Presumed chaetetids in Smithian (early Olenekian, Early Triassic) microbial-sponge limestones, Rock Canyon, Arizona, USA

Outcrops in Rock Canyon (northwestern Arizona) show a Smithian (early Olenekian, Early Triassic) sedimentary succession recording a marine transgression over fluvial conglomerates and leading to several metre-thick microbial limestones. These microbial limestones are associated with a diverse sponge assemblage including keratose demosponges, halichondrids, geodiids, lyssacine hexactinellids, and presumed chaetetids, suggesting that Early Triassic sponge communities were more diverse than previously thought. A presumed chaetetid (hypercalcified demosponge) specimen, identified by its lamellar 3D structure, shows slightly raised, subcircular domal structures formed by narrow, joining tubes arranged in a bush-like pattern. Microscopic observations also reveal residual tabula and sparse spicules. Other presumed chaetetid specimens show intense recrystallization that obliterates most of their characteristic skeletal structure. Although chaetetid sponges are well-documented in the Palaeozoic, they were absent until now from the Late Permian to Late Triassic record. Occurring about 2.5 myr after the Permian-Triassic extinction, the specimens from Rock Canyon would be the first Early Triassic chaetetids and the oldest known in the Mesozoic record. The rarity of Early Triassic chaetetids may be due to sampling and taphonomic biases. Their intense recrystallization and elongated shape exhibit a morphological convergence with large fenestrae commonly found in microbial limestones. During the Early Triassic, buildups composed of microbial limestones and sponges were widespread, occuring in the Panthalassa, Neotethys and Palaeotethys oceans across various depositional settings, from saline lagoons to open marine environments of varying depths. This broad distribution questions the view that these microbial-sponge communities were confined to localized refuge settings or stressful environmental conditions in the aftermath of the Permian-Triassic crisis.