Palaeoecological analysis of a sclerobiont fauna on a single basibiont across the Valanginian of the Neuquén Basin, west-central Argentina

Abstract

Sclerobiont communities have proven their environmental and taphonomic value in extant and past settings; studies are beginning to quantify and evaluate their changes across time. Through the Valanginian of the Neuquén Basin, Argentina, trigonioids of the genus Steinmanella present an almost continuous record. Materials were collected from shales and shell beds across second-order systems tracts (LST, TST and HST). Their sclerobiont communities were characterized and compared across systems tracts and facies. In addition, a link between them and coeval oyster mass occurrences (OMOs) was investigated, since studies on other local sclerobiont communities have consistently shown pronounced oyster dominance. Eleven sclerobiont taxa were found on Steinmanella (bryozoans, bivalves, foraminifers and polychaetes), of which oysters are usually strongly dominant but for the TST, in which their dominance is diminished. The sclerobiont community has a simple structure and interspecific relationships and competition for space seem to have had little importance. Few differences in richness, taxonomy and other parameters were found between facies. Across systems tracts, features of the sclerobiont communities are differentiated mostly from swings in relative abundance of taxa. The lowered oyster dominance in the TST may reflect a source–sink dynamics between OMOs (sources) and Steinmanella as sinks in shallower, soft substrate settings. Likely the sea-level rise temporally drowned the oyster carbonate factory, resulting in decreased larvae emigration to the sinks and lowered oyster dominance during the TST.