Propomatoceros permianus, Sanfilippo & Rosso & Reitano & Insacco, 2017

Sanfilippo, Rossana, Rosso, Antonietta, Reitano, Agatino & Insacco, Gianni, 2017, First record of sabellid and serpulid polychaetes from the Permian of Sicily, Acta Palaeontologica Polonica 62 (1), pp. 25-38 : 29-31

Propomatoceros permianus sp. nov.

Fig. 4 .

Etymology: Named after the Permian period.

Holotype: A unique tube ( MSNC4526 ) attached to a sponge, incomplete at anterior end.

Type locality: Sosio Valley, western Sicily ( Italy).

Type horizon: “Pietra di Salomone” Limestone, Wordian to upper Permian.

Diagnosis.—Tube medium-sized, rapidly increasing in diameter, attached for all its length or seemingly rising above the substrate at its terminal end. Tube triangular in cross-section, with a median slightly denticulate keel. Denticles spaced ca. 0.30 mm from each other, directed towards the aperture. Lateral surfaces with transverse ribs, each developing from a denticle of the keel and directed towards the basal part. Basal lateral rounded thickenings enlarging the contact with the substrate.

Description.—The only available specimen incomplete at anterior end, lacking a portion of the median keel and likely its erect part, consists of a medium-sized tube, 1.6 cm long, which rapidly increases in diameter from 0.2 mm at its posteriormost preserved part to 1.4 mm at the anterior end. The tube is attached for all its length, the intermediate portion curved to form a loop. It shows a triangular cross-section and a median keel bearing barely visible denticles. The lateral surfaces are wrinkled by delicate growth striae and transverse ornamentations consisting of ribs slightly concave towards the aperture, which follow each other at short regular distances. Tube wall thickness about 0.20 mm. Basal part of attachment large and irregularly contoured due to the presence of lateral rounded thickenings in correspondence of transverse ribs. Tube strongly recrystallized.

Remarks.—Serpulid tubes showing a more or less triangular cross-section and a well-developed median keel resembling the genus Pomatoceros Philippi, 1844 (which nowadays is considered a subjective synonym of Spirobranchus Blainville, 1818 ) have a long range in the fossil record, reaching from the Triassic or Early Jurassic ( Ippolitov 2007b; Ippolitov et al. 2014) to the Recent. The genus Propomatoceros was introduced by Ware (1975) for several Cretaceous species resembling the genus Pomatoceros , but differing at least in the anterior tube portion of adult specimens, “in having a further convex and sometimes a free cylindrical stage of variable length” ( Ware 1975: 99). Moreover, Propomatoceros often has a more developed lateral ornamentation. Later, more and more fossil forms have been assigned to Propomatoceros , so that now Propomatoceros includes a plethora of species, most of questionable status or uncertain generic assignment (see Ippolitov 2007b; Vinn and Wilson 2010 for further comments and the general status of this genus).

The rapidly widening tube, the feeble denticulate keel, the conspicuous regularly spaced lateral ribs, and the irregularly thickened and contoured basal flange are the distinctive characters of P. permianus . It shows affinities to Propomatoceros semicostatus Regenhardt, 1961 and P. sulcicarinatus Ware, 1975 from the Lower Cretaceous of Germany, England, and Argentina (see Luci et al. 2013: 218, fig. 3 in part). Like the new species, P. semicostatus has a slender median keel and conspicuous growth striae, but lacks the typical irregularly contoured basal expansions which characterize the tube wall of P. permianus . P. sulcicarinatus is also similar to the present species, but possesses a thicker keel and peristome-like thickenings separated by constrictions.

Placostegus Philippi, 1844 and the attached portions of some species of Filogranula Langerhans, 1884 are similar to Propomatoceros for the keeled tube, triangular in cross-section. The genus Filogranula , documented from the late Early Jurassic to the Recent (Toarcian, ~180 Ma; Ippolitov et al. 2014), has peristomes in the attached portions and more prominent denticulated keels, especially in some one-keeled species. Placostegus known from the Upper Jurassic (Late Oxfordian, ~158 Ma) until today, also differs in the tube growth having a conspicuous erect part, and a more pronounced denticulated keel.

The name Dorsoserpula was introduced by Parsch (1956), who grouped Jurassic serpulid species according to the number of angles in the cross-section of the anterior tube portion respectively by the number of keels, for many different species possessing a median keel or comb and a tube more or less triangular in cross-section. Ware (1975) started to disentangle the Dorsoserpula lot by introducing and defining the genus Propomatoceros ; see above. Jäger (2005) considerably diminished the species content of Dorsoserpula by reducing it to its type-species Serpula delphinula Goldfuss, 1831 , which has a much more circular than triangular cross-section and only a weak keel, and a few other species which are morphologically very similar to the type species and also are more or less circular, although some of them have no keel at all. However, more important than keel or cross-section, the revised Dorsoserpula is characterized by the facultative presence of special morphological features which, unfortunately, in most populations are developed only in some of the individuals: a small longitudinal tube embedded laterally or along the expanded attachment flange, and a tendency to grow forming a spiral case around cylindrical or filiform substrata. To sum it up, after reducing the content of Dorsoserpula to its type species and a few similar species, the general morphological meaning of Dorsoserpula has shifted considerably away from Parsch’s (1956) original concept of systematics of Jurassic serpulids, and also the hitherto known stratigraphical range of Dorsoserpula has shifted from Lower–Upper Jurassic ( Parsch 1956) to Late Jurassic–Late Cretaceous (Middle Oxfordian to latest Maastrichtian, 160–166 Ma; Jäger 2005; Ippolitov et al. 2014).

Stratigraphic and geographic range.—Wordian to upper Permian of Italy.

Figures

Fig. 4. Serpulid Propomatoceros permianus sp. nov. from the Sosio Valley, western Sicily (Italy), Wordian to upper Permian, holotype MSNC 4526. A. General view of the tube encrusting a sponge. B, C. SEM images, general view (B) and detail of the tube (C) showing denticles of the median keel and the equally spaced transverse ribs. D. Detail of the median keel bearing inconspicuous denticles.