The proximal development and thecal structure of the Ordovician graptolites Tylograptus and Sinograptus

A study Of pyritic and three-dimensional specimens of sinograptids from the early Middle Ordovician Ningkuo Formation of western Zhejiang Province, south-east China, indicates that the proximal development of both Tylograptus and Sinograptus is of modified isograptid type. The thecal apertures of the two genera are reconstructed and display a complex morphology more typical of stratigraphically younger graptoloids. Because of differences in its prothecal folds and thecal apertures Tylograptus is distinguished from Holmograptus and regarded as a valid genus. In Sinograptus, a tongue-shaped structure, and constriction of the thecal tube after the metathecal fold, are illustrated for the first time. The proximal development and thecal structures of Tylograptus and Sinograptus imply a possible origin of sinograptids from earlier didymograptids or sigmagraptids. Four species, Tylograptus intermedius, T. globiformis, Sinograptus typicalis and S. rastritoides, are revised.