Morphological and Taxonomic Examination of Rangea Scheiderhoehni from the Shibantan Member, South China

The Ediacara Biota represents a suite of muliticelluar, complex macroscopic taxa with a wide diversity of morphological traits that have been taxonomically and chronologically divided into three distinct assemblages: Avalon, White Sea, and Nama. The diversity of the Ediacara Biota declines sharply at ~550 Ma during the White Sea-Nama transition, a hypothesized extinction event known as the Kotlin Crisis, with ~20% of genera from the White Sea surviving into the Nama assemblage. Among the survivors is Rangea schiderhoehni, a multifolate, fractal, frondose organism reported from Namibia, Russia, and South China. The limestone-dominated late Ediacaran Shibantan Member of the Dengying Formation in South China is unique among Ediacaran fossil localities, as it has been hypothesized to span the White Sea-Nama transition. Despite extensive quarrying, only a single incomplete specimen of Rangea had been found in the Shibantan Member, until recently. Here, we examine the newly discovered specimens from the Shibantan Member in detail and compare them with better-known Rangea specimens from the siliciclastic White Sea-aged beds of Russia and siliciclastic Nama-aged beds of Namibia. We observe numerous morphological characters (e.g., marginal sheath, subisidary primary branches, and hexagonal multifolate shape) which increase the overall surface area/volume ratio. While other frondose taxa survived the Kotlin Crisis, only Rangea has these unique set of characteristics. We argue that these characters maximized and increased the efficiency of Rangea’s surface/volume ratio, affording it resilience during environmental change while also allowing Rangea to have a large paleoecological range. By examining taxa, such as Rangea, that survived the Kotlin Crisis, greater insight can be gained into what caused this extinction/biotic turnover and how this event had wider implications for metazoan communities.