CRETACEOUS–PALEOGENE BOUNDARY ISSUES AND CONTINENTAL MOLLUSKS IN INDIA AND NORTH AMERICA: FINE-TUNED TIMING AND OTHER ISSUES CRITICAL TO INTERPRETING LARGE SCALE EVENTS ON BENTHIC CRITTERS

Since Cretaceous–Paleogene (K/Pg) boundary studies were redefined about 30 years ago, debate over the consequences of the ET impact on extinction and survival has led to changing the scale at which geologists and paleontologists examine the geologic record. Continental mollusks (specifically freshwater, gill-bearing unionoid and sphaeriid clams and snails, and lung-bearing aquatic and terrestrial snails) are environmentally sensitive and can occur in abundance at numerous localities in many depositional settings, making them useful for recognizing local, regional, and wider environmental shifts under documented control.

Independent work by Thierry Adatte and Gerta Keller places two and possibly one reworked Platinum Group Element (PGE) anomalies in the Viri Railroad Section near Anjar, in Kutch, Gujarat, India. These PGE anomalies occur in a few meters between Deccan Traps bearing dinosaurs and continental mollusks a few meters below a not highly resolved K/Pg boundary. Molluscan diversity, disparity, and abundance are unaffected by the occurrence of Anomaly AN14a, the lower of two. If anything, initial results indicate an increase in all parameters by 100%, with the addition of new taxa in the PGE interval. This observation, however, may be biased on better-quality preservation from mudstone to volcaniclastic sediment. No shells were found in carbonaceous mudstone and sandstone beds of the upper anomaly (AN45-49).

Partially buried freshwater mollusks that probably should have survived the K/Pg impact did not. The loss of species subjacent to the boundary, along with changing sedimentological conditions, and lack of significant recovery suggest both a change in environmental circumstances prior to the end of the Cretaceous and compounding additional events dissimilar to those that resulted in, but did not prohibit, recovery of mollusks from faunal turnovers during the Cretaceous. An advancing Western Interior sea, Laramide tectonism, bolide impact, concatenated to bring habitat specialists and an evolutionary orgy of continental mollusks to an end with no immediate recovery in sight.

Both study areas have thriving mollusks within 2 to 4 m of the K/Pg boundary. At this point, high-resolution radiometric dating of specific horizons is required to provide necessary temporal sequence control.