Punctaconchus, 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 : 392-393

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 increasing 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 oblique ridges. Tube interior covered by ripplemark−like transverse ridges that bifurcate and may anastomose.

Discussion.— Punctaconchus differs from serpulid and spirorbid 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 carbonarius Murchison, 1839 , in its size, spirally coiled tube, microlamellar structure and punctae. However, it differs in having much larger punctae and an internal surface ornamented by a pattern of ridges resembling ripplemarks. Punctaconchus tubes also increase in diameter more rapidly than those of Microconchus and the inner whorls are more extensively overlapped by the outer whorls.

The only punctate Mesozoic microconchid hitherto described 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 Palaeoconchus 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 ripplemark−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. ampliporus 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 ontogeny 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.