Subhendu Bardhan

Evidence of superimposition of storm waves on tidal currents in rocks from the Tithonian-Neocomian Umia Member, Kutch, India

Abstract The general opinion is that hummocky cross-stratification is generated from superposition of storm waves on a unidirectional current. However, constraints on the type of the current field have not been clearly established. Sedimentary structures preserved in rocks of the Tithonian-Neocomian Umia Member, Kutch, India, indicate hummocky cross-stratification developed from storm wave interference with ebb-tide currents. Facies indicate repeated progradation of the shoreline and the increasingly important role of sand transport by currents in offshore direction. Stratigraphic analyses identify two major stages of shoreline progradation. Each is composed of distinctive shelf to shoreface transitions. In the tidally influenced shoreface parts of these transitions, the azimuths of units with nearly unidirectional hummocky cross-stratification are consistent with the direction of ebb-tidal cross-stratification, suggesting offshore sand transport by superposition of waves on an ebb-tidal current.

How far did India drift during the Late Cretaceous?—Placenticeras kaffrarium Etheridge, 1904 (Ammonoidea) used as a measuring tape

Abstract India, once a member of the lost supercontinent Gondwana, broke away from it and made a solitary northward excursion and finally collided with Asia. During its long voyage, India remained isolated for 100 Ma and is expected to be characterized by stunning endemic biodiversity. But this is not recognized by the terrestrial faunal and floral content, and their distribution patterns paint no simple scenario. For example, the Inter-trappean vertebrate faunas of India, which lived during “India-in-exile”, do not show any made-in-India assemblages, but rather betray a mixed biota having both Gondwanan and Laurasian affinities. These differential distribution patterns of fauna and flora, and their affinities with those of other areas, prompted many workers to envisage an array of suggestions regarding the time of Indias final separation from Gondwana, the time of northward drifting and different palaeopositions during its long journey. But closer examination of the nature of the vertebrate fossil records reveals that the so-called elusive endemicity of Indian fauna during its sojourn is in fact a product of taxonomic artefact. The majority of the faunas have been described on the basis of poor fossil data, and comparisons for biogeographic correlations are made at higher taxonomic levels, which perhaps masked Indias faunal distinctiveness. Yet, the biological processes that constrain biogeographical distribution operate at the species level. In this paper, we present our own data to reconstruct the palaeoposition of India in the Late Cretaceous, and to estimate the time of its northward migration. The present study is based on a newly recorded ammonite species, Placenticeras kaffrarium Etheridge from the Coniacian horizons in Bagh, central India. The species abounds in Bagh and represents a complete population structure. It resembles significantly the populations described from the coeval horizons of Madagascar and Zululand, South Africa. P. kaffrarium has a stunning display of intraspecific variability represented by at least seven distinct morphotypes reported from Zululand and Madagascar. The Indian population, significantly matches the latter two populations, variant by variant and remarkably, also for the sexual dimorphs within each variant. Geographic range is a biological property of a species and ammonite species have differential distribution patterns depending on the length of larval longevity, oceanic circulation patterns and favourable environments. Placenticeras was a nektobenthic genus living in the shallowest depths of epicontinental seas, and could not cross a narrow strait wider than a few hundred kilometers. Palaeogeographic distribution patterns of all placenticeratid species during the Late Cretaceous support this view and show the wandering nature of the genus over time. However, they always exhibit a latitudinally limited distribution and are restricted mainly to the subtropics north and south of the equator. Thus, it appears that the P. kaffrarium population of Zululand, Madagascar and Bagh constitute a homogenous, single breeding population, implying geographical proximity of these areas, connected by exclusively shallow seas till the Coniacian. Other fossil records of ammonites and geophysical data also support this view. Shortly after this, India started its northward drift. By the Late Maastrichtian, India was completely isolated by a wide oceanic gap from the rest of the Gondwana fragments and the northern part of India crossed the equator. This is evident from strong endemicity of ammonite species assemblages of the northwest and southern Indian subcontinent, which constitute the most diverse communities known.

Predation on Recent Turritelline Gastropods from the Indian Subcontinent and Comparison with a Revised Global Database

ABSTRACT Traces of predation by drilling gastropods and peeling crabs provide important insights about predator-prey interaction in ecological as well as evolutionary times. Predation on turritelline gastropods, in this context, has been frequently discussed in literature. Here, we have estimated the intensity of predation (both drilling and peeling) on Recent turritelline gastropods from the Indian subcontinent, which has been underrepresented in previous studies. Our samples include our own collections from several Indian coasts as well as a vast collection which was locked in the archive of the Zoological Survey of India (ZSI) in Kolkata for the past 150 years. It includes samples from different parts of the Indian subcontinent as well as from many other countries. Drilling frequency (DF) of Indian turritelline species is low compared to average values of global data. We suggest that this is mainly because most of the Indian species are larger (> 4 cm) than species living elsewhere. Smaller species show higher DF and lower values of peeling frequency. Size selectivity of drill holes shows both intra- and interspecific variation. Shell thickness and ornamentation appear to be antipredatory in nature. We have compared our results with a revised global database. Distribution of intensity of predation shows latitudinal variation where both drilling and peeling frequencies increase towards the tropics.

A note on exceptionally high confamilial naticid drilling frequency on Natica gualteriana from the Indian subcontinent

Although common, confamilial naticid predation intensity was not very high in the geological record. Here, we gathered modern confamilial predation data from the Indian coasts and showed that confamilial naticid predation on a naticid species, Natica gualteriana, is exceptionally high at Chandipur, one of our studied areas. We studied the different aspects of confamilial predation from the Indian coasts and showed that the predators in Chandipur were highly efficient as evident from high drilling frequency (DF), site stereotypy and low prey effectiveness. Unusually high DF on N. gualteriana may be attributed to its new arrival in Chandipur where it faced competitive elimination through predation by sympatric naticid predators. Reports of failed invasion are rare. Natica gualteriana is a small invader and therefore its invasion success is threatened by resident populations of large species that extensively drill on young individuals of N. gualteriana just to break the bottleneck of their own offspring from competition.

Intense Naticid Drilling Predation on Turritelline Gastropods from Below the K-T Boundary at Rajahmundry, India.

ABSTRACT Evidence of intense naticid drilling predation occurs on turritelline prey from a turritelline-dominated assemblage (TDA) which lies below the K-T boundary sections in Rajahmundry, India. Previously, it was believed that drilling frequency (DF) on turritelline taxa was low during the Cretaceous. Data from the study area indicates that the Cenozoic level of predation on turritelline taxa already occurred by the Late Cretaceous in the study area. The paleobiogeography of naticid predation is extended from the western world to India, which was located in the southern hemisphere during the Cretaceous. In addition to the high drilling frequency, the Indian fossil record shows that many aspects of naticid behavior; for example, size and site stereotypy, which are characteristic features of Cenozoic predators, were also established by the Late Cretaceous. These data support previous views that the Mesozoic Marine Revolution had minimal influence on morphological change in Late Cretaceous turritelline gastropods and that turritelline gastropods may have developed behavioral and/or physiological antipredatory adaptations. TDAs in general may indicate tremendous fecundity that may act as a buffer against high juvenile mortality due to predation.

A new pleurotomariid gastropod assemblage from the Jurassic sequence of Kutch, western India

ABSTRACT Mesozoic gastropod assemblages currently emerged from Kutch, western India constitute one of the most diverse communities known during the Bathonian–Oxfordian worldwide. The present paper describes part of the Jurassic gastropod faunas of the family Pleurotomariidae from Kutch, comprising ten species belonging to three genera, of which eight species are new. They are: Bathrotomaria reticulata (Sowerby), B. waageni sp. nov., B. calloviana sp. nov., B. buddhai sp. nov., B. prasantai sp. nov., B. dhosaensis sp. nov., B. tewarii (Maithani); Leptomaria daityai sp. nov.; L. asurai sp. nov. and Obornella wuerttembergensis (Sieberer). The assemblage shows a strong Tethyan affinity at generic level, but has a distinctly endemic species content, and thus merits a distinct subprovince within the Indo-Madagascan Faunal Province. The genus Pleurotomaria is discussed in historical perspective and shown to be Mesozoic genus. The six species of Bathrotomaria described herein are considered to have evolved from the early immigrant species, B. reticulata of Europe.

The terebratulid Kutchithyris (Brachiopoda) from the Jurassic sequence of Kutch,western India-revisited

Abstract The Kutch basin developed due to the fragmentation of Gondwana during the Middle Jurassic and hosted diverse endemic fauna, of which brachiopods are one of the chief constituents. The dominant brachiopod faunal element is the terebratulid genus Kutchithyris Buckman. The genus is represented throughout the exposed Middle Bathonian to Oxfordian sequence in Kutch and is also reported sporadically from outside Kutch. The systematics of this small but distinct clade is in a state of flux. The present paper focuses on revising the systematics of the genus and its three dominant species, namely, K. acutiplicata, K. propinqua and K. euryptycha, based on numerous specimens collected from the field with precise stratigraphical and sedimentological background and the type materials. They constitute an evolving lineage, and have been known from the Upper Bathonian rocks of the Pamirs, where they are cited as one of the celebrated examples of rapid speciation. A detailed comparison of the specimens from these two areas reveals that the speciation took place in Kutch and involved cladogenesis. Thus, it provides a good example of the punctuational model of evolution.

Preserved color pattern of a phylloceratid ammonoid from the Jurassic Chari Formation, Kutch, India, and its functional significance

Preservation of pristine pigmentation in fossil shells is a rare phenomenon (Teichert, 1964) and fossilization accompanied by recrystallization often renders the primary color bands indiscernible (Shrock and Twenhofel, 1953). Nevertheless, there are reports on color markings in fossil shells ranging from the Middle Cambrian to Holocene (Hoare, 1978; Mapes and Hoare, 1987; Kobluk and Mapes, 1989).

Sudden origin of ribbing in Jurassic Paracenoceras (Nautiloidea) and its bearing on the evolution of ribbing in post‐Triassic Nautiloids

Ribs appeared cryptically in the Middle Jurassic nautiloid Paracenoceras. These ribs were produced by crowding of growth lirae as a corollary of change in body size during paedomorphic evolution. Initially, they had no direct functional significance. Some other contemporary genera are found to have similar ribbing patterns, partially developed on either the flanks or venter of the adult body chamber. Subsequently in nautiloid phylogeny, ribs spread all around the whorl, becoming analogous to those of contemporary ammonites. Shell rugosity is observed to occur with increasing frequency in post‐Triassic nautiloids, paralleling the trend in ammonites. This is believed to be an outcome of the ‘arms race’ known as the Mesozoic marine revolution. Ribbing that was not at first adaptive in these nautiloids was subsequently co‐opted as a defensive adaptation. The evolution of this structure is a good example of exaptation.

Repair scars on Mactra violacea from the eastern coast of India: A new classification and a model for describing shell breakage on bivalves

Non-lethal shell damage, which is preserved as repair scars on the bivalve shell, can be predatory or non-predatory in origin. When the peeling crabs are the main predatory groups, non-predatory damages are produced by impact from the saltating clasts or by wear and tear during burrowing. In both cases, these repair scars almost look alike, and it is difficult to identify which factor is causally responsible. Because survival of an individual is related to the severity of the shell-break irrespective of the cause, here, we have developed a classificatory scheme to categorize the repaired traces on the basis of intensity of the damage. Moreover, we have provided a model to analyze how the severity of scars can be effectively used to study species’ adaptation against shell breaking causes, by using Mactra violacea as a studied species. Individuals who survive shell breakage may adapt to escalated morphological traits to resist damage in the long term. Subhronil Mondal. School of Geosciences, University of South Florida, 4202 E. Fowler Ave. NES107, Tampa, FL 33620-5250, USA. subhronil.m@gmail.com Subhendu Bardhan. Department of Geological Sciences, Jadavpur University, Kolkata-700032, India. sbardhan12@gmail.com Sumanta Mallick. Department of Geological Sciences, Jadavpur University, Kolkata-700032, India. sumanta.geol87@gmail.com Arindam Roy. Department of Geological Sciences, Jadavpur University, Kolkata-700032, India. addy.geol@gmail.com