MICRINA AND TANNUOLINA FROM THE LOWER CAMBRIAN OF EASTERN YUNNAN, SOUTH CHINA

The Tannuolinidae, including two genera, Micrina and Tannuolina, represents a group of Early Cambrian phosphatic skeletal fossils that are characterized by bimembrate components known as mitral and sellate sclerites, basal-internal accretional growth, spaced growth lamellae, carinae (teeth), and non-uniformly distributed pores and canals. The occurrence of Micrina in South China extends the geographic range of this genus, previously known only in Australia. The new material of Tannuolina includes composite sclerites: two left- and right-handed mitral sclerites are juxtaposed along their decrescent sides, and a smaller sellate is ontogenetically merged with the sella of a larger sellate. The decrescent, sella, as well as the duplicature, are also characterized by negative allometry and sparse distribution of pores and canals, perhaps as results of these sides being articulation surfaces. The new fossils indicate that the scleritome of Tannuolina consists of four anterior-posterior files of sclerites: two opposingly oriented mitral rows are flanked by two imbricated sellate series. Other bimembrate mitrosagophorans, such as Micrina and Camenella, may have a similar sclerite organization.

The phylogenetic positions of tannuolinids have not been resolved. It is unlikely, given their differences in sclerite morphology, mineralogy, histology, and orientation, that the tannuolinid mitral and sellate slerites are homologous to the anterior and posterior shells of Halkieria evangelista. Unless it can be demonstrated that the mitral and sellate sclerites are from different animals, tannuolinid sclerites are probably not homologous to brachiopod shells, either; the similarities in biomineralization and skeletal microstructure between the multisclerite tannuolinids and brachiopods are probably due to convergent evolution. The phylogenetic position of the tannuolinids, although most likely in the lophotrochozoan clade, can only be resolved with future discovery of fully articulated scleritomes that can be subject to cladistic analysis.