Evolution and palaeoenvironmental migration of a Cambrian undermat miner

Abstract

Microbial mats sealed the sea bottom, becoming the food supply of many early metazoans, which exploited this resource in multiple ways during the late Ediacaran and earliest Cambrian. The onset of highly penetrative bioturbation in shallow-marine environments later in the early Cambrian removed this seal, resulting in the protracted restriction of microbial mats to marginal- and deep-marine environments. In this study, we demonstrate that the undermat-miner trace fossil Oldhamia followed the pattern of matground restriction to deep-marine environments. This is shown by a dataset compilation of Cambrian Oldhamia based on a comprehensive review of the literature with addition of recently discovered Oldhamia in the Cambrian strata of North and South China. Oldhamia is one of the most distinctive and widely distributed ichnogenera in early to middle Cambrian siliciclastic successions. It consists of semi-permanent, very shallow-tier, undermat-miner structures produced by stationary vermiform organisms. Despite its importance, details of Oldhamia ichnodiversity trajectories at ichnospecies level, environmental trends, and potential palaeogeographic patterns have not been explored. Our results demonstrate that Oldhamia displays high diversification in Fortunian shallow- and deep-marine settings, representing a signal of the Cambrian Information Revolution. Oldhamia alata and O. geniculata only occurred in shallow-marine settings, O. curvata and O. flabellata were restricted to the deep sea, and O. antiqua and O. radiata ranged from shallow- to deep-marine settings. The producers of Oldhamia retreated into deep sea since the end of the Fortunian mirroring the restriction of matgrounds as a response of intense bioturbation in shallow-marine deposits during the Agronomic Revolution. The producers of Oldhamia originated from the Iapetus Ocean and then migrated into other oceans, most likely through dispersal assisted by ocean currents.