Transitional Ediacaran–Cambrian small skeletal fossil assemblages from South China and Kazakhstan: Implications for chronostratigraphy and metazoan evolution

•Transitional Ediacaran–Cambrian ​skeletal fauna found in South China and Kazakhstan.•New fauna first occurs in basal Cambrian cherty sediments (Daibu and Aksai members).•Cloudinids (Cloudina and Rajatubulus gen. nov.) persisted into the early Cambrian.•New fauna reveals a less abrupt faunal change at the Pc–C boundary than thought.

The Ediacaran–Cambrian transition records distinct evolutionary changes of metazoans. The Ediacaran fossils (i.e., Ediacara-type biota and tubular fossils) are contrasting with the diverse small skeletal fossils (SSFs) from the early Cambrian. The apparent dissimilarities hindered studies deciphering their evolutionary relationships. This also led to a popular assumption that there exists a mass extinction of the Ediacara biota and cloudinids at the Precambrian–Cambrian (Pc–C) boundary. Here we report for the first time a transitional fauna which consists of typical elements of Ediacaran, i.e. cloudinids and related tubicolous organisms, together with Cambrian SSFs including protoconodonts, anabaritids and siphogonuchitids from South China and Maly Karatau (Kazakhstan). The sediments yielding the transitional fauna are characterized by siliceous rocks in both regions and were previously considered to be unfossiliferous. Their chronostratigraphic assignment in South China has been debated for years. Based on the new fossil assemblage, the siliceous strata of the Daibu Member (Northeast Yunnan, South China) and the basal Kuanchuanpu Formation (South Shaanxi, South China) can be assigned to the earliest Cambrian SSF biozone (Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone) and thus can be considered of early Cambrian in age. A new subzone of the earliest SSF zone in eastern Yunnan (South China) is proposed herein defined as Ganloudina symmetrica–Rugatotheca typica Interval Subzone. The new fauna demonstrates that the cloudinids, originally confined to the late Ediacaran, persisted into the Cambrian Fortunian, and no major extinction event occurred at the Pc–C transition.