Microconchida Weedon, 1991

Wilson, Mark A., Vinn, Olev & Yancey, Thomas E., 2011, A new microconchid tubeworm from the Artinskian (Lower Permian) of central Texas, USA, Acta Palaeontologica Polonica 56 (4), pp. 785-791 : 786-787

Order Microconchida Weedon, 1991 Genus Helicoconchus nov.

Type species: Helicoconchus elongatus sp. nov.; by monotypy, see below.

Etymology: Combination of helico, spiral and conch, tubicolous shell.

Diagnosis.—Small calcitic tube with planispiral, dextrallycoiling attachment surface, tube diameter increasing rapidly; after one to two whorls the tube becomes erect, helical, very long and its diameter remains relatively constant. Well−developed umbilicus absent in planispiral portion of juvenile attached tube. Tube wall microlamellar with no punctae or pseudopunctae. Tube interior with diaphragms, many of which have central pits; spacing of diaphragms changes with growth from infrequent to an average of one every two mm in erect portion of tubes; tube interior walls smooth. Tube exterior with very fine growth lines. Tubes show lateral budding and binary fission budding with the interior connections between parent and daughter tubes apparently repaired by secretion of new wall. Gregarious habits, some forming small reefs up to two meters in diameter of radial, tightly packed, branching tubes and others remaining as isolated tubes.

Discussion.— Helicoconchus , like all microconchids, can be distinguished from serpulid and spirorbid polychaetes by its microlamellar shell structure and the closed proximal end of the tube ( Vinn and Mutvei 2009). This new genus differs from Punctaconchus Vinn and Taylor, 2007 ; Microconchus Murchison, 1839 ; and Pseudobrachidium Grupe, 1907; by its lack of punctae, budding origin of daughter tubes, and greatly extended late growth erect tube. Species of Microconchus with an erect helical adult tube part, such as M. advena ( Salter, 1863) (Late Devonian–Carboniferous) and M. aberrans ( Hohenstein, 1913) (Middle Triassic) , are normally less extended than the tubes of new genus. Helicoconchus also lacks the annulated shell of Annuliconchus Vinn, 2006 , and the pseudopunctae of Palaeoconchus Vinn, 2006 . These latter two microconchid genera also do not have the extended, budding helical tube that distinguishes Helicoconchus .

Helicoconchus aggregations superficially resemble the “serpulid” Serpula helicalis Beus, 1980 , found in the Upper Devonian (Frasnian) of Arizona, USA, with their narrow helical tubes and distinct exterior growth lines. S. helicalis , however, appears to have no internal features, no known budding, nor a planispiral attachment surface. The only known specimens are silicified, so its shell microstructure remains unknown. The “tabulate coral” Spirapora Copper, 1981 , of the Upper Ordovician (Ashgill) of Ontario, Canada, looks even more like Helicoconchus as a colony of helical, budding tubes of the same general size and shape. Spirapora , though, has no internal structures or a planispiral attachment surface. Serpula helicalis and Spirapora are almost certainly not serpulids or corals, and they deserve further study. They are not apparently related to Helicoconchus .

Stratigraphic and geographic range.— Helicoconchus is thus far known only from the Wichita−Albany Group (Lower Permian, Artinskian–Kungurian, of central Texas), beginning with occurrences through an interval of Artinskian restricted marine strata including the Elm Creek Limestone ( Walsh 2002: fig. 5.9), Valera Shale and basal Bead Mountain Formation and at a higher level of Kungurian age restricted marine deposits in the Leuders Limestone and Lytle Limestone of the lower Clear Fork Group (Peter Holterhoff, personal communication 2010). Age assignments of north−central Texas formations are from Wardlaw (2005).