Discovery of ventrally directed spiralia in a Permian spiriferellid brachiopod and implications for its feeding system

Spiralia are lophophore-supporting, coiled internal structures developed in some extinct brachiopods. In spite of considerable variations in their orientation, the spiralia of most spiriferide and spiriferinide taxa are known to be laterally directed. Recent studies have shown that these brachiopods consistently have a median inhalant and lateral exhalant feeding system. Here, we report a Permian spiriferellid brachiopod fossil (Spiriferella protodraschei) bearing ventrally directed spiralia in its interior. Using the serial sections of the specimen, we have reconstructed the detailed morphology and orientation of the spiralia. Each spiralium in the specimen does not show the apically tapering pattern supposedly universal in all the known types of spiralia: instead it maintains a similar diameter even at its last whorl. The spiralia appear to have directly developed from strong and anteriorly extended crura, consisting of ten whorls in one side and 13 whorls in the other side. As the morphology and orientation of spiralia are immediately associated with the arrangement of spirolophous lophophore within the mantle cavity, the extraordinary orientation and form of the spiralia indicate that this brachiopod likely had developed a considerably modified feeding pattern with respect to most other spirolophous brachiopods. It is postulated that the inhalant/exhalant current circulation of the species (and its descendants) would be considerably different from that of other spiriferide taxa. In particular, the combination of the vertically oriented life posture (free-lying with thickened ventral apex bottom) and ventrally directed spiralia resembles both fossil atrypide and modern rhynchonellide brachiopods in the orientation of spirolophe, suggesting that some spiriferellid brachiopods may have developed a lateral inhalant/median exhalant feeding current system. A few spiriferide and spiriferinide brachiopod taxa with a weakly transverse but strongly convex ventral valve are noted to exhibit similar modifications in their spiralia, possibly due to the spatial limitation of their mantle cavities.