Patterns of biodiversity and faunal rebound following the K–T boundary extinction event in Austral Palaeocene molluscan faunas

Abstract

Palaeocene molluscan faunas are characterised by complex evolutionary histories following the Cretaceous–Tertiary (K–T) boundary extinction event and exhibit dramatic, distinct signatures of composition and biodiversity levels relating to extinction and post-extinction recovery processes. This paper is the first to document and survey the entire Palaeocene record of the Southern Hemisphere, which comprises at least 515 recorded molluscan taxa from Australia, New Zealand/Chatham Islands, Antarctica, and South America. The record is much richer than previously recognised. The K–T boundary event was a prime mechanism of change for the composition of Palaeocene faunas, but Palaeocene diversity patterns were shaped also by the final break-up of Gondwana, concomitant changes in climate and in oceanic circulation, and faunal recovery processes. The ‘flip-flop’ in diversity of bivalves and gastropods across the K–T boundary stems from a rapid, evolutionary burst of speciation in the Danian for gastropods, especially carnivorous forms with planktotrophic development, which infilled ecospace vacated by the extinction. In the Southern Hemisphere fossil record, deposit feeders were less affected by the extinction event and seemingly more extinction-resistant, but other important factors related to stratigraphic range and spectrum of life habits/habitats affected survivorship success. Suspension feeders, especially epifaunal forms, were hard hit by the extinction, but had bounced back within a few million years by late Danian time at the latest, but at lower diversity than during the Late Cretaceous. The extinction was not as marked in most southern regions, as reflected in the Antarctic record. In the Antarctic K–T boundary interval, diversity drops suddenly ca 50 m before the boundary, and while suspension feeders remain at low diversity for at least 300 kyr, gastropods still dominate the molluscan assemblages. Most of the K–T boundary molluscan survivors were bivalve species (66%) and all of these were members of representative genera that displayed extensive stratigraphic and geographic ranges in the Cretaceous or earlier in the Mesozoic. The majority of earliest Danian Antarctic molluscs belong to newly evolved species within surviving genera (58%), but by late Danian time this trend had changed to a dominance of new species in new genera. Many new groups arose during the Palaeocene, especially by late Danian time, and these faunas are highly distinctive at both genus and species level. For example, the Wangaloa Formation fauna of late Danian age (ca 63–61 Ma) is dominated by new species in new genera, ranging in values from 62.5 to 81%. A systematic catalogue of all known Palaeocene Austral species is presented herein in the Appendix for the first time with details of formation recorded, age, and inferred life habits.