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The Bajocian tubeworm Spirorbis midfordensis, previously regarded as a spirorbid polychaete, is …


Biology Articles » Paleobiology » Paleozoology » Microconchid tubeworms from the Jurassic of England and France » Systematic palaeontology

Systematic palaeontology
- Microconchid tubeworms from the Jurassic of England and France

Class Tentaculita Bouček, 1964

Order Microconchida Weedon, 1991

Genus Punctaconchus nov.

Type species: Punctaconchus ampliporus sp. nov.

Derivation of the name: Combination of puncta and conch (tubicolous shell).

Species included: Punctaconchus ampliporus sp. nov., P. midfordensis Richardson, 1907, P. palmeri sp. nov.

Diagnosis.—Minute calcitic tubes, planispiral, up to three whorls, dextrally coiled (clockwise), diameter of tube increas− ing rapidly and evenly, last whorl almost totally enveloping inner whorls, umbilicus narrow. Tube wall microlamellar, penetrated by large, circular or elliptical punctae distributed over entire surface, lamellae deflected outwards around pores. Tube exterior smooth or ornamented by longitudinal or obli− que ridges. Tube interior covered by ripplemark−like trans− verse ridges that bifurcate and may anastomose.

Discussion.—Punctaconchus differs from serpulid and spi− rorbid polychaetes in its microlamellar shell structure, the presence of numerous small pores (punctae) in the tube wall and the closed origin of the tube. The new genus resembles the type species of the Microconchida, Microconchus carbo− narius Murchison, 1839, in its size, spirally coiled tube, microlamellar structure and punctae. However, it differs in having much larger punctae and an internal surface orna− mented by a pattern of ridges resembling ripplemarks. Pun− ctaconchus tubes also increase in diameter more rapidly than those of Microconchus and the inner whorls are more exten− sively overlapped by the outer whorls.

The only punctate Mesozoic microconchid hitherto de− scribed is Pseudobrachidium germanicum Grupe, 1907 from the Late Triassic of Germany (Warth 1982). It differs from the new genus in having much smaller pores, and in this respect resembles Microconchus. Unlike Punctaconchus, tubes of Pseudobrachidium can become uncoiled during late growth stages, as in some Microconchus. Indeed, it is possible that Pseudobrachidium is a junior synonym of Microconchus. The new genus resembles Palaeoconchus Vinn, 2006, and Annuliconchus Vinn, 2006, both from the early Palaeozoic of Baltoscandia. However, both of these genera lack punctae and Annuliconchus also has an annulated tube. Neither Palaeo− conchus nor Annuliconchus has the distinctive, ripplemark− like tube interiors characteristic of Punctaconchus. Spirorbis midfordensis Richardson, 1907 is reassigned to Punctaconchus because of its microlamellar shell structure characteristic of microconchids, the presence of large punctae penetrating the tube wall at regular intervals and the ripple− mark−like ornament of the tube interior.

The three species of Punctaconchus (P. ampliporus, P. midfordensis, and P. palmeri) can be distinguished using the characters of external ornamentation and porosity. P. ampli− porus has relatively smooth tubes with large, dense punctae, P. midfordensis has tubes with sharp longitudinal striations, and P. palmeri tubes have oblique ornamentation in early on− togeny and sparse punctae.

Stratigraphic and geographic range.—Middle Jurassic, Late Aalenian to Late Bathonian of England, and Normandy, France; also questionably occurring in the Early Jurassic, Toarcian of England.

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 holo− type; AN747, polished, etched section; Other material: AN748–AN757, numerous individuals encrusting other shells from the same locality as the types. AN758, six indi− viduals associated with a paratype of the bryozoan Repto− clausa porcata Taylor, 1980 (NHM D7526) and encrusting a brachiopod shell, Upper Aalenian, L. murchisonae Zone, Inferior Oolite Group, Birdlip Limestone Formation, Crick− ley Member, near Stroud, Gloucestershire, England; A7698, more than 50 individuals encrusting a derived pebble dred− ged 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. Ex− ternal surface smooth, evenly pitted by closely spaced, very large punctae. Diameter of tube increasing rapidly and regu− larly, umbilicus relatively narrow.

Description.—Tube small in size, dextrally coiled (clock− wise), containing up to three whorls (Fig. 2A–C). Outline ap− proximately 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. 2A2), umbilical part moderately sloping, tube aperture subpentagonal, lumen oval in cross section. Tube interior covered by faint ripplemark−like ridges perpen− dicular to growth direction, numbering about six per 0.1 mm, sometimes bifurcating and anastomising to give a net−like appearance (Fig. 2C2). Tube wall relatively thick (0.10–0.15 mm), microlamellar. Punctae very large (15–20 μm wide), closely and evenly spaced, distances between neighbouring punctae 10–20 μm (Fig. 2E). Shell laminae deflected out− wards by about 5 μm around edges of punctae (Fig. 2D). In longitudinal section wall laminations appear wavy, individ− ual laminae averaging 1.4 μm in thickness.

Dimensions.—Maximum diameter of tubeworm: 1.08–1.65 mm; diameter of aperture: 0.54–0.66 mm. Number of speci− mens 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 com− posed of calcite platelets or laths, that of P. midfordensis is fi− brous (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 Aale− nian) 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, Eng− land. Questionably Toarcian of Huntingdonshire, England (probably a derived fossil).

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, In− ferior Oolite Group, Salperton Limestone Formation, Clypeus Grit Member (labelled “Bradfordian [Truellii], Upper Coral− bed”), road section, Midford, near Bath, Avon (Lindsall Rich− ardson Colln, purchased March 1915). NHM A1817–1819, AN764 (sample), Upper Bajocian, P. parkinsoni Zone, Infe− rior Oolite Group, Salperton Limestone Formation, Clypeus Grit Member (labelled “Bradfordian [Truellii], Upper Coral− bed”), Worgan’s Quarry, near Stroud, Gloucestershire, Eng− land (Lindsall Richardson Colln, purchased March 1915). NHM A11925–A11929, eleven specimens, Lower Bajocian, Witchellia laeviuscula Zone, Lincolnshire Limestone Forma− tion, Kirton Shale Member, Kirton−in−Lindsey, Lincolnshire, England.

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

Description.—Tube small, dextrally (clockwise) coiled, com− prising up to three whorls (Fig. 3A1, B). Outline approxi− mately circular. Tube diameter increasing rapidly and regu− larly. Umbilicus relatively narrow, moderately sloping. Broad base of tube cemented to the substrate commonly over its en− tire length but in some specimens the aperture is slightly raised. Tube exterior with sharp, strong longitudinal ridges (10–20 μm wide) spaced 30–100 μm apart, convex, pitted by punctae (Fig. 3A3). Aperture subpentagonal to oval, lumen oval in cross section. Tube interior covered with well devel− oped 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), lami− nated (Fig. 3E), microstructure comprising fibres 8–13 μm long, 1–2 μm thick and oriented transversely (Fig. 3C). Lami− nations wavy in longitudinal section, individual laminae about 1.4 μmthick on average. Pores large (12–15 μm wide), spaced 10–20 μm apart. Shell lamellae deflected outwards by 5–7 μm around pores.

Dimensions.—Maximum diameter of tubeworm: 0.90–1.89 mm; diameter of aperture: 0.42–0.81 mm. Number of speci− mens measured: 9.

Remarks.—Punctaconchus midfordensis resembles the Trias− sic species Microconchus phlyctaena Brönnimann and Zani− netti, 1972, but differs in having strongly developed longitudi− nal ridges and large punctae. All known specimens of this spe− cies are detached from their substrates, unlike the other two species of Punctaconchus in which all specimens are pre− served firmly cemented to hard substrates. The identity of the substrates used by P. midfordensis is unknown. Possibilities include plants or soft bodied animals, or alternativelymolluscs 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 Lincoln− shire, England.

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; Cail− lasses 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 inte− rior with well developed ripplemark−like ridges transverse to growth direction.

Description.—Tube small, dextrally coiled (clockwise), com− prising up to three whorls (Fig. 4A, B, C1, D, E1). Outline ap− proximately circular. Tube diameter increasing very rapidly and regularly. Umbilicus relatively narrow, moderately slop− ing. 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, C2) with strong oblique ridges (10 μm wide) spaced 30–40 μm 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 μm wide), transversely elliptical (Fig. 4E). Aperture oval, lumen oval in cross section. Tube interior covered with well devel− oped ripplemark−like ridges (about five per 0.1 mm) perpen− dicular to growth direction, sometimes bifurcating (Fig. 4D). Tube wall 0.08–0.10 mmthick at the aperture in mature speci− mens. Lamellae of tube wall wavy (Fig. 4F), reflecting the ripplemark−like ridges on the tube interior, heights of the waves being 5–6 μm. Growth increments mollusc−like in lon− gitudinal 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 speci− mens 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 pal− meri differs from P. midfordensis in lacking prominent lon− gitudinal 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 com− prises two species, S. cingulata Nielsen, 1931, and S. tortilis Nielsen, 1931, which are either spirorbids or coiled ser− pulids. They are unrelated to microconchids.

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


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