Cretaceous chaetetids (Porifera: Demospongiae) from Mexico: Systematics, palaeoecology, palaeobiogeography, stratigraphy and perspectives

A review on the findings of chaetetid sponges in Mexico is made. Up to date, two localities house the whole diversity of Mesozoic chaetetids in the country consisting of three species, all of them from the Cretaceous. The studied localities are Arroyo La Cruz, in the state of Oaxaca, and El Gorrión, in the state of San Luis Potosí. At Arroyo La Cruz, outcrops from the San Isidro Formation (of a Hauterivian-Barremian age) can be found, whereas Coniacian to Santonian deposits from the Tamasopo Formation can be located at El Gorrión. Acanthochaetetes huauclillensis Sánchez-Beristain and García Barrera is the only species that can be found in the state of Oaxaca. In contrast, two species of the genus Blastochaetetes, namely Blastochaetetes flabellum Michelin and Blastochaetetes reitneri nov. sp. Sánchez-Beristain and García-Barrera, make up the diversity of the El Gorrión locality.

Blastochaetetes reitneri nov sp. Sánchez-Beristain and García-Barrera can be differentiated from other species in the genus by the presence of intertabular spaces measuring no less than 2 mm, and thick tabulae (as much as 0.13 mm). These are unique features among the genus. For its part, Blastochaetetes flabellum Michelin is easily recognisable by the pattern formed by parallel concentric bands on the surface of the specimens. On the other hand, Acanthochaetetes huauclillensis Sánchez-Beristain and García-Barrera can be distinguished from other members of the genus Acanthochaetetes by the scalariform pattern formed by the tabulae at adjacent calicles.

Palaeoecologically, Mexican chaetetid sponges more or less show the same growth forms, from domical to laminar, both of which could be related to constant sedimentation rates in a low-energy environment (reef-or reef-like settings). However, microbialites are often associated to Acanthochaetetes huauclillensis, whereas no hint to these deposits can be found related to Blastochaetetes.

Mexican chaetetids also have important palaeobiogeographic and stratigraphic implications. Evidences of Blastochaetetes for the country represent the most recent record for the genus. This could mean that it dispersed westward from Europe, where the oldest records can be found. On the other hand, the finding of Acanthochaetetes in Mexico during the Hauterivian to Barremian implies its dispersion to the Western Tethyan Realm prior to its diversity peak in Europe in the Albian.

The fact that Mesozoic deposits are widespread in Mexico implies the possibility of finding multiple specimens in the future, which would deepen the knowledge on these organisms. Moreover, since one of the genera studied here -Acanchochaetetes-is an extant taxon, the possibility of finding well-preserved post-Cretaceous chaetetid specimens for further studies is high.

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Paleoecológicamente, los quetétidos mexicanos muestran más o menos las mismas formas de crecimiento -de dómica a laminar-, las cuales podrían estar relacionadas con tasas de sedimentación constante en un ambiente de baja energía (arrecife o ambientes similares a arrecifes). Sin embargo, es posible encontrar microbialitas a menudo asociadas con Acanthochaetetes huauclillensis, mientras que no se pueden encontrar indicios de este tipo de depósitos relacionados con Blastochaetetes.

Los quetétidos mexicanos también tienen importantes implicaciones paleobiogeográficas y estratigráficas. Las evidencias de Blastochaetetes para el país representan el registro más reciente para el género. Esto podría significar que se dispersó hacia el oeste desde Europa, donde se pueden encontrar los registros más antiguos. Por otro lado, el hallazgo de Acantochaetetes en México en el intervalo Hauteriviano - Barremiano implica su dispersión al Reino del Tethys Occidental antes de su pico de diversidad en Europa durante el Albiano. El hecho de que los depósitos mesozoicos estén muy extendidos en México implica la posibilidad de encontrar múltiples ejemplares en el futuro, lo que profundizaría el conocimiento sobre estos organismos. Además, dado que uno de los géneros estudiados aquí -Acanchochaetetes-es un taxón existente, la posibilidad de encontrar especímenes de quetétidos post-Cretácicos bien conservados para estudios adicionales, es alta.