The fossil record of Mesozoic and Cenozoic freshwater bivalves: Are Asian lakes the cradle?

With a standing diversity of almost 1400 species, extant freshwater bivalves account for approximately 14% of modern bivalve biodiversity, occurring from 55°S to beyond the Arctic Circle, on all continents except Antarctica. While molecular phylogenies of freshwater bivalves are rapidly improving, the underlying causes of diversification and the modes of expansion are still poorly understood. The fossil record of freshwater bivalves, albeit much less comprehensive than for marine taxa and biased towards lake environments, helps to explore these issues. We have started databasing fossil freshwater bivalve names in a constrained effort in spring 2020, within the Aphia environment of MolluscaBase (www.molluscabase.org), where the data underlying this study are available. As of 9 April 2022, when the database was last queried, we record 652 genus-group names, as well as 7665 species-group names, of which 1926 are treated as valid; the validity of another 2282 species-group names is currently unassessed. We estimate that we are nearing completeness, with approximately 95% of all species-group names recorded. Of these, 3443 relate to the Palaeoheterodonta, with 454 names in Trigonioidoidea, 211 in Pseudocardinioidea and 2778 in Unionida. Of 4222 species-group names in Euheterodonta, 1932 relate to Lymnocardiinae, 573 to Sphaerioidea, 692 to Cyrenoidea, 963 to Dreissenoidea and 62 to freshwater Corbulidae. We currently recognize three main types of bias. Several Mesozoic families and genera, particularly in the Unionida and Sphaerioidea, are poorly defined. Taxonomic over-splitting, commonly at the species-level, is rife. Sampling is likely biased towards the global north. Notwithstanding the above, we predict that the general diversity patterns will stand.

We recognise at least 18 independent invasions of freshwater, in ten orders within the Bivalvia. At least six of these occurred in the Mesozoic; another nine have no fossil record. Two main centres of diversification are obvious. (1) The large Late Triassic to Late Cretaceous lakes of East and Central Asia hosted the bulk of the Mesozoic freshwater bivalve diversity and potentially are the origin of several major freshwater bivalve clades. These include the Pseudocardinioidea, which have no record outside Asia; the Trigonioidoidea, which have their oldest records in East Asia; and several clades within the Unionida, which have their highest diversity and earliest representatives reported from the area. Unionida as a whole, however, if rooted in Silesunionoidea, as currently accepted, have Triassic records on several continents, and their origin is unclear. (2) Cenozoic freshwater bivalve diversity peaked in the Neogene lakes of the wider Paratethys region, which are the cradle of the freshwater Lymnocardiinae and the Dreissenidae. While the former did never expand beyond Europe, the latter include several invasive species, which continue to expand.