Tides of rebirth: A stratigraphic perspective on the Lazarus effect in freshwater bivalves in the Aptian-Albian Romualdo Formation, Araripe Basin, Brazil

Abstract

The Aptian-Albian Romualdo Formation was deposited in a confined aquatic setting marked by at least three distinct marine ingression events. Most paleontological knowledge of this unit derives from the shale-dominated interval in its middle part, while fossil data from the sandstone-dominated facies in the uppermost part of the formation remain scarce. This paucity of data arises because the overlying deposits were either eroded or are currently obscured by Cenozoic gravity-flow deposits. Nonetheless, the uppermost Romualdo Formation preserves a tide-dominated delta succession, documenting the basin gradual return to continental conditions. In the Sobradinho section, a new mollusk-dominated assemblage is identified in an intraclast-supported conglomerate located ∼10 m above the upper shell bed interval. The assemblage includes bone fragments, plant remains, and marine/brackish-water mollusks. Additionally, freshwater bivalves, previously known only from the underlying Crato Formation, are also recorded. All macroinvertebrates exhibit a consistent preservational pattern, with infill composed of the rock matrix. Therefore, the freshwater bivalves are not reworked fossils from older sediments within the conglomerate. Their presence suggests that, for much of the Romualdo Formation depositional history, these freshwater bivalves thrived in riverine and/or low-salinity transitional or coastal environments. Without this new record, their known vertical distribution would remain largely confined to the bivalve-rich mudstones of the basal third of the Crato Formation and to a siltstone bed within a confined bay succession ∼100 m below the newly reported assemblage. This discovery significantly expands their stratigraphic range as facies-controlled Lazarus taxa.