The Kaili Formation named for nearby city of Kaili in the Guizhou province of southwest China, the Kaili formation is more than 200 meters thick with boundaries dated from the Lower–Middle Cambrian boundary to the early Middle Cambrian (~513 to 506 million years ago). The Kaili Formation subdivided into three trilobite zones: Bathynotus holopygous–Ovatoryctocara granulate Zone, Oryctocephalus indicus Zone, and Olenoides jialaoensis Zone.[1] Ocryctocephalus indicus and Ovatoryctocara granulata, which were suggested to be index species for the GSSP of the Lower–Middle Cambrian boundary, occur at the similar stratigraphic horizon in Siberia and China. FAD of the trilobites Oryctocephalus indicus and Ovatoryctocara granulate are suggested as a two alternative standards of the Lower–Middle Cambrian boundary.[2] Kaili Formation correlated with Amgian Stage in Siberia.
The biggest middle part of the Kaili Formation, Oryctocephalus indicus Zone, contains a Burgess Shale-type Lagerstätte with many well-preserved fossils known collectively as the Kaili Biota. This biota is located between the two most important and famous Cambrian Lagerstätten: the middle Lower Cambrian Maotianshan Shale (containing the Chengjiang Biota) and the Middle Cambrian Burgess Shale.[3][4]
The faunal assemblage is highly diverse, comprising some 110 genera among 11 phyla; of these, some 40 genera are also found in the Burgess Shale, and some 30 are also found in the Maotianshan Shale. Trilobites and eocrinoids with hard parts that are easily preserved are the most common fossils, but many animals with only soft tissues are also preserved. For example, an arthropod similar to the Ediacaran biota Parvancorina of the Neoproterozoic age Ediacara Hills of South Australia has been found at the Kaili site.[5] Some other notable fossils discovered at Kaili are putative invertebrate eggs and embryos[6], trace fossils of the genus Gordia[7], naraoiids, chancellorids, Microdictyon, Wiwaxia, and Marrella.[4]
The depositional environment of the Kaili formation is not entirely known, and there are two hypotheses for its formation. It may have been a nearshore marine environment with 'normal' levels of oxygenation; or it may have been a deeper water environment further from the shore, on the open continental shelf; in this setting oxygen would not be available below the surface layers of the deposited sediment.[7] The trace fossil assemblages in the formation suggest that it was below wave base and reasonably well oxygenated.[7]
|
||||||||||||||||||||||||||