 million years ago.
The discovery of a Golunda species (tatroticus) dated to ~4.5-3.5mya challenged this theory. Now the discovery of Golunda mohandensis has given support to the reverse hypothesis, that Golunda first evolved in South Asia and moved westwards.
“Our findings support the hypothesis that Golunda originated in the Indian sub-continent before dispersing into Africa, although other biogeographic scenarios are also possible”, the researchers wrote.
Dr. Ningthoujam Singh continues the discussion in our email interview. “Our specimen is from a magnetostratigraphically dated horizon that is approximately 4.9 million years old, making it the oldest Golunda fossil reported worldwide. In Africa, no Golunda fossils older than 3.5 million years have been found.”
“Currently, we can assert that Golunda likely evolved on the Indian subcontinent and then migrated to Africa, based on the ages of fossil records.”
However, Dr. Singh also urges caution. “Molecular studies indicate that the sister group to both the Indian and African bush rats is Otomyini, an African clade that diverged around 9 to 7 million years ago. Therefore, the hypothesis that Golunda dispersed from Africa to South Asia cannot be ruled out”, he says.
The researchers also note in their paper that resolution of this issue will depend on future fossil collections and analyses of contemporaneous fossil specimens from both Africa and South Asia. “Continued paleontological exploration of the Miocene–Pliocene transition in the Siwalik deposits is essential to further clarify the origin and evolutionary history of this lineage in South Asia.“