Diverse Macroids and Rhodoliths from the Upper Pleistocene of Baja California Sur, Mexico

Abstract

BAARLI, B.G.; SANTOS, A.; DA SILVA, C.M.; LEDESMA-VÁZQUEZ, J.; MAYORAL, E.; CACHÃO, M., and JOHNSON, M.E., 2012. Diverse macroids and rhodoliths from the Upper Pleistocene of Baja California Sur, Mexico.Small multitaxonomical nodules, characterized as rhodoliths, balanuliths, coralliths, bryoliths, and nodules composed of vermetids “vermetuliths,” are described from one horizon in carbonate sand from the Upper Pleistocene Mulegé Formation at Playa La Palmita, Baja California Sur, Mexico. Such a diversity of fossil, free-rolling biota is seldom described in the literature. This is the first time vermetuliths are reported in the fossil record; in addition, the coral Astrangia has not been reported to constitute coralliths before. These nodules and their associated firm-ground were generated in a shallow bay near rocky shores. Break up of a firm-ground during a sedimentary hiatus provided fragments of loosely consolidated, carbonate sandstone for organic nucleation. Fast growers, like balanids, vermetids, and bryozoans, settled on these sandstone fragments or on bioclasts. Initial rapid growth of pioneer organisms was succeeded by a period of bioerosion, and finally, encrustation with a thin, crustose to lumpy cover of coralline red algae in the climax stage of succession. These were insipient rhodoliths, where the thin cover of coralline red algae reflects a short residence time. Also evident is a rich crypto- and endofauna that lived within and an epifauna that lived on the nodules.