UNVEILING THE LOWER MIOCENE GASTROPOD DIVERSITY TREND OF KUTCH, INDIA

The lower Miocene molluscan assemblages of Kutch, India are known for long. Previous studies suggested that gastropod diversity increased within the lower Miocene. However, a number of studies showed that species richness is dependent on sample volume or specimen number. In other words, the previously reported increase in gastropod species richness through the lower Miocene may be due to small-sized samples and non-standardised analytical protocol.

We, here, present a new dataset involving a total of more than 16,000 specimens collected from the Khari Nadi and Chhasra formations, which are equivalent to the Aquitanian and Burdigalian ages. To test the hypothesis of increase in diversity in the lower Miocene, we collected bulk samples that provide relative abundance distribution of species.

The revised systematic analysis reveals the presence of 72 species, covering both the Aquitanian and Burdigalian ages. Contrary to previous reports, our dataset suggests that gastropod species richness decreased from 59 in the Aquitanian to 39 in the Burdigalian time. This pattern also holds true for the genus-level diversity trend. We used individual-based rarefaction method to standardise our samples, which enables us to compare diversity of two time planes at a specific specimen number. Our rarefied curves, at a standardized cut off of 1648 gastropod specimens, also suggest that gastropod diversity of Kutch rather declined statistically significantly from the Aquitanian (35 species) to the Burdigalian (19 species) time. This decline is evident even when we include broken but identifiable specimens of bulk and random surface collections.

We conclude that the lower Miocene of Kutch experienced a decline in gastropod diversity at both genus and species levels. We believe that previous studies may be confounded with imprecise stratigraphic framework and sampling constraints. Present study employs a standardized sampling regime and provides the faithful picture of gastropod diversity in the lower Miocene of Kutch. In future, the causal factors for the diversity decline will be explored.