The environmental characterization of the Lower Permian mesosaur−bearing strata of the Mangrullo Formation (Paraná Basin,

ABSTRACT. Priohybodus cf. P. arambourgi is reported for the ®rst time from the Tacuarembo  Formation of Uruguay. This species is a hybodontid shark known previously only from Upper Jurassic and Lower Cretaceous deposits of Saharan Africa and the Arabian Peninsula. The material (22 isolated teeth and a dorsal ®n spine) was found in a thin bone bed, associated with abundant bone fragments, scales and teeth of semionotiform ®shes and theropod dinosaurs. Until now, the age of the Tacuarembo  Formation has been dif®cult to determine because its fossil content lacked useful biostratigraphic indicators. The ®nding of Priohybodus cf. P. arambourgi in that unit greatly expands the palaeobiogeographic range of the species, and allows us to propose a Late Jurassic±Early Cretaceous age for the Tacuarembo  Formation.

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