The global fossil record of Mesozoic–Cenozoic freshwater bivalves

Bivalves are an extraordinarily successful group that has invaded freshwater environments multiple times across multiple clades throughout Earth history. Today, freshwater bivalves occur with approximately 1400 species almost across the entire globe. Many groups are well researched because they are important bioindicators and provide ecosystem services. Their fossil record is comparatively poorly known. Previous reviews of fossil freshwater bivalves focused on selected clades, regions or time intervals. A comprehensive and critical synthesis of the diversity dynamics, distributions, and first occurrences of all freshwater bivalve clades was never attempted. Here, we present a first review of the global freshwater bivalve fossil record from the Triassic to the pleistocene. We record 3926 valid, fossil-only species (of 5733 available species-group names in total) in 418 genera, 33 families, 14 superfamilies, and 8 orders. Overall, we recognise at least 20 independent invasions of fresh water in ten orders within the Bivalvia. At least six of these occurred in the Mesozoic; another ten have no fossil record. The dominating group in the fossil record is the Unionoidea with 1506 valid species, followed by Cardioidea (Lymnocardiinae) with 875, Sphaerioidea with 363, Cyrenoidea with 301, Dreissenoidea with 297, and Trigonioidoidea with 233 species. While today freshwater bivalves have centres of diversity in large, ancient river systems, the fossil record is biased towards lake environments. Fossil diversity hotspots are i) the large lakes in the Late Triassic to Late Cretaceous of East and Central Asia, hosting a large part of the Mesozoic freshwater bivalve diversity and potentially the origin of several major clades, and ii) the Neogene lakes of the Paratethys and adjacent regions in Europe, which are the cradle of Lymnocardiinae and Dreissenidae. Current diversity reconstructions, as well as alleged first occurrences of major clades must, however, be treated with caution. Several bivalve genera and families, especially from the Mesozoic, in Unionida and Sphaerioidea, are poorly defined. Taxonomic over-splitting and a sampling bias towards the global north additionally distort the picture. Despite these biases, we estimate that the overall diversity trends and patterns are real.