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Abstract
BackgroundSeveral clades of bivalve molluscs have invaded freshwaters at various times throughout Phanerozoic history. The most successful freshwater clade in the modern world is the Unionoida. Unionoids arose in the Triassic Period, sometime after the major extinction event at the End-Permian boundary and are now widely distributed across all continents except Antarctica. Until now, no freshwater bivalves of any kind were known to exist in the Early Triassic.Principal FindingsHere we report on a faunule of two small freshwater bivalve species preserved in vertebrate coprolites from the Olenekian (Lower Triassic) of the Burgersdorp Formation of the Karoo Basin, South Africa. Positive identification of these bivalves is not possible due to the limited material. Nevertheless they do show similarities with Unionoida although they fall below the size range of extant unionoids. Phylogenetic analysis is not possible with such limited material and consequently the assignment remains somewhat speculative.ConclusionsBivalve molluscs re-invaded freshwaters soon after the End-Permian extinction event, during the earliest part of the recovery phase during the Olenekian Stage of the Early Triassic. If the specimens do represent unionoids then these Early Triassic examples may be an example of the Lilliput effect. Since the oldest incontrovertible freshwater unionoids are also from sub-Saharan Africa, it is possible that this subcontinent hosted the initial freshwater radiation of the Unionoida. This find also demonstrates the importance of coprolites as microenvironments of exceptional preservation that contain fossils of organisms that would otherwise have left no trace.
Highlights
The End-Permian extinction event at the end of the Permian Paleozoic Era is widely acknowledged to be the most severe biotic crisis to hit the Earth during the entire Phanerozoic Eon [1]
If the specimens do represent unionoids these Early Triassic examples may be an example of the Lilliput effect
Since the oldest incontrovertible freshwater unionoids are from sub-Saharan Africa, it is possible that this subcontinent hosted the initial freshwater radiation of the Unionoida. This find demonstrates the importance of coprolites as microenvironments of exceptional preservation that contain fossils of organisms that would otherwise have left no trace
Summary
The End-Permian extinction event at the end of the Permian Paleozoic Era is widely acknowledged to be the most severe biotic crisis to hit the Earth during the entire Phanerozoic Eon [1]. One of the characteristics of this event is that its onset was sudden [2,3], the environmental conditions remained harsh for several million years into the succeeding Triassic Period. But largely overlooked, occurrences of freshwater unionoids in the Middle Triassic (Anisian) Manda and Ntawere Formations of Tanzania and Zambia, respectively [15,16]. These are probably no more than 10 million years younger than the extinction event. No freshwater bivalves of any kind were known to exist in the Early Triassic
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