Biological decay by microorganisms in stems from the Upper Triassic Ischigualasto Formation (San Juan Province, Argentina): A striking microbial diversity in Carnian-Norian terrestrial ecosystems

We studied degradation patterns and microorganisms preserved in permineralized stems of Rhexoxylon piatnitzky, Protojuniperoxylon ischigualastense, and Agathoxylon argentinum from the Valle de la Luna Member of the Ischigualasto Formation (Upper Triassic, San Juan, Argentina). All three species show loss of middle lamella, thinning, and whitening of tracheid cell walls, and detachment of the S3 layer, consistent with selective delignification by white rot. This type of rot is the product of lignin and cellulosic degradation by Basidiomycetes and Ascomycetes. Rhexoxylon piatnitzkyi also shows small cavities, more or less circular in outline, on the wall of tracheids. This is known as soft rot and can be produced by some Ascomycetes, the anamorphic phases of diverse fungi, and tunneling bacteria. Additionally, abundant and diverse microorganisms were found. Rhexoxylon piatnitzkyi shows fragments of septate hyphae, evidence of bacterial activity, and structures of unknown affinity. Protojuniperoxylon ischigualastense presents structures common to anamorphic phases of diverse fungi, such as chlamydospores and conidiophores. Lastly, Agathoxylon argentinum bears coiled and septate hyphae, and a fan-shaped mycelium, whose significance is discussed. The microbiological study was complemented by the assessment of sedimentary and taphonomic data of the fossil-bearing beds. A gradual increment in humidity for the Valle de la Luna Member is interpreted, which concludes with a humidity peak in its upper section. This contribution represents the first detailed study of xylophagous and saprophyte micro-communities for the Ischigualasto Formation. The abundance and diversity of microorganisms described illustrate the complexity of wood-inhabiting micro-communities from the Carnian-Norian interval of Argentina.