Vertical zonation or allogenic succession? Sclerobionts on a coral patch-reef from the Lower Cretaceous Agrio Formation, Neuquén Basin, west-central Argentina

Reefs are hotspots of diversity, modifying their surroundings and fostering numerous biotic interactions: in the fossil record, they preserve valuable paleoecological and paleoenvironmental information. Lower Cretaceous reefs are understudied; this paper analyzes the sclerobiont fauna of a Hauterivian patch-reef from the Neuquén Basin (Argentina) to interpret its life history and paleoenvironmental significance. Massive and ramose forms of paleoautoecologically homogeneous genera form this reef; corals bore zooxanthellae and presented features indicative of a tolerance for high-sedimentary input. Sclerobionts were analyzed separately for ramose and massive corals. In addition, the upper and lower surfaces of massive corales were also logged separately. The sclerobiont fauna comprised mainly thecideide brachiopods, oysters, serpulids, cyclostome bryozoans and calcareous sponges. In addition, in ramose corals only, microbial crusts are also recorded and encrusted by sclerobionts. Massive corals presented lower diversity indices as compared to ramose ones. In both coral morphotypes, bioclaustrated sclerobionts were registered: in particular, serpulids established mutualistic relationships with the corals. Massive corals' undersides hosted a much more abundant, sciaphyllous sclerobiont fauna. This patch reef dwelled in a mid-ramp setting under relatively high sedimentation rates; massive corals were buried more quickly than ramose forms. The microbialites settled only on ramose corals after the patch reef began to dwindle; massive ones were already buried, but ramose ones remained in life position. The arrival of microbialites indicates lowered sedimentation rates, likely due to nutrient enrichment. Thus, the pattern of sclerobiont distribution observed in these corals reflects not a vertical zonation of coeval sclerobionts, but rather an allogenic succession that evidences the progressive burial of the patch-reef. At Early Cretaceous mid-latitude settings, corals were adapted to suboptimal conditions, such as a relatively high sedimentation, sustaining a moderately diverse and dynamic sclerobiont fauna composed mostly of heterotrophic organisms.