Annuliconchus siluricus Eichwald, 1860, a microconchid worm from the Silurian of Gotland.
SEM photograph of the laminar shell ultrastructure of a microconchid from the Silurian of Gotland.
The spiral shells cemented to the various substrates similarly to spirorbids (serpulid polychaetes) are very common in the Late Palaeozoic (Weedon 1990, Dreesen & Jux 1995). In Treatise (1962) the genus Spirorbis is mentioned to appear in Ordovician marking the earliest record of spirorbid polychaetes. However the Devonian and Lower Carboniferous spirorbids were later reinterpreted as the vermiform ‘gastropods’, and more recently assigned to new order Microconchida, Weedon 1991 of class Tentaculitoidea, Bouček 1964 (Weedon. 1991). They are completely dissimilar in fabric from the microstructure of recent spirorbids and so easy to identify (Taylor & Vinn 2006). The microconchids are reported from variety of rocks ranging from Upper Ordovician to Middle Jurassic (Taylor & Vinn 2006, Vinn 2006). Microconchids occupy a similar niche in the Upper Palaeozoic to that of the oyster/serpulid association of the Mesozoic and Tertiary (Burchette & Riding 1977). All Palaeozoic 'spirorbids' are the microconchids and not the serpulid polychaetes (Taylor & Vinn 2006). The microconchid worms are very commonly encrusting the various fossils in the Upper Ordovician and Silurian of Baltoscandia (Vinn 2006). They appear in the Vormsi Stage in Estonia. Lindström (1884) described a form from Gotland as Autodeteus (=Anticalyptraea) calyptrata Schrenk and referred it to phorid gastropods, but more likely it is representing a microconchid.
Burchette, T.P. & Riding, R. 1977. Attached vermiform gastropods in Carboniferous marginal marine stromatolites and biostromes. Lethaia 10, 17-28.
Dreesen, R. & Jux, U. (1995): Microconchid buildups from Late Famennian peritidal lagoonal settings (Evieux Formation, Ourthe Valley, Belgium). - N. Jb. Geol. Paläont. Abh., 198 (1/2), 107-121, 5 Abb.; Stuttgart.
Lindström, G. 1884. On the Silurian Gastropoda and Pteropoda of Gotland. K.Sven. Vetenskaps-Akad. Handl. 19, 1-250.
Taylor, P. D. & Vinn, O. 2006. Convergent morphology in small spiral worm tubes ('Spirorbis') and its palaeoenvironmental implications. Journal of the Geological Society, London 163, 225-228.
Vinn, O. 2006. Two new microconchid (Tentaculita Bouček 1964) genera from the Early Palaeozoic of Baltoscandia and England. Neues Jahrbuch für Geologie und Paläontologie 2006 (2), 89-100.
Weedon, M. J. 1990. Shell structure and affinity of vermiform ‘gastroppods’. Lethaia 23, 297-309.
Weedon, M. J. 1991. Microstructure and affinity of the enigmatic Devonian tubular fossil Trypanipora. Lethaia 24, 227-234.