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Archaeocyaths, the first metazoan reef builders of the Phanerozoic, faced their ultimate demise during early Cambrian Age 4 but the exact timing of their local extinctions varied globally. In this study, we report archaeocyaths in strata that overlie the last robust archaeocyathan reefs of the western United States (Laurentian Cordillera). These are found in small microbial mounds in the upper unit of the Mule Spring Limestone near Split Mountain, Clayton Ridge, Nevada, as well as in storm beds in the Thimble Limestone Member of the Carrara Formation in Echo Canyon of the Funeral Mountains, Death Valley, California. Thin-section analysis revealed the presence of modular archaeocyaths, with Archaeocyathus being the only genus present. The small microbial mounds of the lowermost upper unit of the Mule Spring Limestone preserve frame-building Archaeocyathus in situ, a few meters above the well-known Bristolia beds of the Mule Spring Limestone. As some of the youngest known archaeocyaths of the western US, these occurrences represent the last gasp of archaeocyaths in the early Cambrian of Laurentia and one of the latest occurrences globally of archaeocyaths. We thus interpret Archaeocyathus in these units as an example of a dead clade walking—some of the last true archaeocyaths that locally persisted into the later Age 4 Cambrian following the disappearance of diverse archaeocyath reefs in western Laurentia. These last archaeocyath communities exhibited low diversity and disparity before the total extirpation of this reef-building hypercalcified sponge and their ecosystem. 

Abstract The terminal Ediacaran Period is signaled worldwide by the first appearance of skeletonizing tubular metazoan fossils, e.g.,CloudinaGerms, 1972 andSinotubulitesChen, Chen, and Qian, 1981. Although recent efforts have focused on evaluating the taxic composition and preservation of such assemblages from the southwestern United States, comparable forms reported in the 1980s from Mexico remain to be re-examined. Here, we reassess the latest Ediacaran skeletal materials from the La Ciénega Formation of the Caborca region in Sonora, Mexico, using a combination of analytical methods: optical microscopy of extracted fossils, thin-section petrography, scanning electron microscopy and energy dispersive X-ray spectroscopy, and X-ray tomographic microscopy. From our examination, we conclude that the La Ciénega hosts a polytaxic assemblage of latest Ediacaran tubular organisms that have been preserved through two taphonomic pathways: coarse silicification and calcareous recrystallization preserving finer details. Further, these fossils show signs that their shells might not have been inflexible or completely mineralized in vivo, and that they might also record tentatively interpreted predation traces in the form of drill holes or puncture marks. This work, along with ongoing efforts around the world, helps to provide a framework for biostratigraphic correlation and possible subdivision of the Ediacaran Period, and further shapes our view of metazoan evolution and ecology in the interval directly preceding the Cambrian explosion.