FIDELITY OF MARINE INVERTEBRATE DEATH AND FOSSIL ASSEMBLAGES IN A COASTAL MARINE ECOSYSTEM, BAHIA BIQUE, PANAMA

Along Bahia Bique on Panama’s Pacific coast, an unusual fossil assemblage has intrigued paleontologists for many years. Here, fossils are typically preserved within small, irregularly-shaped, dark-colored, siltstone or sandstone concretions, many of which appear to be burrow in-fills, with most encrusted by Recent epibionts. It was previously assumed that these fossils were dumped spoil from the excavations of the Culebra Formation (lower Miocene) of the nearby Panama Canal. However, no Miocene fossils were recognized in this assemblage.

Fossils include abundant mollusks and crustaceans, but also rare shark teeth, catfish skulls and spines, a ray jaw, turtle fragments, and mangrove rhizophores. All 18 species of fossil mollusks are found in the local death assemblage (= 52 species), and occur with similar relative abundance. Of the most common taxa in the death assemblage, the bivalves Saccostrea palmula, Cumingia adamsi, and Protothaca asperrima, and gastropod Nassarius angulicostis are also common as fossils. Fossil crabs are very abundant with one species of fiddler crab (Uca ornata) predominating. This is the first report of this tropical eastern Pacific species in the fossil record, and one of the few for the genus Uca. Preservation of specimens is remarkable with relatively complete carapaces in normal, or non-traumatic positions, with chelae closed and held along the anterior portion of the carapace and pereiopods often relaxed laterally. Evidence suggests they were rapidly entombed in their burrows. In the area today, U. ornata are found in association with other fiddler crab species (e.g., U. saltitanta, U. oerstedi, U. festae). However, none of these other taxa were identified in the fossil assemblage, only rare remains of Callinectes sp. aff. C. arcuatus, Hepatus sp., Grapsus sp. aff. G. grapsus, and Cardisoma crassum.

Unfortunately, the fossil faunule lacks stratigraphic control being found ex-situ in intertidal gravels along Bahia Bique. However, all of the fossil taxa are still living today, and their association is typical of an extant estuarine setting along Panama Bay. Therefore on the basis of similarity in composition and relative abundance to the present-day death assemblage of Bahia Bique, it is most parsimonious to assume that this fossil fauna is Holocene or late Pleistocene in age.