The Middle Ordovician tergomyan mollusc Pygmaeoconus: an obligatory epibiont on hyolithids
Radvan J. Horný
The Middle Ordovician Pygmaeoconus porrectus (BARRANDE in PERNER, 1903) is described as the first known molluscan obligatory epibiont on hyolithids. Its biology is similar to that of the coral Hyostragulum and some trepostome bryozoans described by Galle and Parsley (2005). The antero-posterior axis of its cap-shaped shell in the life position was aligned with the longitudinal axis of the hyolithid conch, faced with the anterior side against the current for food and oxygen. Internal moulds bear three to four periodically repeating circular sets of six pairs of discrete pedal muscle scars that relocated by saltation and a pair of cephalic scars. Pygmaeoconus is considered a tergomyan mollusc of the family Kirengellidae, subfamily Pygmaeoconinae subfam. n.