Morphogroups, test shape variability and test asymmetry in Pellatispira crassicolumnata UMBGROVE from the Eocene of Surat-Bharuch (India)

摘要

Bilaterally symmetrical and planispirally evolute Pellatispira crassicolumnata UMBGROVE from the middle and upper Eocene shelf successions of Vagalkhod-Tarkesvar area (Gujarat) record variations in test shape, symmetry and coiling, and contain rare joined tests. These aspects are investigated to explore their development. Based on semi-quantitative measurements of test shape parameters from different stratigraphic levels, four categories of test shape were recognized, whereby the flattened lenticular and subglobular tests predominate in the marl and arenaceous marlite while lenticular and inflated lenticular tests show wide distribution. Based on test shape and host lithology, four morphogroups (A-D) are recognized. Morphogroup-A comprising flattened lenticular tests with thin lateral walls, closely spaced short vertical pillars and low apertural face thrived on the soft substrate towards the lower part of the photic zone. Morphogroup-D, characterized by subglobular tests with thick lateral wall, long thick vertical pillars and high apertural face, dwelled on the hard substrate towards at the upper part of the photic zone where high energy and sufficient light prevailed. Morphogroup-B and morphogroup-C comprising respectively of lenticular and inflated lenticular forms generally thrived between the upper (shallow) to lower (deep) levels of the photic zone, though the former dominated on the soft substrate and the latter on the hard substrate. Both apertural face and pillars appear to reflect the light requirements at different depths. Asymmetric tests occur in all morphogroups and sediment types. Asymmetrical tests are shown to originate by modifying growth during ontogeny, or by merging tests in the adult stage.