Punctaconchus midfordensis ( Richardson, 1907 )

Vinn, Olev & Taylor, Paul D., 2007, Microconchid tubeworms from the Jurassic of England and France, Acta Palaeontologica Polonica 52 (2), pp. 391-399 : 395

Punctaconchus midfordensis ( Richardson, 1907)

Fig. 3 .

1907 Spirorbis midfordensis sp. nov.; Richardson 1907: 435, fig. 7.

2006 Microconchus midfordensis (Richardson) ; Taylor and Vinn 2006: fig. 1F, G.

Material.—NHM A1814–1816 (five specimens, possibly syntypes), Upper Bajocian, Parkinsonia parkinsoni Zone, Inferior Oolite Group, Salperton Limestone Formation, Clypeus Grit Member (labelled “Bradfordian [Truellii], Upper Coralbed”), road section, Midford, near Bath, Avon (Lindsall Richardson Colln, purchased March 1915). NHM A1817–1819, AN764 (sample), Upper Bajocian, P. parkinsoni Zone, Inferior Oolite Group, Salperton Limestone Formation, Clypeus Grit Member (labelled “Bradfordian [Truellii], Upper Coralbed”), Worgan’s Quarry, near Stroud, Gloucestershire, England (Lindsall Richardson Colln, purchased March 1915). NHM A11925–A11929, eleven specimens, Lower Bajocian, Witchellia laeviuscula Zone , Lincolnshire Limestone Formation, Kirton Shale Member, Kirton−in−Lindsey, Lincolnshire, England.

Emended diagnosis.—Tube small, dextrally coiled (clockwise). External surface with strong longitudinal ridges, pitted regularly by large punctae. Internal surface with transverse ridges, sometimes bifurcating, forming a ripplemark−like pattern. Tube diameter increasing rapidly and regularly, umbilicus relatively narrow.

Description.—Tube small, dextrally (clockwise) coiled, comprising up to three whorls ( Fig. 3A 1 , B). Outline approximately circular. Tube diameter increasing rapidly and regularly. Umbilicus relatively narrow, moderately sloping. Broad base of tube cemented to the substrate commonly over its entire length but in some specimens the aperture is slightly raised. Tube exterior with sharp, strong longitudinal ridges (10–20 ̊m wide) spaced 30–100 ̊m apart, convex, pitted by punctae ( Fig. 3A 3 ). Aperture subpentagonal to oval, lumen oval in cross section. Tube interior covered with well developed ridges (about four per 0.1 mm) perpendicular to growth direction, sometimes bifurcating, resembling ripplemarks ( Fig. 3D ). Tube wall relatively thick (0.10–0.15 mm), laminated ( Fig. 3E ), microstructure comprising fibres 8–13 ̊m long, 1–2 ̊m thick and oriented transversely ( Fig. 3C ). Laminations wavy in longitudinal section, individual laminae about 1.4 ̊m thick on average. Pores large (12–15 ̊m wide), spaced 10–20 ̊m apart. Shell lamellae deflected outwards by 5–7 ̊m around pores.

Dimensions.—Maximum diameter of tubeworm: 0.90–1.89 mm; diameter of aperture: 0.42–0.81 mm. Number of specimens measured: 9. Remarks.— Punctaconchus midfordensis resembles the Triassic species Microconchus phlyctaena Brönnimann and Zaninetti, 1972 , but differs in having strongly developed longitudinal ridges and large punctae. All known specimens of this species are detached from their substrates, unlike the other two species of Punctaconchus in which all specimens are preserved firmly cemented to hard substrates. The identity of the substrates used by P. midfordensis is unknown. Possibilities include plants or soft bodied animals, or alternatively molluscs with aragonitic shells which are typically lost through leaching in these Middle Jurassic carbonates.

Stratigraphic and geographic range.— Middle Jurassic, Lower– Upper Bajocian , Avon, Gloucestershire and Lincolnshire, England .

Figures

Fig. 3. Microconchid Punctaconchus midfordensis (Richardson, 1907). Scanning electron micrographs (back−scattered electron images) of uncoated specimens. A, D, E. Bajocian, Clypeus Grit Member, Worgan’s Quarry, Gloucestershire, UK. A. NHM A1817, detached individual from upper side (A1) showing longitudinal ridges on tube exterior, underside (A2) showing closed tube origin exposed by erosion of basal wall, and detail of punctate tube surface (A3). D. NHM A1818, ornament on tube interior near aperture. E. NHM A1819, polished, etched section of tube wall showing a few punctae filled by calcite, and diagenetic mineral infilling of tube interior (top left). B, C. Bajocian, Kirton Shale Member, Kirton−in−Lindsey, Lincolnshire, UK. B. NHM A11925, tube with external ornament of longitudinal ridges. C. NHM A11926, transversely oriented fibres on tube exterior. Scale bars 500 ̊m (A1, A2, B), 100 ̊m (D, E), 50 ̊m (A3, C).

Fig. 1. Microconchid Punctaconchus ampliporus sp. nov. Several individuals, including the holotype (NHM A12009; arrowed), encrusting a bivalve shell together with a serpulid, thecidacean brachiopods and a bryozoan. Bathonian, White Limestone Formation, Foss Cross Quarry, Gloucestershire, UK. Scale bar 2 mm.