EVALUATING THE DIVERSITY, AFFINITY, AND CONSTRUCTION OF TERMINAL EDIACARAN TUBIFORM FOSSILS FROM THE LA CIÉNEGA FORMATION, SONORA, MEXICO

The terminal Ediacaran Period is marked by the groundbreaking appearance of shelly metazoan fossils, belonging to such tube-dwelling taxa as Cloudina and Sinotubulites. While abundant and cosmopolitan, making sense of the phylogenetic relationships of these organisms and what their ecosystems looked like remain important and largely open questions for understanding early animal evolutionary patterns. Here, we reevaluate shelly fossils that were first reported in the 1980s from the La Ciénega Formation of the Caborca region in Sonora, Mexico, in an effort to compare them to other reinvigorated and similar fossil deposits in the southwest US. Using a wide range of approaches applied to fossiliferous hand samples and extracted fossils, including standard microscopy, thin-section preparation, and electron and x-ray microscopies, we find that these Mexican skeletons: (1) represent multiple distinct groups of animals; (2) are preserved in different ways, both coarsely silicified and with finer-scale calcareous preservation; (3) show signs of early deformation, indicating that their shells might not have been overly rigid or inflexible; and (4) show tentative indications that predators may have punctured or drilled into their shells. This work, along with ongoing efforts around the world, help to provide a framework for biostratigraphic correlation and possible subdivision of the Ediacaran Period—further shaping our view of metazoan evolution and ecology in the interval directly preceding the Cambrian explosion.