Reassessing Early Triassic wrinkle structures from moderate-high latitudes: An updated interpretation of metazoan colonization in matground ecosystems after the Permian-Triassic mass extinction

Abstract

Wrinkle structures have occurred on our planet for 3.2 billion years. They, therefore, if proved to be biogenic origin, could represent one of oldest life forms on Earth. Such distinct microbial mats were commonly present in the Precambrian, and were also widespread in shallow marine siliciclastic settings in the Cambrian and aftermath of major Phanerozoic mass extinctions, but their interactions with metazoans remain varied. Here, I reassess abundant wrinkle structures which were documented previously from the Lower Triassic successions of two moderate-high latitudinal localities, the southern Qilian basin of west China, and the northern Perth Basin of Western Australia in both northern and southern hemispheres. Wrinkle structures commonly occurred at the interface between a fine sandstone bed and an overlying siltstone or mudstone bed. Both burrowing and shelly benthos were sporadically distributed in wrinkle structures, contrasting with the biomass in non-wrinkled layers. Although wrinkle structures may be preserved in well‑oxygenated habitable habitats together with diverse benthos, they proliferated during anoxic transient periods or when storm-influenced sediments were deposited. The co-occurrence of wrinkle structures with trace fossils and benthos such as bivalves reflects an animal-sediment interaction, instead of an animal-mat interaction, and caution therefore should be made when documenting the interactions between microbes and metazoans in fossil records. Grazing activities by both infaunal deposit-feeders and bivalves were absent on the wrinkled surfaces, while horizontal burrows and infaunal bivalves cross-cut and disrupted the wrinkled surfaces, implying that trace fossils post-dated the formation of the wrinkle structures, and the organisms mainly exploited in buried microbial mats. The distribution of trace fossils and benthos on wrinkled surfaces from these two studied localities do not support the concept of microbial refugia or the preferential preservational window hypotheses. MISS including wrinkle structures and microbialites have been documented from both low- and high-latitude regions, and their relatively sparse occurrences in moderate- to high-paleolatitudes regions may be due to sampling bias. The proliferation of microbes such as cyanobacteria in the aftermath of the mass extinction was not related to selectivity of climate zones and seawater temperature in shallow marine settings.