Dinosaur extinction and subsequent mammalian recovery during the Cretaceous-Paleogene (K/Pg) transition in the Nanxiong Basin

Abstract

Geochemical analysis of the vertebrate fossils (dinosaur eggshell fragments and chelonian bones) from the K/Pg boundary section exposed in the Nanxiong Basin, Guangdong Province, South China shows that at least three geochemical environmental events, marked by Ir anomalies, took place during the latest Cretaceous (Maastrichtian) and early Paleocene. The first geochemical environmental event occurred at about 67 Ma. In this event dinosaur diversity was hardly reduced, although it appears to have led to an increased incidence of eggshell pathology. The second event was larger, and occurred at or near the pollen-defined K/Pg boundary interval. Enrichment of other trace elements, such as Cr, Ni, Co, Zn, Pb, Cu, Mn and V, within the eggshells occurs mostly at and near the interval, and eggshells in this stratigraphic zone also show positive oxygen-18 excursions and anomalies in thickness and microstructure. Abundant Ir and other trace elements in the dinosaur eggshells reflect changes in their original mineral composition that took place before fossilization began. The anomalously abundant elements were taken into the dinosaurs' bodies in food, and then passed into their eggs. This process would have affected the normal physiological functioning of the dinosaurian body, as well as causing the observed eggshell pathologies. The extinction of the dinosaurs in the Nanxiong Basin did not occur instantaneously, but was spread out within 250 ka with major extinction beginning at the boundary interval. The cause may have been environmental poisoning and adverse climatic changes, as indicated by geochemical analyses of the dinosaur eggshells. Through the food chain, trace elements were taken into the dinosaurian body in excessive amounts, causing imbalances in trace element levels. This affected the dinosaurs' reproduction, leading to the formation of pathologic eggshells. The eggshells became so brittle that they failed to protect the developing embryos, contributing to the extinction of the dinosaurs. The third geochemical environmental event occurred in the early-middle Paleocene, the time interval represented by the bottom layer of the Shanghu Formation. This event may have significantly delayed the full recovery of non-marine ecosystems and mammals after the dinosaur extinction. The stepwise extinction and the Paleocene mammalian recovery observed in the Nanxiong Basin nearly coincide with the duration of the Deccan volcanism seen in India. Recent advances in Deccan volcanic studies indicate that the volcanic eruptions occurred in a series of rapid pulses, grouped into three main phases. The initial set of eruptions, phase 1, occurred at ~67.4 Ma. This phase accounts for only about 6% of the total volume of the Deccan Traps, probably because the eruptions were less intense in this phase than in the later ones. Phase 2 included the largest eruptions, took place prior to the K/Pg boundary, and accounts for ~80% of the total volume of the Deccan Traps. Phase 3 began in the early Danian, or about 270 ka after the K/Pg boundary, and accounts for ~14% of the total volume of the Deccan traps. Therefore, it seems likely that the K/Pg events that occurred in the Nanxiong Basin can be explained in terms of the effects of the Deccan volcanism on an already stressed biosphere.