Evidence and implications of marine invertebrate settlement on Eocene otoliths from the Moodys Branch Formation of Montgomery Landing (Louisiana, U.S.A.)

Microscopic analysis of 3,256 fish otoliths from the Eocene Moodys Branch Formation at Montgomery Landing, Louisiana, U.S.A., revealed 93 specimens with evidence of marine invertebrate settlements, primarily encrustings and boreholes. Although size, abundance, shape (stability), durability, and surface residence-time influenced the use of otoliths, key factors were size, abundance, and surface time. Invertebrates affecting otoliths were cnidarians (scleractinian solitary corals), bryozoans (cheilostome species), molluscs (mainly gastrochaenid bivalves), and annelids (serpulids), noted by larval form settlement, encrustation, and drilling. The size of the scleractinian corals, the time duration of the serpulids, encrustation by cnidarians and serpulids and paucity of other epifauna such as bryozoans, and the lengths of Gastrochaena borings appear to indicate that the otoliths did not remain exposed on the sea floor for an extended period. Surface residence-time may also explain why the abundant, diverse invertebrates present affected only about 3% of the otoliths.