In the last ten million years before the Cambrian Explosion, groundbreaking early animals started to make shells. Fossils of these shells can be found worldwide, but making sense of how these animals might have been related and what their ecosystems looked like remain important questions. Here, we reevaluate shelly fossils that were first reported in the 1980s from northwestern Mexico in an effort to compare them to other reinvigorated and similar fossil deposits in the southwestern USA. Using a wide range of approaches, including standard microscopy, thin-section preparation, and electron and x-ray microscopy, we found that these Mexican shells: (1) represent multiple distinct groups of animals; (2) are preserved in a couple of different ways; (3) show signs that their shells might not have been overly rigid or inflexible; and (4) might also show indications that predators had punctured or drilled into their shells. Along with ongoing efforts elsewhere around the world, studies of the fossil record just before life's first large diversification event will help to provide a framework for global correlation and illuminate the earliest evolutionary and ecological dynamics of animals.

The terminal Ediacaran Period is signaled worldwide by the first appearance of skeletonizing tubular metazoan fossils, e.g., Cloudina Germs, 1972 and Sinotubulites Chen, 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.