New fossil discoveries in the Chilhowee Group (southern Appalachians, USA): evidence for the Ediacaran-Cambrian transition, 'Cambrian Agronomic Revolution', and earliest trilobites at the southern margin of Laurentia

The Chilhowee Group in the southern Appalachian Mountains consists of a rift-to-drift sequence of late Neoproterozoic to early Paleozoic clastic rocks, recording break up of Pannotia and early 
development of the Iapetus Sea. The lower part of the Chilhowee is traditionally considered essentially unfossiliferous – only a few nondescript ichnofossils have been noted in previous stratigraphic studies. After a decade of examining these rocks, we can add significant details to this latest Ediacaran-earliest Cambrian transition in the form of new fossil discoveries. The oldest fossils we observed (probably terminal Ediacaran) are associated with very thin mudstone partings (marine inundation) in thick feldspathic arenite beds (coastal braidplain deposits). These and other recent discoveries of trace fossils at higher levels in the Chilhowee (earliest Cambrian), together with a re-evaluation of previous reports of fossils from the entire succession, allow us to delineate a series of ichnoassemblages associated with certain depositional environments. These co-occurrences of trace fossils appear to represent a series of key events in the Ediacaran-Cambrian transition in the southern Appalachians, including: (1) the earliest marine inundations; (2) the Proterozoic-Phanerozoic transition; (3) early Cambrian (Fortunian) interaction of metazoans with microbial mats in shallow marine environments; (4) evidence of the ‘Cambrian Agronomic Revolution’ in this part of Laurentia (Cambrian Stage 2?); and (5) appearance of the familiar Paleozoic fossil record in the upper part of the Chilhowee, including a nevadiid trilobite and early arthropod traces (probably Cambrian Stage 3). Ichnotaxa identified in this interval include ?Archaeonassa isp., Cruziana cf. C. tenella, Diplocraterion parallelum, Helminthoidichnites-like structures, Palaeophycus tubularis, ?Planolites isp., ?Psammichnites isp., Rosselia isp., Rusophycus cf. R. nabataeica, Syringomorpha isp., ?Torrowangea isp. ‘Treptichnus’ isp., and ?Trichophycus isp. Body fossil discoveries include rare tubular compression body fossils, possibly Tyrasotaenia, and a new nevadiid trilobite.