Paleoecology of the foraminifer Acruliammina longa (Tappan, 1940) from the upper Hauterivian of the Neuquén Basin (northern Patagonia, Argentina)

Abstract

Herein the epibiont agglutinated placopsilinid foraminifer Acruliammina longa from the upper Hauterivian of the Neuquén Basin is discussed. This is the first record from South America of A. longa, already known from the upper Valanginian to lower Turonian? of North America and Europe. The studied material consists of foraminiferal tests forming macroids and encrusting the valves of Ptychomya koeneni, coming from poorly lithified claystones/siltstones to marlstones forming high frequency (6th-order) depositional sequences within the upper Hauterivian Agua de la Mula Member of the Agrio Formation. This new record extends the paleobiogeographic distribution of A. longa, and allows us to investigate the paleoenvironmental and paleoecological significance of this species in a mixed carbonate and siliciclastic ramp. Energy dispersive spectroscopy and X-ray computed microtomography were used to evaluate taphonomic features of foraminiferal tests, highlighting differences between tests that encrust bivalves and those forming macroids, which also differ in their position within the 3rd-order sedimentary sequences. A. longa tests encrusting bivalves are less damaged, show coarser grain size, and occur throughout the 3rd-order regressive systems tracts, whereas tests forming macroids show higher breakage and finer grain size, occurring throughout the 3rd-order transgressive systems tracts. This distribution allows us to interpret changes in rates of terrigenous input at the time of encrustation, and also in energy conditions, highlighting the opportunistic behavior of A. longa. Low sedimentation rates represent the main paleoenvironmental condition that favored foraminiferal propagule settlement and the growth of A. longa, both on bivalve fragments exposed on the seafloor in the case of macroids, and on living Ptychomya koeneni shells.