Foraminiferan tests and dasycladalean thalli as cryptic microhabitats for thaumatoporellacean algae from Mesozoic (Late Triassic–Late Cretaceous) platform carbonates

Abstract

Thaumatoporellacean algae are widespread constituents in Middle Triassic–Cretaceous shallow-marine carbonates of the Tethyan realm. Based on various examples from Mesozoic limestones of Mediterranean platforms (e.g., Dinaric, Apenninic, Apulia) and rare records of Iberia (Pyrenees), Saudi Arabia and Mexico, it is shown that thaumatoporellaceans commonly dwelt as cryptoendoliths in the tests of larger benthic foraminifera and the thalli of dasycladalean algae. Their high morphological plasticity allowed the test invasion and the adaptation to the available interior spaces (chambers, apertures). The temporal distribution of cryptoendolithic thaumatoporellaceans with first records in the Late Triassic, shows acme intervals in Early–Middle Jurassic and Early–Late Cretaceous times. Within the foraminiferans, the thaumatoporellaceans were erroneously considered as an integral part of the test, respectively, phrenoteca-like structures (species Biokovina gradacensis) in the Lower Jurassic and trematophore (species Scandonea? mediterranea) in the Upper Cretaceous. Therefore, the presence of phrenoteca-like structures in the Biokovinidae, being part of the family diagnosis, is challenged. The comparably thin walls of the cryptoendolithic thaumatoporellacean algae are interpreted as an adaptation to the poorly illuminated microhabitats (photoadaptation) in order to maximize light capture for photosynthesis.