“IRREGULAR THALASSINOIDES” AT EOCENE UNCONFORMITIES IN THE U.S. GULF COASTAL PLAIN

Marine unconformities in the middle Eocene (Claibornian) Lisbon Formation and its equivalents in southwestern Alabama and southeastern Mississippi are often marked by conspicuous network burrows. These burrow systems, made conspicuous because they are filled with material from the overlying beds, may extend as deeply as 46 cm beneath an unconformity. In current terminology, the burrows can be classified as Thalassinoides because they consist of passively filled networks of roughly cylindrical segments joined at triple junctions. However, they differ from the classic search image in being highly labile in diameter, outline, and orientation. This morphology can be interpreted as defensive against predators that may become lost in an irregular maze. Modern analogs are made by various crustaceans, e.g., alphaeid shrimp Alpheus heterochaelis, in relatively firm substrates. Modern analogs of more regular Thalassinoides are made by elongate callianassid shrimp such as Callichirus major in soft to firm substrates. There seems to be little or no intergradation between the two morphologies. Moreover, the regular forms tend to penetrate much more deeply than the relatively shallow irregular forms, which gives the former an advantage in preservation. While “irregular Thalassinoides” is probably not confined to unconformities, it is preferentially preserved below them in the studied sections, and is a useful indicator of firmgrounds.