Mimospira llangynogensis, Ebbestad & Cope, 2021

Ebbestad, Jan Ove R. & Cope, John C. W., 2021, A low diversity Sinuites gastropod community from the Floian, Early Ordovician, of South Wales, Acta Palaeontologica Polonica 66 (2), pp. 319-335 : 324-326

Mimospira llangynogensis sp. nov.

Fig. 4 .

Zoobank LSID: urn:lsid:zoobank.org:act:E9919E4F­34D0­4B67­870E­C466C435A7B6

Etymology:After the Llangynog Inlier, where the type specimen is found.

Type material: Holotype ( NMW 2017.15 G.58, Fig. 4A ), laterally flattened internal mould and its external mould . Paratypes ( NMW 2017.15 G.59–66, Fig. 4C ), internal and external moulds; from the type locality.Two of the paratype specimens occur on NMW 2017.15 G.60i, ii .

Type locality: Dan­lan­y­Castell quarry, Carmarthen , Wales, UK .

Type horizon: Merlinia selwynii Trilobite Zone, Moridunian regional Stage (lower Floian Stage, Fl 1) .

Material.— Type material only.

Diagnosis.—A large species of Mimospira with a small number of sharp ribs inclined at about 20° relative to the suture.

Description.—Shell is conispiral with 5–6 whorls, width slightly less than height, which can reach at least 12 mm. Pleural angle is about 55°. Sutures sharply incised but not deep (external view). Whorl surface steeply inclined and evenly convex between sutures. Aperture gently prosocline, simple and thin near suture, but likely getting thicker adumbilically. Ornamentation consist of regularly spaced ribs with rounded edges, separated by weakly concave interareas. The ribs are regularly spaced, with slightly increased spacing in later ontogeny. Each whorl show 5–6 ribs in lateral view, inclined at about 20° relative to the suture. Base of whorls rounded, convex, sloping gently into shallow axial area.

Remarks.—All specimens are laterally compressed internal or external moulds, but the large size and ornamentation allows erection of a new species. The holotype is flattened laterally, but is selected as it shows the apertural margin, which seems to be simple without any thickening ( Fig. 4A ). The inner lip of the aperture is not preserved, but one internal mould show an excavated ledge along the base (arrow in Fig. 4E ) suggesting the presence of a thickened margin towards the umbilical area. Fig. 4C 2 shows the only preserved basal view, which is an internal mould of the partial counterpart of the lower specimen in Fig. 4C 1 . The rounded base gives a shallow area towards the axis, but no true umbilicus. The hole seen in the base indicates the space resulting from a dissolved columella.

Only Mimospira sp. from the Tremadocian (Lower Ordovician) Vogtendorf Formation of the Franconian Forest in Germany, tentatively assigned to the genus by Sdzuy et al. (2001), attain a similar size. The single imperfect German specimen differs from M. llangynogensis sp. nov. in having more densely spaced ribs that also become much denser near the base of the whorls. Other Tremadocian species are found in the Prague Basin, with M. cf. helmhackeri in the Trenice Formation and M. aff. helmhackeri and M. sp. in the overlying Mílina Formation, although only the quite fragmentary Trenice specimen has been figured ( Kraft et al. 2013, 2015). Another Tremadocian species is Mimospira atava ( Moberg and Segerberg, 1906) from the Bjørkåsholmen Formation of southern Sweden, which is a small species with densely ribbed ornamentation (see further discussion in Wängberg­Eriksson 1979; Sdzuy et al. 2001).

The ornamentation in the Late Ordovician species M. kallholnensis Wängberg­Eriksson, 1979 , from the Boda Limestone of Sweden is similar in their distinction and spacing compared to that of M. llangynogensis sp. nov., and the shell reaches a height of 7 mm. The two differ in the ribs being more cord­like in M. kallholnensis and its shell being proportionally narrow and slender. The type species of Mimospira , M. helmhackeri from the Darriwilian of Bohemia, reaches 8 mm in height, and the ornamentation is similar to that of M. kallholnensis , albeit more densely spaced. The base of the last whorl in the type species forms a flange­like extension not seen in other species of the genus.

Bates (1963) described Matherella? acuticostata from the Fennian (Dapingian; Middle Ordovician) Treiorwerth Formation at Trefor in the county of Anglesey, north­west Wales.The species was placed with Mimospira by Wängberg­Eriksson (1979), and consists of a tall shell reaching 8 mm in height but with much more densely spaced oblique lirae than seen in M. llangynogensis sp. nov.

Stratigraphic and geographic range.—Ordovician; Illinois USA), Tallinn ( Estonia), Carmarthen, Wales ( UK).

Figures

Fig. 4. Clisospirid mimospiridan Mimospira llangynogensis sp. nov. from the Floian, Lower Ordovician Bolahaul Member of the Ogof Hên Formation, Dan­lan­y­Castell quarry, Wales, UK. A. NMW 2017.15G.58, holotype, laterally flattened internal mould in apertural (A1) and lateral (A2, A3) views. B. NMW 2017.15G.59, latex cast of external (B1) and internal (B2) mould.C. NMW 2017.15G.60i, ii, two specimens partly preserved as external and internal moulds (C1); counterpart of the internal mould (arrow in C1) showing its base (C2). D. NMW 2017.15G.61, silicon cast of external mould, showing ornamentation and sutures. E. NMW 2017.15G.62, internal mould, arrow points to excavated ledge along the base. Scale bars 5 mm.

Fig. 2. Carcassonellid tergomyan Carcassonnella cf. vizcainoi Horný and Peel, 1996 from the Floian, Lower Ordovician Bolahaul Member of the Ogof Hên Formation, Dan­lan­y­Castell quarry, Wales, UK. A. NMW 2017.15G.1, silicon cast, in lateral (A1), posterior oblique (A2), and ventral (A3) views. B. NMW 2017.15G.2, internal mould in lateral view. Scale bars 5 mm.

Fig. 1. A. Moridunian stratigraphy (after Fortey and Owens 1978). B–D. Simplified maps of Wales (B), the southeastern part of the country (C), and the area south of Carmarthen (D) showing the location of the study area and outcrop. E. Simplified geological map with position of the Dan­lan­y­Castell quarry (modified from Cope 1982).