Ambient inclusion trails: Their recognition, age range and applicability to early life on earth

Abstract

Ambient inclusion trails (AITs) are a distinct class of microtubular structure, first reported almost half a century ago from Precambrian rocks. Classification of a structure as an AIT implies formation by means of migration of a mineral crystal through a lithified substrate. Historically, AITs have been problematical in two ways. Firstly, they have caused confusion in studies of early life because they are similar, morphologically, to both filamentous microfossils and microborings made by endolithic bacteria. Secondly, their formation mechanism has not been rigorously studied, so there has been much debate about whether AITs are purely an inorganic phenomena or whether a biological component is necessary. In this paper we review all reported examples of AITs, documenting the criteria necessary for their recognition, their taphonomic preservation in the rock record, their differentiation from modern contamination, and recent advances made in the Pilbara Craton of Western Australia which appear to support a biogenic formation mechanism for at least some AITs.We also report what we believe are the youngest examples of AITs in Phanerozoic rocks. These come from the Middle Devonian of Northern Scotland. Their excellent preservation within phosphatic fish scales not only gives further insights into AIT formation, it extends their known range forward by almost 200. Ma. Clearly, AITs were not just a Precambrian phenomenon.