Youngest ambient inclusion trails from Middle Triassic phosphatized coprolites, southwestern China: new insights into an old intriguing phenomenon

Copious microtubular structures are reported, for the first time, from the coprolites of early Middle Triassic Guanling Formation in Luoping, Yunnan Province, southwestern China. Multiple salient characteristics of these microtubes, including curved to coiled grooves with longitudinal striations, angular turns, cross-cuttings, and a terminal rounded pitting at one end of a groove, support their strong affinities with ambient inclusion trails (AITs) that are more commonly found in pre-Ediacaran sedimentary rocks. These newly found microtubular structures are here interpreted to represent the youngest occurrence of AITs in the geological record. A review of all AITs and similar microtubular structures reveals their common occurrences in Ediacaran phosphorite and Phanerozoic phosphatized fossil substrate. Similar to the AITs found from the post-Ediacaran strata, the AITs from the Luoping coprolites are also preserved in a phosphatized substrate. We suggest that a microbial-induced process might have contributed to the phosphogenesis of faeces and AITs formation. This process is interpreted to have involved the precipitation of phosphate minerals by microbial communities, and the movement of pyrite through and within phosphatized substrate gel driven by fluid pressure. This new interpretive model differs from the existing scenario invoked to explain the formation mechanism of pre-Ediacaran AITs, in which pyrite propulsion is thought to be more likely driven by metamorphism-induced fluid pressure. We thus hypothesize that the transformation in AITs' formative substrate, from chert to phosphate, as well as their formation mechanism, might have been triggered by a significant change in ocean biogeochemistry linked to a phosphogenic event in the Neoproterozoic.