“Paleo-mangrove forest” biomes of the lower Cretaceous Romualdo Formation, northeastern Brazil: A multi-proxy analysis

• Isotope analysis of non-pristine fossils helps characterize Mesozoic environments. • Romualdo Lägerstatte wetlands were characterized by frequent transitionality. • We describe the first record of Mesozoic “paleo-mangroves” in western Gondwana. The Early Cretaceous Romualdo Formation, Lagerstätte bearing a remarkably diverse faunal/floral assemblage commonly associated with shallow marine paleoenvironments established during the opening of the South Atlantic Ocean. However, compared to open sea biotas, several groups of its microfossil record present assemblages dominated by typical freshwater/transitional taxa. Our new carbon, oxygen and strontium isotope data in calcareous concretions and carapaces of the ostracod Harbinia micropapillosa , coupled with a reevaluation of the “upper Romualdo” strata vertebrate and plant biotas, indicate strata of this unit were deposited in estuarine bay paleoenvironments, with very marginal marine influence. The unusual presence of large marine fish taxa would not result from their proximity to open sea regions, but rather from being utilized as a seasonal breeding ground, similar in ecology to present-day mangrove forests – the first record of such ecosystems occurring during the Mesozoic of South America.