Discoidal carbonaceous compressions from the early Mesoproterozoic Gaoyuzhuang Formation: a fossil example of microbial biofilm

Abstract

Discoidal carbonaceous compressions are the most common type of Precambrian macrofossil, with a long temporal range starting from the late Paleoproterozoic. However, their unsolved biological nature restricts our understanding of the early evolution of macroscopic life. Here, we report an assemblage of well-preserved discoidal carbonaceous macrofossils from the early Mesoproterozoic Gaoyuzhuang Formation in North China, which provides insights into this problem. They are preserved in round to elliptical shapes ranging in size from millimetres to several centimetres. Petrographic thin sections show that the macrofossils consist of laminated structures with alternating laminae of organic matter, along with clay minerals and dolomites. Neither cellular structures nor individual microfossils were identified within them, but their regular shape, internal structures and associated mineral constituents suggest that they are probably the remains of microbial biofilms, rather than multicellular organisms. This assemblage presents a well-preserved fossil example of macroscopic microbial biofilms with a regular overall morphology. It implies a possible origin of microbial biofilms for some of the early carbonaceous macrofossils and calls for a detailed re-examination of those macrofossils to exclude such a possibility. Our findings are consistent with previous studies that showed biofilms may have had an important role in the survival of early microorganisms in the Precambrian ecosystem. Thematic collection: This article is part of the The North China Craton as a window to Earth’s middle age collection available at: https://www.lyellcollection.org/topic/collections/the-north-china-craton-as-a-window-to-earths-middle-age