Bivalve borings in phosphatic coprolites and bone, Cretaceous-Paleogene, northeastern Mali

Bivalve borings are described for the first time in coprolites. They occur along with bored bone from Cretaceous through Eocene phosphatic conglomerates in the Taoudeni and Iullemmeden Basins of northeastern Mali. Coprolites are extensively penetrated by flask-shaped borings (Gastrochaenolites): the oldest known occurrence of the ichnospecies G. ornatus preserves mechanical scratch traces at the base of some early Eocene coprolite borings. The alleged tracemaker, a pholad bivalve of the Subfamily Martesiinae, is preserved as an external mold in one of the early Eocene coprolites. It is the first occurrence of rock-borers in this subfamily from the Paleocene of West Africa. Bored coprolites were very firm to fully lithified (paleocoprolites) when bored by the pholads, indicating early-diagenetic phosphogenesis of the clasts. The intense and repeated bioerosion of coprolites and bone by pholads suggests a long residence time in shallow-marine waters for the clasts. Therefore, the vertebrate fossil assemblages in these Mali conglomerates are considered highly time-averaged. Recognition of Gastrochaenolites borings in vertebrate remains provides useful taphonomic and paleoenvironmental information, despite their destructive impact on the fossils.