Bivalve borings in phosphatic coprolites and bone, cretaceous-paleogene, northeastern Mali

摘要

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 Iul-lemmeden 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.