Habrotrocha angusticollis from Miocene sediments and their extant forms in the Indian sub-continent

Microscopic biotic remains in Upper Miocene clayey sediments of Sevok group (Siwaliks) present along the Sevok river, Oodhlabari area, Darjeeling district was studied. The sedimentary section constitutes grey marly-clay, nodular siltstone along with intermittent fine grained sandstone. The flask-shaped fossil shells were observed in clayey sediments which resembled the shells of their nearest living counterpart Habrotrocha angusticollis found as a rare rotifer in lakes of NEI. We also studied the modern H.angusticollis in high rainfall region of Western Ghats, Karnataka and found their shells in abundance along with testate amoebae, diatoms and desmids in moss cushions from Virajpet and Madikeri. The shells of extant forms in Western Ghats were of one type but the fossil shells from Upper Miocene in north-eastern part of India (NEI) were of about 7 types. These vary from round-bottom flask to pear shaped body suggesting more diversity in NEI in the past that survived in wetter conditions and moderate temperatures very similar to misty climate in rainforests of Western Ghats and NEI. We infer that shells of Habrotrocha angusticollis serve as indicators of terrestrial (palaeo) ecology and (palaeo) climate of highly moist (≥90% humidity) under-storey substrate in rainforest region.