SPECIES DISCOVERY AND CHANGING SPECIES INCLUSIVENESS IN CENOZOIC MOLLUSCS – WHAT EMERGES FROM A POPULAR HANDBOOK AFTER MORE THAN 50 YEARS OF NEGLECT?

Over the past 30 years study of benthic marine invertebrate species, a general pattern has emerged that shows that many common and widespread (‘traditional’) morphologically-based species fall into more narrowly distributed allopatric clusters when DNA sequencing of standard gene fragments is undertaken. These DNA clusters typically correspond with more narrowly defined morphological species – so-called cryptic species. Almost all neontological taxonomists choose to recognize these newly separated clusters as species. Invertebrate palaeontology pays little attention to species concepts and it seems likely that ‘species-as-taxon’ concepts are now diverging among zoological and palaeontological workers. Few have paused to consider whether this is important, perhaps because little compiled data exists.

Updating a handbook of common British Cenozoic fossils first published in 1960 for a new edition published in 2016 involved fully revising the identification, taxonomic name and stratigraphic range data given for 266 molluscan species therein. I will use this small but highly vetted data set to get insight into whether the species taxon concepts of commonly found molluscs have changed over the past 56 years. Sophisticated morphological analyses or in-depth taxonomic studies have not been undertaken on these species. Nevertheless, I will examine how current views of the morphological inclusiveness of species has changed in response to; a) improved taxonomy and nomenclature, b) increased sampling, c) improved stratigraphic resolution of sampled rock units, and d) novel discoveries. I will determine to what extent these changes have led to stratigraphic or geographic range extensions or contractions. Finally, I will consider to what extent the stratigraphic duration of the newly delimited species compares with data derived from other taxonomic and geographic studies of marine molluscs in the Cenozoic.