Punctaconchus palmeri, 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 : 395-398

Punctaconchus palmeri sp. nov.

Fig. 4 .

1981 Spirorbula sp. ; Palmer and Fürsich 1981: 7, pl. 2: 12.

Derivation of the name: After Timothy J. Palmer who, along with Franz T Fürsich, described the palaeoecology of the sponge reefs in which the new species is found.

Holotype: NHM AN759(1), tube attached to bivalve shell.

Type locality: St Aubin−sur−Mer, Calvados, Normandy, France.

Type horizon: Upper Bathonian, Aulacosphinctes hollandi Zone ; Caillasses de la Basse Ecarde Formation, sponge reefs.

Material.— Holotype: NHM AN759(1). Paratypes: AN759(2)– AN759(7) (six individuals on same substrate as holotype), AN760–762 (22 individuals on three substrates), AN763 (polished and etched section).

Diagnosis.—Tube small, dextrally coiled, cemented to the substrate over its entire length, aperture slightly raised in some specimens. Diameter of the tube increasing rapidly and regularly, umbilicus relatively narrow. External surface with strong semiperpendicular ridges in juveniles, becoming faint in adults, pitted by transversely elliptical punctae. Tube interior with well developed ripplemark−like ridges transverse to growth direction.

Description.—Tube small, dextrally coiled (clockwise), comprising up to three whorls ( Fig. 4A, B, C 1, D, E 1 ). Outline approximately circular. Tube diameter increasing very 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 of juveniles ( Fig. 4B, C 2 ) with strong oblique ridges (10 ̊m wide) spaced 30–40 ̊m apart, often broken into nodes. Mature specimens externally covered with weakly developed, faint perpendicular ridges, 6–8 per 0.1 mm. Tube convex, sparsely pitted by punctae (5–10 ̊m wide), transversely elliptical ( Fig. 4E ). Aperture oval, lumen oval in cross section. Tube interior covered with well developed ripplemark−like ridges (about five per 0.1 mm) perpendicular to growth direction, sometimes bifurcating ( Fig. 4D ). Tube wall 0.08–0.10 mm thick at the aperture in mature specimens. Lamellae of tube wall wavy ( Fig. 4F ), reflecting the ripplemark−like ridges on the tube interior, heights of the waves being 5–6 ̊m. Growth increments mollusc−like in longitudinal sections of tubes ( Fig. 4F ).

Dimensions —Maximum diameter of tubeworm: 0.66–1.50 mm; diameter of aperture: 0.33–0.66 mm. Number of specimens measured: 10.

Discussion.— Punctaconchus palmeri is similar to both P. ampliporus and P. midfordensis in having punctate tube walls and a ripplemark−like ornament on the interior surface of the tube. It resembles P. ampliporus in lacking prominent longitudinal ridges on tube exteriors, but differs in having sparser and much smaller pores and a stronger ripplemark−like ornament on the tube interior. It also differs in showing a more rapid increase in tube diameter. Punctaconchus palmeri differs from P. midfordensis in lacking prominent longitudinal ridges, and in having sparser and much smaller punctae.

This species was assigned by Palmer and Fürsich (1981) to the genus Spirorbula Nielsen, 1931 . The original material of Spirorbula comes from the Danian of Denmark and comprises two species, S. cingulata Nielsen, 1931 , and S. tortilis Nielsen, 1931 , which are either spirorbids or coiled serpulids. They are unrelated to microconchids.

Stratigraphic and geographic range.—Middle Jurassic, Late Bathonian, Calvados, Normandy, France.

Figures

Fig. 4. Microconchid Punctaconchus palmeri sp. nov. Scanning electron micrographs (back−scattered electron images) of uncoated specimens. Bathonian, Caillasses de la Basse Ecarde Formation, St Aubin−sur−Mer, Normandy, France. A. Holotype, NHM AN759(1). B. Paratype, NHM AN759(2), small specimen viewed obliquely to show aperture and ornament on initial whorl. C. Paratype, NHM AN760, entire specimen (C1) and detail of external ornamentation (C2). D. Paratype, NHM AN761, broken tube with ripplemark−like ornament visible on interior of basal wall. E. Paratype, NHM AN762, bored individual (E1) and detail of punctae on tube outer surface (E2). F. Paratype, AN763, polished and etched section of tube wall showing wavy wall lamination and growth increment bounded by deeply etched laminae; tube exterior is at top of image. Scale bars 500 ̊m (A, C1, D, E1), 200 ̊m (B), 100 ̊m (C2, E2), 50 ̊m (F).

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).