Paleocene Thalassinidea colonization in deep-sea environment and the coprolite Palaxius osaensis n. ichnosp. in Southern Costa Rica

Palaxius osaensis n. ichnosp., a new ichnospecies of crustacean coprolite is described. The coprolite is preserved in a 200-m thick Paleocene sequence in Southern Costa Rica that is largely dominated by pillow basalts. The studied sample is part of a seamount formed in the Pacific Ocean that was accreted to the Central American isthmus during the Eocene. The absence of lava vesicles, shallow-water deposits, and detrital sediments in the section suggest that the coprolites were deposited in a deep environment during the first stage of the development of the seamount. This represents one of the deepest occurrences of Thalassinidea coprolites reported in the literature and indicates that the producers of the coprolites, presumably some shrimps, developed the aptitude to colonize abyssal environments at least since Early Tertiary. The crustacean coprolites were encountered at a site which apparently lacked a food supply, although hydrothermal processes are believed to have provided the opportunity for a chemotrophic community to develop on the deepest part of the seamount. P. osaensis n. ichnosp. is also found in Colombia in late Cretaceous shallow-water sediments that notably contain Palaxius caucaensis coprolites (Micropaleontology 41 (1995) 85-88). Occurrences of P. osaensis n. ichnosp. deposited at both shallow and deep levels may possibly be related to an aptitude of some thalassinid organisms to have developed in various biotopes during the late Cretaceous-Paleocene.