New offset-associated structures in some Carboniferous rugose corals

Abstract

Tubular structures termed ducts, which extend from the base of offsets axially and upwards through the disscpimental tissue of the parent corallite, are described from some Carboniferous rugose corals. Two ducts are usually associated with each offset, one from each extremity where the offset wall meets that of the parent. The ducts may merge to form a tangentially elongate slit at higher levels in the disscpimcntarium of the parent. The structures served to prolong gastric and nervous communication between parent and daughter polpys following budding. Ducts and slits are well developed in cerioid Lonsdaleia but are absent in fasciculate Lonsdaleia, a distinction lending credence to the recognition of the former group of species as a distinct genus, Actinocyathas, within the Axophyllidac. In addition, less well developed ducts are present in at least some cerioid species of Lithostrotion, whereas they are absent from fasciculate species, supporting separation of the latter from Lithostrotion sensu stricto and their assignment to the genus Siphonodendron. The form and distribution of duets also confirms polyphyly in Thysanophyllum. The separation of T. pseudovermiculare (as Dorlodotia), without ducts, from Thysanophyllum sensu stricto, with weak ducts similar to those in some Lithostrotion spp., is supported. Both arc assigned to the T. pmedictum has well developed ducts, however, which reinforces other evidence suggesting its removal from Thysanophyllum and classification in the Axophyllidac in close affinity with Actinocyathus, Rugosa, offsets, ducts, functional morphology, taxonomy, Carboniferous.