DIOPATRICHNUS IN AN EOCENE SHALLOW MARINE CONDENSED SECTION, LISBON FORMATION, ALABAMA

The ichnogenus Diopatrichnus refers to vertical to subvertical, typically straight, cylindrical burrows, most commonly lined with bivalve shells but also other mollusks and platy materials. Such burrows, generally attributed to tube-constructing annelids, have received little attention in the literature. Diopatrichnus is abundant in exposures of the Eocene Lisbon Formation along the Conecuh River in southern Alabama. The Diopatrichnus-bearing interval of the Lisbon is a shallow marine shelf condensed section dominated by weakly consolidated, thoroughly bioturbated, fossiliferous, glauconitic, muddy sands with subordinate thin mud and shell beds. Diopatrichnus occurs as part of a softground ichnofossil assemblage representing the Cruziana ichnofacies. The assemblage also includes abundant Thalassinoides and Rosselia, common Asterosoma, Teichichnus, and Planolites, and rare Ophiomorpha. Differential erosion of the muddy sands provides three-dimensional views of Diopatrichnus. At this locality, Diopatrichnus have an average interior diameter of 2.9 cm and an average exterior diameter of 4.4 cm; lengths of lined burrow segments range up to 16 cm or more. The linings of the Lisbon Diopatrichnus are commonly monospecific, dominated by small, whole, predominately concave-up, nested bivalve shells. However, some linings are constructed from more diverse shell assemblages that include corals. Ongoing studies of these burrows and associated sediments are leading to a better understanding of the construction and paleoenvironmental significance of Diopatrichnus.