Article: A sinus-bearing monoplacophoran and its role in the classification of primitive molluscs
The discovery of symmetrically disposed multiple muscle scars in Bellerophon acutilira Hall, a middle Devonian form assigned to the bellerophontoid genus Sinuitopsis Perner, indicates that Sinuitopsis is a cyclo-myan monoplacophoran. Deployment of the muscle scars shows affinity with the cyclomyan genera Yochelso-nellis (Horny) and Cyrtolites Conrad. A ridge-furrow complex is interpreted as a reflection on the inside of the shell of a dorsally situated rectal tract. The presence of a deep anal sinus and notch in S. acutilira demands a reconsideration of monoplacophoran-gastropod relationships. Such a sinus or slit can no longer be always considered anterior, and thus, evidence of torsion in fossil archeogastropods. Quite possibly sinus-bearing cyclomyans were the direct ancestors of the primitive archeogastropods (i.e. the Bellerophontina). The use of the sinus as a tool for the recognition of bellerophontids is discouraged in favour of parietal deposits and posterior trains. A posterior anal sinus was probably advantageous in the achievement of maximum separation of respiratory currents and excretory products.