Abundant assemblage of Ostracoda (Crustacea) in Mexican Miocene amber sheds light on the evolution of the brackish-water tribe Thalassocypridini

Amber inclusions of fully aquatic animals are rare, most of them isolated specimens of water-bound insect larvae. In contrast, we here describe an ample, speciose fauna of 262 fully aquatic individuals of Ostracoda (Crustacea) from Mexican amber of Early Miocene age. This fauna was trapped whilst under water swimming around trunks of resin-producing trees in a brackish lagoon. Ostracod crustaceans are typically around a millimeter in length, and are known for their mostly well-calcified bivalved carapaces that account for their unparalleled fossil record in arthropods. However, in the Chiapas amber we found representatives of the tribe Thalassocypridini, which are characterised by lightly calcified carapaces and therefore lack a substantial fossil record. Embedded in amber, this 'drawback' becomes a clear benefit because the unobstructed view onto the appendages otherwise hidden in the carapace allowed us to identify nine ostracod species, six of them new to science. The exceptional number of individuals permitted insights into population composition, reproduction, taphonomy and into micro-environmental parameters of the amber's place of formation. Based on ecological data available for Recent Thalassocypridini species, we posit that this tribe reached a peak radiation in open lagoonal settings during the Miocene, and shifted towards anchialine environments by the Recent.