Agerostrea

Abstract: The Upper Cretaceous — Early Palaeogene sediments of Meghalaya, mostly restricted along the southern part are represented by the Mahadek and Langpar formations and host marine invertebrate fossils. The Cretaceous-Palaeogene (K/Pg) mass extinction boundary is recorded within the Langpar Formation based on foraminiferal zonation. The present study is restricted to the Upper Cretaceous sediments, where the mega-invertebrate fossils were studied and attempt was made to study the lithological variation together with faunal assemblages in order to review the stratigraphy. Two marker horizons were identified from the Late Maastrichtian sediments that have characteristic lithology and fauna and are helpful in regional correlation. The Mahadek Formation, a medium to coarse grained calcareous sandstone unit with two characteristic echinoid genera Gongrochanus sp. and Stygmatopygus sp. and the bivalve Agerostrea sp. represent one marker horizon. The study indicates that it is the transition zone in the upper shelf environment. The other marker horizon within the lower part of the Langpar Formation, consist of carbonaceous shale and calcareous subarkose intercalation host heteromorph ammonites Eubaculite sp., Glyptoxoceras sp. etc. and prolific gastropods along with burrows. This indicates a comparatively deeper shelf environment. The presence of pyrite frambroids of size less than 5µm from the marker horizon of the Langpar Formation indicate suboxic to anoxic conditions and their presence give substantial evidence for the toxicity in the environment due to anoxic condition and thus the role of anoxia during the K/Pg mass extinction is strengthened by the record. Several sections were studied and the abundant echinoids (Gongrochanus sp. and Stygmatopygus sp.) were observed in Mawsynram (Phlang Mawsyrpat and Phud Japung area), whereas prolific Agerostrea sp. in Noh Kalikai, Sohkha and Muktapur-Amlarem sections and heteromorph ammonites along with gastropods in Sohra and Pynursla sections. The shell bed successions of echinoids and bivalves provide evidence of local episodic storm events in the upper shelf. The lithology and faunal assemblage together with the tectonic set up of the basin led to conclude that the reason for the difference in faunal diversity was probably due to the difference in the palaeo-depth of the basin.

Abstract: The micro-morphological shell characters (both in optical microscope and SEM) have been used to discriminate in between oyster’s sub-families Pycnodonteinae Stenzel 1959, Exogyrinae Vyalov 1936, Ostreinae Refinesque 1815 and Lophinae Vyalov 1936. These sub-families are represented by six constituent genera Phygraea (Phygarea) vesicularis (Lamarck 1806), Hyotissa semiplana (Sowerby1813), Curvostrea rouvellei (Coquand 1862), Ceratostreon pliciferum (Dujardin 1837), Agerostrea ungulata (Schlotheim 1813) and Rastellum (Arctostrea) pectinatum (Lamarck 1810) in the late Cretaceous (Maastrichtian) sediments of the Ariyalur area of Tamil Nadu, south India. The optical microscopic observations and SEM studies of the shells of these six genera clearly indicate that all the four sub-families consist of distinctive set of shell-microstructures. Sub-family Pycnodonteinae is characterized by predominance of vesicular, exogyrinae by prismatic, Ostreinae by cross foliated and Lophinae by foliated shell microstructures. Besides their characteristic shell microstructures, some additional microstructures are also visible in the shells of some of the genera of these four sub-families.