FIRST FORMAL REPORT OF A CRAB IN AMBER FROM THE MIOCENE OF CHIAPAS AND OTHER UNCOMMON CRUSTACEA

The first formal report of a crab preserved in amber is presented for a small sesarmid crab from the Lower Miocene of Chiapas. A second specimen is known from a book illustration, stating the piece belongs to a private collector, and only a bad drawing is presented. The specimen belongs to the genus Sesarma, a widely distributed genus found in caves and estuaries of the Caribbean and Indo-Pacific regions. The exceptional preservation of the specimen is ideal to observe most diagnostic features, including chelae and fine setae of appendages. The small size of the specimen (about 4 mm in width) suggest it may be a juvenile, and details as corrugated cuticle plus an additional detached leg indicate that the specimen just had molted before it got trapped in the amber. This may explain the rare occurrence of this kind of crustaceans, as this is the first time a crab in amber can be illustrated and described in detail. Right after the molt, cuticle is still weak, and if tapped in a sticky surface, the crab would not be able to escape easily. This occurrence also helps understand the paleoenvironment in which amber was deposited during Miocene times in this portion of Mexico, as other unusual crustaceans included in amber correspond to amphipods and ostracods. Three specimens of amphipods are under study, and although there is one formal report of one amphipod from the amber of Chiapas, the new specimens belong to different species. The ostracods and one amphipod are found in a piece of amber which includes sand (mainly quartz grains). The amber must have been soft enough to include grains and ostracods that probably lived as part of the meiofauna in an estuarine to shallow marine environment. Although there is no control on the geographic provenance of the fossil-bearing amber from Chiapas, it seems that it was deposited in different environments of the ancient coast, as some pieces include only terrestrial organisms, such as fungi, arachnids, land snails, terrestrial isopods, insects, frogs, lizards and bird feathers, while in other regions, only aquatic organisms are found. Research of this kind of rare fossils is difficult, due to commercialization of pieces that can reach high prices, such as the crab here presented, which if not rescued by the local government, will be sold for no less than 30,000US.