ИСТИНА

Brachiopods of the order Strophomenida (Rhynchonelliformea) often have two openings in the posterior part of the shell: between valves in the middle of area and apical foramen or tube (pedicle sheath) on the umbo of ventral valve. The latter is generally supposed to accommodate peduncular outgrowth. We studied first representatives of the order Strophomenida from the Ordovician of Leningrad Region with pedicle sheath located within first-formed region of ventral valve and shaped as thin hollow outgrowth with inner diameter of 20-30 µm at the apex. The gap between the valves, which is used as pedicle opening in articulated brachiopods, is absent at juvenile stages and develops later in the ontogeny, that testifies to the presence of well developed pedicle in strophomenid ancestors. According to C. Nielsen, the ancestor of brachiopods folded on ventral side and their dorsal and ventral valves in fact are both dorsal. Therefore the anus of brachiopods initially should be located on ventral side and the pedicle sheath may be interpreted as tubular outgrowth of the ventral valve accommodating anal papilla. The anus of recent brachiopods is never ventrally located. However, possible homologue of pedicle sheath of fossil strophomenids is the pedicle of recent discinids shaped as outgrowth of soft tissues in the aperture within the ventral valve. Natively this outgrowth was anal papilla, but later anus moved to the right side of the body and ventral outgrowth transformed into anchoring organ. We suppose that discinid pedicle is not homologous to the pedicle of lingulids and Rhynchonelliformea.