Oldest parastacid molariform fossils from South America: Maastrichtian record from northern Patagonia, Argentina

The fossil record of freshwater crayfishes (Astacoidea and Parastacoidea) is globally scarce and dominated largely by trace fossils and bio-gastroliths, particularly in Gondwana. In South America, Mesozoic records have been restricted to ichnofossils, with unequivocal body remains known only from the Paleogene. Here we describe isolated mandible cephalic molar fields attributable to Parastacidae from the Maastrichtian Los Alamitos Formation of northern Patagonia, Argentina. These specimens, recovered from historical collections of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” and previously misidentified as vertebrate teeth, represent the oldest confirmed parastacid body fossils from South America and the second Cretaceous record of the clade in the Southern Hemisphere. The material consists of five incompletely preserved molariform structures, including one nearly complete cephalic molar field, exhibiting a distinctive combination of features such as a subrectangular outline, prominent lateral cusps, a longitudinal central depression, and a thick enamel-like apatite coating. Comparative analysis indicates that these remains cannot be referred to any extant South American genus and likely represent a distinct, unnamed parastacid lineage, possibly showing closer affinities to Malagasy taxa. The Patagonian record reported here fills a critical temporal gap between Early Cretaceous parastacids from Australia and Paleogene records from South America and Australasia, supporting a widespread Gondwanan distribution of parastacids during the Late Cretaceous and reinforcing hypotheses of early high-latitude diversification followed by subsequent dispersal.