Ichnofaune du Cambrien basal de Normandie (Formation du Rozel) et diversification des premiers metazoaires

The Ediacaran-Cambrian transition (ca. 539 Ma) represents a crucial moment in the history of life on Earth, mainly characterized by the rapid diversification of the first skeletal metazoans and the establishment of modern ecosystems. However, the number of fossil sites remains limited to the sedimentary deposits that have yielded extraordinary fossils (i.e. Cambrian Lagerstätte). This interval is also locally rich in trace fossils preserved in siliciclastic successions, which provide a wealth of information to track the evolution of early behaviours and the diversification of modern animals. Recently, the Rozel Formation in Northwestern Cotentin of Normandy has provided a trace fossil assemblage that was previously poorly documented, but that remains particularly well preserved. This assemblage is composed principally of the ichnogenera Archaeonassa Fenton et Fenton, 1937, Nereites Mac Leay, 1839, Psammichnites Torell, 1870, and Treptichnus Miller, 1889. The aim of our research is to re-evaluate the age of the Rozel Formation, to understand the behavioural evolution of the first metazoans from the trace fossil record throughout the Ediacaran-Fortunian sedimentary deposits of the Armorican Massif. The complexity of the trace fossil record (sophisticated horizontal trace fossils; large and bilobed burrows; three-dimensional burrows) supports that the Rozel Formation assemblage is typical of the Fortunian Stage of the Cambrian Period. This period is characterized by the development of bioturbated sediments in which the bioturbation became more pervasive, penetrative, and drastically diversified in relation to the first appearance of metazoans (i.e. Cambrian Explosion)