AMMONITES ON THE BRINK OF EXTINCTION: DIVERSITY, ABUNDANCE, AND GEOGRAPHIC RANGE OF THE ORDER AMMONOIDEA AT THE CRETACEOUS-PALEOGENE (K/PG) BOUNDARY

We examined the stratigraphic and geographic distribution of ammonites around the world to tabulate the generic and specific diversity of these animals just prior to, at, and just above the K/Pg boundary. Based on our compilation, ammonites are abundant and diverse in the last 0.5 million years of the Maastrichtian. They are distributed across all four Cretaceous suborders (Phylloceratina, Lytoceratina, Ammonitina, and Ancyloceratina) comprising six superfamilies (Phylloceratoidea, Tetragonitoidea, Desmoceratoidea, Acanthoceratoidea, Turrilitoidea, and Scaphitoidea) and 31 (sub)genera. They include 56 species; if specimens in open nomenclature are also included in the count, the tally increases to 91 species. The ammonites are distributed differently around the globe depending on their environmental setting. Shallow-water environments (<100 m deep), represented by deposits in northern and central Europe and North America, favor sphenodiscids, diplomoceratids, and fast-evolving scaphitids and baculitids. Deeper-water settings (>100 m deep), represented by deposits in the Bay of Biscay, the Tunisian Trough, Antarctica, and Sakhalin, favor desmoceratoids and slow-evolving phylloceratids and lytoceratids, with occasional diplomoceratids and baculitids. In terms of numbers of individuals, baculitids are probably the most abundant ammonites at the end of the Cretaceous. Their wide distribution with respect to environmental settings suggests that they were facies independent. Following the Alvarez hypothesis, it is now generally accepted that the disappearance of ammonites at the end of the Maastrichtian was due to the asteroid impact. Because ammonites probably followed a planktic mode of life immediately after hatching, their extinction has been linked to a transient episode of surface water acidification. However, recent data from the Atlantic Coastal Plain, the Netherlands, and Denmark indicate that some ammonites (e.g., scaphitids and baculitids) may have survived for as much as several hundred thousand years after the impact. This pattern of extinction and short-term survival may be related to the geographic distribution of species or genera. Alternatively, it may be related to variation in the degree of surface water acidification in shallow- versus deeper-water settings.