PART OF A POSSIBLE BRACKISH COMMUNITY PRESERVED IN CHIAPAS AMBER (LA PRIMAVERA MINE, LOWER MIOCENE; STATE OF CHIAPAS, MEXICO)

Reconstructions of amber forests frequently depict them as contiguous with aquatic environments, such as rivers, estuaries, or seas. Yet, only one possible intertidal animal, a juvenile, decapod (possibly Grapsidae) crab, has been reported for amber. Knowing what aquatic organisms existed in close proximity to ancient amber forests is essential to understand the structure and dynamics of these paleocommunities. We report three new records of fossil animals, all belonging to different phyla, in amber. These organisms lived next to or in the Chiapan (México) amber forest. The substrate on which these organisms dwelled (or were deposited) consists of a fine layer of gray, carbonate mud sediment, most likely microcrystalline calcite or micrite (x-ray crystallography) with some quartz (petrographic microscopy). This substrate is consistent with a shallow lagoon depositional environment. The first fossil, is a barnacle base (Crustacea: Cirripedia, possibly Balanus sp. or Chthalamus sp.) 1.8 mm in diameter, with bilaterally symmetrical arrangement of plates. The second fossil type consists of about half a dozen burrowing, tube-forming organisms, possibly tubeworms (Annelida: Polychaeta), inferred from several exposed, well-preserved, small (up to 3.5 mm long x 0.5 mm diameter at opening), tapering mud cases. The case inhabitants are not visible. As barnacles, polychaetes are typically marine although several species have been reported for brackish waters. Barnacles and polychaetes co-occur in modern mangroves, which are shallow aquatic plant communities relatively close to the sea. The third fossil, a fingernail clam valve (Mollusca: Pisidiidae, probably Sphaerium sp.) 1.0 mm long, typically a freshwater organism, is also in the piece. Furthermore, a slimy-looking film, whose biological identity has not yet been determined in cross sections prepared, covers part of the substrate. Finally, an oyster (possibly Oystrea sp.) is partially covered by the resin as typically found in La Primavera mine. This Mesoamerican forest was located 600-1200 m below its current (fossilized) location some 15-20 Ma. This fact is consistent with the active geological activity of this region.