LOWER AND MIDDLE EOCENE BRACHYURA FROM CHIAPAS, SOUTHERN MEXICO; NEW DATA FROM TWO DIFFERENT ASSEMBLAGES

Lower Cretaceous to lower Miocene rocks of Chiapas state, southern Mexico preserve a suite of decapod crustaceans. Recent fieldwork on two different Eocene localities, Veinte de Noviembre and Mesa de Copoya, from central Chiapas, have added new crustacean taxa. Age for the Veinte de Noviembre locality has been previously considered middle Eocene. However based on Sr/Sr analysis of nautiloid shells the age has been corrected to lower Eocene. Here, a diverse decapod crustacean assemblage including species of Callianassidae, Raninidae, Necrocarcinidae, Leucosiidae, Portunidae, Goneplacidae, Xanthidae, and Pinnotheridae families have been collected. Age correction and lithology suggest that these deposits are part of the El Bosque Formation, defined mainly as alluvial plains associated with marginal continental deposits and shallow marine environments. All specimens are molts. Many carapaces have signs of deformation and/or fractures depicting transportation before deposition. Comparatively, Mesa de Copoya, near Tuxtla Gutiérrez, Chiapas, assigned to the middle Eocene San Juan Formation, does not have the high diversity as Veinte de Noviembre. Crab remains are also fractured and/or deformed. Species of the Diogenidae, Calappidae, Raninidae, Hepatidae, and Portunidae families have been collected. Paleonvironment is interpreted as lagoonal, linked to a deltaic system, with fluvial influence. Interestingly, not a single genus is common between these two localities. These studies confirm that Chiapas contains the most diverse paleocarcinofauna from Mexico, as decapod crustaceans are found from Lower Cretaceous to lower Miocene deposits.