Palaeoecology and Depositional Environment of the Extraordinary Val d’Ambria Fossil Locality (Southern Alps, Italy) and the Effects of Global Warming on Kungurian Continental Settings

The continental basins from the early Permian of the Southern Alps are renown for the quantity and quality of vertebrate and invertebrate ichnofossils, as well as of plant remains. Also, sporomorphs and invertebrates were found. The availability of radioisotopic datings provides robust age constraints on the fossil associations, so that they are a reference for studies on biostratigraphy and climate-related ecosystem changes. The recently discovered Val d’Ambria fossil locality (Valtellina, N Italy), yielded so far the most diverse tetrapod ichnoassociation of the Southern Alps and one of the best in the world during the Permian. It belongs to the Pizzo del Diavolo Formation of the Orobic Basin, dated as Kungurian. The tetrapod footprint association is diverse and includes 10 ichnogenera: Batrachichnus and Limnopus (temnospondyls or lepospondyls), Amphisauropus (seymouriamorphs), Dimetropus (sphenacodonts or caseids), Tambachichnium (varanopids), Hyloidichnus (captorhinomorphs), Varanopus (bolosaurians), Erpetopus (nyctiphruretids or acleisthorinids), Pachypes (nycteroleters) and Dromopus (araeoscelids or varanopids). The preservation quality of the icnofauna is remarkable, trackways are common and in some cases scaly skin is observed, both on footpints and on a body impression. Moreover, tetrapod swimming traces were found (Characichnos). Also, sets of converging scratch traces prove the existence of tetrapod burrows. The invertebrate ichnoassociation is less diverse and dominated by superficial traces, especially arthropod tracks such as Diplichnites, Lithographusand Stiaria. The plant fossil assemblage includes conifer shoot fragments, impressions of arboreous fern stems as well as seeds. Overall, the facies associations include six different lithofacies, defined according to the sediment grain size, mineralogical composition, sedimentary structures and stratigraphic position; FPDa: massive or weakly stratified conglomerates; FPDb: massive sandstones and sandstones with flow structures; FPDc: laminated pelites alternated with sandstones; FPDd: lacustrine carbonates and siltstones; FPDe: reddish sandstone and siltstone; FPDf: volcanic sandstones with tuffs and cinerites. The depositional environment can be interpreted as an ephemeral lake in the basin depocentre, bordered by mudflat and sandflat areas, and surrounded by basement reliefs generating alluvial fans that fed the plain with coarse- to-fine sediments. The presence of a vast mudflat area in the centre, characterised by flooding and desiccation, very likely reflects the conditions typical of endoreic basins in semi-arid climates. The palaeoecosystem is dominated by footprints of stem reptiles (Erpetopus, Dromopus), and amniote footprints are by far more abundant and diverse than anamniote footprints. The high occurrence of drought-adapted forms is consistent with seasonality and the absence of perennial water bodies. The predominance of superficial traces is consistent with frequent emersion and submersion of the sediment. The occurrence of tetrapod burrows is coherent with seasonal conditions. This assemblage is peculiar since it represents the first stem impressions of ferns so far described from the Permian of the Alps. The mixture of drought-resistant conifers and fern stem impressions supports a wet and dry climate. All these considerations point to a very seasonal palaeoenvironments with ephemeral water bodies, consistent with the global warming trend observed in the early Permian and culminated with the Artinskian Warming Event and the establishment of greenhouse conditions that characterised the Kungurian.