Some observations on an atypical planktic foraminifer from the Middle Eocene of Kutch, India

An atypical planktic foraminifer having close resemblance with <em>Orbulinoides</em> Blow &amp; Saito, 1968 is recorded from a lignite mine section in Kutch, western India. It is practically indistinguishable from <em>Orbulinoides</em> under optical microscope due to general similarities in chamber morphology, coiling mode, aperture number and character, but differs in wall texture when observed under scanning electron microscope (SEM). The wall is characterized by pores of variable size and irregularly distributed pustules unlike the spinose wall of normal <em>Orbulinoides</em>. Based on the SEM micrographs some specialist opinion suggested that the taxon might be a benthic foraminifer. The morphological features of the studied foraminifera are, however, distinctly different from benthic foraminifera with a planktic stage. The associated larger benthic foraminifera correlate the section with the carbonates of Middle Eocene (Bartonian) age in the adjoining areas of Kutch. The accompanying foraminiferal assemblage suggests a near-shore, brackish-water, semi-enclosed depositional environment. It is suggested that the aberrant wall texture was possibly due to environmental stress developed in the restricted basin. Due to incongruous wall texture and to its occurrence as the only planktic foraminifera in the section, the nomenclature of the reported foraminifera is kept open.

一种与Orbulinoides Blow & Saito, 1968形态高度相似的非典型浮游有孔虫，发现于印度西部库奇地区的一处褐煤矿床剖面中。在光学显微镜下，该标本与Orbulinoides属几乎无法区分，二者在房室形态、旋卷方式、壳口数量及特征上均具有总体相似性，但在扫描电子显微镜（scanning electron microscope, SEM）下观察时，其壳壁结构存在显著差异。该标本的壳壁以大小不一的孔隙及不规则分布的小瘤为特征，与典型Orbulinoides属的刺状壳壁截然不同。基于SEM显微图像，部分专家认为该分类单元可能属于底栖有孔虫。但本次研究的有孔虫的形态特征，与具有浮游阶段的底栖有孔虫存在明显区别。伴生的大型底栖有孔虫类群，将该剖面与库奇周边地区的中始新世（巴顿阶）碳酸盐岩地层进行了时代对比。伴生的有孔虫组合指示了近岸、半咸水、半封闭的沉积环境。研究认为，该异常的壳壁结构可能是局限盆地中产生的环境胁迫所致。由于其壳壁结构与典型类群存在显著差异，且该剖面中仅发现此一种浮游有孔虫，因此本次报道的有孔虫的分类命名暂未确定。