FOSSIL PANTRIES: CRUSTACEAN DEBRIS STORAGE CHAMBERS IN LATE CENOZOIC ESTUARINE SANDS

Spectacular displays of hemispherical trace fossils including basal compartments packed with plant debris occur in Pliocene and Pleistocene estuarine sands on the northeastern Pacific coast. This trace has recently been named Maiakarichnus currani (Verde and Martinez, 2004) from Miocene examples in Uruguay. There it is associated with Ophiomorpha-Thalassinoides and was interpreted as brood chambers of mud shrimp (Thalassinidea). We studied examples of M. currani preserved in great numbers; a few chambers were associated with horizontal Thalassinoides or with equilibrium structures. The hemispherical chambers are 3-5 cm in diameter with shallow cone-shaped bases that contain plant debris and rare foraminiferal tests. The chamber wall is unlined and 30-50 very narrow shafts radiate outwards from the sphere, extending 1-2 cm into the enclosing sand. Each chamber is extremely fragile and was apparently constructed by mucous-secretions cementing sand grains. The shafts probably anchored the chamber in the shifting sands and the plant debris indicates that these are food storage structures and not brood chambers.

Thalassinidea inhabit burrows that are actively irrigated. Hypotheses on the function of such shrimp-burrow debris chambers include: a) passive accumulation of plant debris; b) active accumulation as an alternative to ejection of such particles out of the burrow; c) hoarding food; and d) microbial farming. Living Bermudan mud shrimp, Corallianassa longiventris, actively collect and store comminuted plant matter within debris chambers. Abed-Navandi et al. (2005) analyzed the nutritional value of this plant matter (largely seagrass) showing that it provides a substantial nutritional source for the shrimp. For the tracemaker of Maiakarichnus currani, similar pantries fixed the burrow in shifting sands within dynamic estuarine settings, and may also have supplied food in times of intermittent or reduced supply.