Systematics, shell structure and affinities of the Palaeozoic Problematicum Cornulites

The genus Cornulites, with the type species C. serpularius Schlotheim, 1820, from the Silurian of Gotland, comprises annulated, conical or tubular calcite shells, often found attached to the hard parts of other organisms. No consensus has ever been reached over the zoological affinities of the taxon, and no examples of soft-part preservation are known: detailed examination of shell structures and growth patterns provide the only means of assessing its systematic position. Using transverse and longitudinal thin sections of C. serpularius Vine, 1882, and C. cellulosus sp. nov., from the Much Wenlock Limestone Formation of England, the shell structure of Cornulites is shown to be lamellar, but with conspicuous internal chambers (camerae) at the apical end of the shell and, particularly in C. cellulosus, numerous smaller vacuities (cellulae) between the lamellae in the apertural shell region. Growth of the shell was by the secretion of low-magnesian calcite increments within one another, giving a cone-in-cone structure, with the prominent development of cellulae in C. cellulosus probably a constructional feature relating to an upright life position. By comparison of morphology and shell structure with other taxa, the zoological affinities of Cornulites are re-examined; previously suggested affinities with annelids, foraminifers, molluscs and poriferans can be ruled out. Specific shell structures, most notably pseudopuncta similar to those of bryozoans and brachiopods, have led some recent workers to interpret cornulitids as lophophorates. However, it is shown that they can be interpreted alternatively as solitary, aseptate members of the stem-Zoantharia (Cnidaria: Anthozoa). Four cornulitid species are recognized in the Much Wenlock Limestone Formation: C. cellulosus sp. nov., C. gremialis sp. nov., C. scalariformis and C. serpularius. In the absence of the type material, C. serpularius is here restricted to cornulitids closely resembling the specimens originally figured by Schlotheim.