Functional analysis of the extraordinary shell of Sherbornia mirabilis Iredale, 1917 (Caenogastropoda) and discussion on its current placement within the Pickworthiidae

In 1917 Iredale described Sherbornia mirabilis as a peculiar gastropod from Christmas Island in the Indian Ocean. Its remarkable shell morphology leads to questions on func­tions and adaptations. The greatly expanded apertural shield, three canals and protoconch morphology sug­gest relationships to different genera and families, which prompts the question whether morphological adaptations of this species are evolutionary polyphyletic. Based on shell morphology (studied by scanning electron microscope and micro-tomography) we discuss the placement of this spe­cies and the closely related extinct species Faxia macros­toma. Shell shape, deep sinus in the protoconch, and round aperture support its current allocation within the Cerithi­oidea. Within this superfamily the canals are unique, as are the expanded peristome/apertural shield, thus supporting placement in a separate family Sherborniidae.