Punctaconchus ampliporus, Vinn & Taylor, 2007

Vinn, Olev & Taylor, Paul D., 2007, Microconchid tubeworms from the Jurassic of England and France, Acta Palaeontologica Polonica 52 (2), pp. 391-399 : 393-395

Punctaconchus ampliporus sp.nov.

Figs. 1 , 2 .

Derivation of the name: In reference to the large (Latin amplus) pores (Latin pori).

Holotype: NHM A12009(1), tube attached to bivalve shell.

Type locality: Foss Cross Quarry, Gloucestershire, England.

Type horizon: Middle or Upper Bathonian, M. morrisi or P. hodsoni zones; Great Oolite Group, White Limestone Formation.

Material.— Holotype: A12009(1). Paratypes: A12009(2)– A12009(13), twelve individuals on same substratum as holotype; AN747, polished, etched section; Other material: AN748–AN757, numerous individuals encrusting other shells from the same locality as the types. AN758, six individuals associated with a paratype of the bryozoan Reptoclausa porcata Taylor, 1980 (NHM D7526) and encrusting a brachiopod shell, Upper Aalenian, L. murchisonae Zone, Inferior Oolite Group, Birdlip Limestone Formation, Crickley Member, near Stroud, Gloucestershire, England; A7698, more than 50 individuals encrusting a derived pebble dredged from the River Nene at Stebbington, Huntingdonshire, England, and reputedly Toarcian, Lytoceras jurense Zone , Haugia variabilis Subzone.

Diagnosis.—Tube small, dextrally coiled, cemented to the substrate over its entire length, lacking a free distal part. External surface smooth, evenly pitted by closely spaced, very large punctae. Diameter of tube increasing rapidly and regularly, umbilicus relatively narrow.

Description.—Tube small in size, dextrally coiled (clockwise), containing up to three whorls ( Fig. 2A–C ). Outline approximately circular. Diameter of tube increasing rapidly and regularly, umbilicus relatively narrow. Broad base of tube cemented to the substrate over its entire length, lacking a free distal portion. Tube external surface smooth, convex, pitted by pores ( Fig. 2A 2 ), umbilical part moderately sloping, tube aperture subpentagonal, lumen oval in cross section. Tube interior covered by faint ripplemark−like ridges perpendicular to growth direction, numbering about six per 0.1 mm, sometimes bifurcating and anastomising to give a net−like appearance ( Fig. 2C 2 ). Tube wall relatively thick (0.10–0.15 mm), microlamellar. Punctae very large (15–20 ̊m wide), closely and evenly spaced, distances between neighbouring punctae 10–20 ̊m ( Fig. 2E ). Shell laminae deflected outwards by about 5 ̊m around edges of punctae ( Fig. 2D ). In longitudinal section wall laminations appear wavy, individual laminae averaging 1.4 ̊m in thickness.

Dimensions.—Maximum diameter of tubeworm: 1.08–1.65 mm; diameter of aperture: 0.54–0.66 mm. Number of specimens measured: 5.

Discussion.—This new species resembles Punctaconchus midfordensis in the size of the tubes and presence of large punctae in the tube walls. However, it differs in having a smooth tube exterior and slightly fainter ridges on the tube interior, as well as more densely packed punctae. The very dense porosity of the tubes of P. ampliporus is particularly evident in a specimen ( Fig. 2E ) in which the punctae have been cast naturally by diagenetic minerals. Whereas the microlamellar tube of the new species appears to be composed of calcite platelets or laths, that of P. midfordensis is fibrous ( Fig. 3C ), although this apparent difference may be diagenetic.

In his description of Spirorbis midfordensis, Richardson (1907) mentioned being shown a specimen of the brachiopod Pseudoglossothyris simplex from the Pea Grit (Upper Aalenian) encrusted by numerous Spirorbis . These are likely to have been Punctaconcus ampliporus which is represented at this stratigraphical level in the NHM collections.

Stratigraphic and geographic range.—Middle Jurassic, Late Aalenian–Middle/Late Bathonian of Gloucestershire, England. Questionably Toarcian of Huntingdonshire, England (probably a derived fossil).

Figures

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.

Fig. 2. Microconchid Punctaconchus ampliporus sp. nov. Scanning electron micrographs (back−scattered electron images) of uncoated specimens. A–D. Bathonian, White Limestone Formation,Foss Cross Quarry, Gloucestershire, UK.A. Holotype,NHM A12009(1), entire individual (A1) and detail of tube surface showing pores, many partly infilled (A2). B. Paratype with broken aperture, NHM A12009(2). C. Worn paratype, NHM A12009(3), entire individual (C1) and detail of ornament on tube interior near aperture (C2). D. Paratype, NHM AN747, polished, etched section of tube wall showing large punctae filled by calcite, with wall laminae deflected outwards towards tube exterior (bottom right) around punctae, and coarse diagenetic infill of tube interior (top left). E. Toarcian, Stebbington, Huntingdonshire, UK, NHM A7698, corroded specimen (E1) with natural casts of punctae (E2). Scale bars 500 ̊m (A1, B, C1, E1), 100 ̊m (C2, E2), 50 ̊m (A2, D).

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).