Diversity, taphonomy and palaeoecology in the Ediacaran of Newfoundland

Pre-Cambrian outcrops in the Bonavista and Avalon peninsulas of Newfoundland (CA) record a diverse fossiliferous assemblage, dominated by macroscopic organisms presenting extinct Bauplans. In particular, frondose organisms such as the Rangeomorpha and the Arboreomorpha represent the most diverse groups and are typically composed by a number of self-repeating branch orders and sometimes a stalk or a stem. Despite traces of early metazoan life being present, the position of the Ediacaran groups in the tree of life is not always well defined. Moreover, their taxonomy and taphonomy – and in turn their palaeobiology and palaeoecology – are poorly understood. In this work we describe three taxa, two rangeomorphs (Culmofrons plumosa and Charnia ewinoni gen. et sp. nov.) and one problematic metazoan (Lydonia jiggamintia gen. et sp. nov.), with a comprehensive overview of their phylogenetic position, their taphonomy, their auto-ecology and population dynamics. Morphometric analyses allow us to formally separate the three species from other taxa, and to erect two new species endemic to Newfoundland (C. ewinoni and L. jiggamintia). For the two rangeomorphs species, we find evidence for a reclining lifestyle and symbiotic relationships with the microbiota of the underlying sediments, supported by taphonomical evidence and palaeonvironmental considerations, in contrast with traditional reconstructions that would put the rangeomorphs erect in the water column. The exceptional preservational quality of the Bonavista material further allow us to identify never described before reproductive structures, as well as to further our understanding of rangeomorphs developmental models. Specimens from the Bonavista peninsula, previously identified as the pseudofossil Blackbrookia, are here reassigned to Lydonia jiggamintia gen. et sp. nov., on the basis of morphometrical analyses and of structures indicating the presence of a metazoan-grade aquiferous system. The analyses of the specimens of a single fossiliferous surface allow us to propose population and growth dynamics models which are consistent with poriferan-grade organisms. The resulting picture of the Avalon Assemblage of Newfoundland is that of a dynamic environment, with a large diversity and a complex trophic net, involving micro- and macroscopic organisms. Moreover, we suggest that the presence of metazoans in the Ediacaran is largely underestimated in the literature, potentially due to a taphonomical bias.