New insights into the early evolution of horizontal spiral trace fossils and the age of the Brioverian series (Ediacaran-Cambrian) in Brittany, NW France

In northwestern France, the Brioverian series is a thick siliciclastic succession deposited during the Cadomian cycle (c. 750-540 Ma). In the uppermost Brioverian beds, previous studies unravelled an assemblage dominated by simple horizontal trace fossils associated with microbially stabilized surfaces. Here, we report Spirodesmos trace fossils - one-way, irregular and regular horizontal spirals - from Crozon (Finistère, Brittany), Montfort-sur-Meu and St-Gonlay (Ille-et-Vilaine, Brittany). After reviewing the literature on horizontal spiral trace fossils, an Ediacaran-Fortunian Spirodesmos pool is identified from marginal-marine to shelf settings, while an Ordovician-Recent trend formed in the deep-marine realm. These results suggest that an onshore-offshore migration in Spirodesmos took place during Ediacaran-Fortunian to Ordovician time, similar to what happened in graphoglyptids. In addition, the age of the uppermost Brioverian beds (Ediacaran or early Cambrian) is still a pending question. Here, we report two new U-Pb detrital zircon datings from sandstone samples in St-Gonlay, giving maximum deposition ages of 551 ± 7 Ma and 540 ± 5 Ma. Although these results do not discard an Ediacaran age for the uppermost Brioverian beds, a Fortunian age is envisioned because the new dating corroborates previous dating from Brittany, Mayenne and Normandy. However, the intervals of error of the radiometric dating, and the dominance of non-penetrative trace fossils associated with matgrounds (an ecology more typical of the Ediacaran Period), do not allow definitive conclusions on the age of the uppermost Brioverian beds.