A new stemmed echinoderm from the Furongian of China and the origin of Glyptocystitida (Blastozoa, Echinodermata)
Résumé
The Furongian (late Cambrian) is an extremely poorly sampled time in the history of echinoderms, with only few localities yielding complete specimens. Here, we document an exquisitely preserved stemmed echinoderm from the Furongian Sandu Formation in South China that provides important new data illuminating the origin of Glyptocystitida, a common Palaeozoic clade of echinoderms. Sanducystis sinensis n. gen. n. sp. displays an organized theca bearing three circlets of plates (basal, infralateral and lateral), a laterally positioned periproct in the CD interray, a lack of respiratory pectinirhombs and a stem divided in two parts, with expanded inner and outer columnals proximally and narrow, elongate, homeomorphic columnals distally. A phylogenetic analysis places Sanducystis more derived than the columnal-bearing eocrinoid' Ridersia and sister group of a clade encompassing Macrocystella-like glyptocystitoid rhombiferans from the Furongian onwards. By filling in an important morphological gap, Sanducystis provides a clear understanding of character evolution within Glyptocystitida.