The first report of “chaetetids” from the Cretaceous of North America and their palaeoecological implications

Abstract

The recently discovered Upper Cretaceous locality El Gorrión (SI48, San Luis Potosí, central Mexico) yielded three specimens of chaetetids. All specimens can be attributed to the genus Blastochaetetes Dietrich due to the discontinuities in the calicle walls. In addition, one of them could be classified as Blastochaetetes flabellum (Michelin) based on its distinctive internal dimensions. The remaining two specimens were assigned to an undetermined species taxon in open nomenclature, namely Blastochaetetes sp. indet. 1. Three growth forms were identified for the chaetetids of El Gorrión. Blastochaetetes flabellum has a laminar growth form, which may be an adaptation to survive high-energy conditions, depending on whether the chaetetid was fixed to its substrate or not. The two remaining specimens show either a smooth, nonenveloping or a ragged, high-domical morphotypes, which respond to different environmental conditions. The material is strongly silicified and therefore poorly preserved. The specimens lack spicules, so it is not possible to place them into any of the known sponge groups. Nevertheless, these fossils, as chaetetids, are clearly sponges. This finding is of great importance since such organisms have been neither reported for the Mesozoic in Mexico, nor for the Cretaceous in North America. Furthermore, it constitutes the second report of chaetetids for the Mesozoic of both North and Latin America.