TUBULAR FOSSILS FROM THE LATE EDIACARAN LA CIÉNEGA FORMATION, SONORA, MEXICO

The terminal Ediacaran Period hosts a diversity of cloudinomorphic and other millimeter-scale tubular fossils, which are thought to be among the earliest biomineralizing metazoans. Selectively to fully silicified cloudinomorph and tubular fossils are preserved as dense polytaxic packstone/wackestone accumulations in a <5-meter interval of cross-bedded dolostone within the terminal Ediacaran La Ciénega Formation in Sonora, Mexico. Samples of this unit were dissolved in dilute acetic acid and the residues were imaged using a scanning electron microscope. Block samples were also volume imaged using x-ray tomographic microscopy. Over 100 extracted specimens of tubular fossils and fragments were examined and sorted into groups by morphology. Specimens which had blunt or flared nested funnels (n = 27 and 26, respectively) are tentatively grouped as two distinct morphologies of Cloudina, and annulated tubes (n = 35) were grouped as probable Sinotubulites. Other specimens were smooth-walled and appear similar to Cambrotubulus, which were grouped into either straight or curved morphologies (n = 20 and 11, respectively). Additionally, there were at least 25 ambiguous samples that we could not assign to any generic group. Specimens grouped as Cloudina- and Sinotubulites-like occasionally demonstrated twisted and compressed structures as well as elliptical cross sections, which we interpret as plastic deformation prior to taphonomic silicification, suggesting that these tubes were likely flexible in vivo.