Micro-CT reveals 3D endosiphuncular structure in Late Ordovician actinoceratid cephalopod from the Prague Basin (Czech Republic)

The first unequivocal actinoceratid cephalopod from the late Katian (Late Ordovician) shales of the Prague Basin (Central Bohemia), Adamsoceras? hanusi sp. nov., is described. The only specimen contains several fragmented but undeformed siphuncular segments. The shell wall, septa, ectosiphuncle and annulosiphonate deposits are dissolved but their original shape is reflected by cavities left after them inside the specimen. The endosiphuncular canal system inside the siphuncle is preserved, including the central canal, radial canals and perispatium. The shape of the empty spaces and structures inside the siphuncle indicate that the growth of the annulosiphonate deposits was irregular and uneven; the deposits pushed back the original soft tissue inside the siphuncle and the present irregular structure of the endosiphuncular system was thus formed. The preservation of the central and radial canals additionally shows that the walls of the individual blood vessels running through the canals must have been soft, elastic, and probably had a higher preservation potential than the rest of the soft tissue. Consequently, the shape of the endosiphuncular system is different in each median section and each segment, and assignment to a particular system type is thus not possible. We conclude that definition of taxa based on previously established types of endosiphuncular systems might be artificial in at least some actinoceratids.