An Early Cretaceous sponge meadow from the Neuquen Basin, west-central Argentina: unsuspected hosts of a dynamic sclerobiont community

Sponges, especially Calcarea, are minor components of benthic associations, especially during the Mesozoic. In the Lower Cretaceous of the Neuquen Basin, small calcareous sponges have been found building a small monospecific meadow. It is restricted to a marlstone lens-shaped bed in a quiet outer-ramp setting in the Cerro Marucho Locality (Picun Leufu depocentre), above a shell bed of small exogyrid oysters; oysters and sponges were the only preserved macrobenthic faunal elements. Individual sponges were small, under 4 cm high, and presented a sub-cylindrical morphology with one or more rounded, apical osculi, many inhalant openings and triactine spicules. Specimens studied here were assigned to Endostoma sp. aff. Endostoma nodosa. These sponges are quite commonly encrusted by exogyrid oysters, serpulids, sabellids, agglutinating foraminifers and cyclostome bryozoans. Overgrowths among sclerobionts were common, though no undoubtedly in vivo interaction has been recorded. Disarticulated left oyster valves were frequently bioclaustrated by the sponges, showing that in vivo settlement upon sponges was common. Many oysters settled in the periphery of the osculum suggesting a commensal relationship. The study of this sponge meadow and its sclerobiont community allowed the identification of different stages of ecological succession. The pioneer stage was characterised by sponge settlement on oyster valves, within an otherwise soft consistency bottom. High sedimentation or high nutrient inputs, either individually or in combination, could explain the great abundance of oysters. During the climax stage, sponges thrived and harboured several sclerobiont taxa, developing a relatively dynamic palaeocommunity. Finally, an intensification in either sedimentation rates or nutrient input (or both) past the tolerable threshold for sponges may have been the cause(s) of the meadow's demise. Endostoma and similar forms were up to now reported mostly from the Jurassic and Cretaceous of Europe as accessory builders, or as accompanying fauna in reefal settings. This new record shows that in rare occasions they could form low-relief meadows on their own.