Response of the Oligo-Miocene Bivalve Fauna of the Kutch Basin (Western India) to Regional Tectonic Events

ABSTRACTTectonic changes has influenced the evolution of the marine community by changing the land and seaway configuration through time. Two such tectonic events during Oligo-Miocene times — the closure of the Tethyan seaway due to development of the Gomphotherium-Landbridge leading to separation of the Arabian Sea from proto-Mediterranean Sea (∼19 Ma) and significant uplift of the Tibetan plateu marking the initiation of the monsoon (∼16 Ma) — represent a classic case of tectonic shift influencing the regional environment of the Indian subcontinent. We investigated the taxonomic and body size related response of the shallow marine fauna to this regional change using bivalves from 11 time-constrained shellbeds of the Kutch Basin (western India) from three formations — Maniyara Fort (Chattian), Khari Nadi (Aquitanian) and Chhasra (Burdigalian-Langian) representing a time span of ∼9 Ma (24.4 – 15 Ma).Our collection of over 2000 individuals represents a total of 15 families and 61 morphospecies. The fossils are predominantly calcitic in nature and families of aragonitic composition are often preserved as molds indicating a potential negative effect of diagenesis. The taphonomic nature, however, does not vary substantially across shellbeds and hence, less likely produced a temporal pattern. The five most abundant species, Ostrea latimarginata, Ostrea angulata, Talochlamys articulata, Anomia primaeva and Placuna lamellata occur in all the formations. The species composition of the Maniyara Fort formation is substantially different from those of the younger formations implying the possible effect of biogeographic separation. Moreover, the absence of proto-Mediterranean taxa in Oligocene shellbeds support a limited faunal exchange as early as ∼24.4Ma (Chattian) ago. We observed a monotonic increase in the overall rarefied species richness and a decrease in evenness from the Maniyara Fort to the Chhasra Formation. However, shellbed analyses show a dominantly conservative behavior of diversity and body size without a strong directional trend through time. Although it is difficult to rule out the negative influence of taphonomy on the diversity of the studied fauna, Oligo-Miocene marine bivalve fauna of the Kutch Basin demonstrate little or no influence of the Tethyan closure and Himalayan upliftment.

A replacement name for the poorly known illegitimate Paleogene genus Psilamonocolpites Y.K.Mathur (Ginkgoales)

The monotypic genus Psilamonocolpites Mathur (1966: 40) was established to classify a new species of gymnosperm’s pollen fossil, within the informal taxon Ginkgo-group, from Western Kutch, in India. Psilamonocolpites longicolpatus Mathur (1966: 40), collected in Paleogene Suppratrappean sediments of Kutch Basin, is the type species by monotypy. Despite the fact that some subsequent contributions to the original description of the species have been focused on increasing knowledge about the palynological paleoflora of Kutch Basin and other Paleogene sedimentary environments located in the southern hemisphere (Venkatachala & Kar 1968, Saxena 1977, Askin 1990, Zavada & de Villiers 2000, Gengwu et al. 2006, Mathews et al. 2013), the occurrence of Psilamonocolpites longicolpatus Y.K.Mathur has not been documented since, and remained largely ignored in the scientific literature, partly because it is known only from the holotype collected approximately 56 years ago. No additional species to the type species of the genus has been described to date.

An updated generic classification of Cenozoic pleurotomariid gastropods, with new records from the Oligocene and early Miocene of India

Although taxonomically distinct, the Cenozoic pleurotomariids are the bottlenecked remnants of the Mesozoic members of the family in terms of morphology, with only conical forms surviving the end-Cretaceous mass extinction. Here, we propose an updated classification scheme for the Cenozoic representatives of this group, based on data from the entire Cenozoic pleurotomariid fossil record. We consider all conventional as well as several new characters so that this scheme can readily help to distinguish Cenozoic pleurotomariid genera. Following the new classification scheme, a revision of the generic status of Cenozoic species previously assigned to ‘Pleurotomaria’ Defrance, 1826 is presented. Only a few Cenozoic pleurotomariid gastropods have been reported from the Indian subcontinent. Here we report four species from the Oligocene of the Kutch Basin and the early Miocene (Burdigalian) of the Dwarka Basin of Gujarat, western India, of which two are described as new: Perotrochus bermotiensis n. sp., Entemnotrochus kathiawarensis n. sp., Entemnotrochus cf. E. bianconii, and Entemnotrochus? sp. 1. UUID: http://zoobank.org/89b6ff67-2834-477f-862b-67691104aca4