Ptychonema agyris Ebbestad, 2007

Popov, Leonid E., Ebbestad, Jan Ove R., Mambetov, Amanbek & Apayarov, Farid Kh., 2007, A low diversity shallow water lingulid brachiopod-gastropod association from the Upper Ordovician of Kyrgyz Range, Acta Palaeontologica Polonica 52 (1), pp. 27-40 : 34-36

Ptychonema agyris Ebbestad , sp. nov.

Figs. 5, 6 , 7 .

Derivation of the name: From Greek agyris , meaning gathering, pertaining to the occurrence in coquinas, whether fortuitous or not.

Holotype: A near complete specimen with patches of shell preserved ( NMW 98.66 G.946; Fig. 5A–H). Preserved height and maximum width of the specimen are both 9.0 mm.

Type locality: Locality 510, northern bank of the Almaly Gorge, western Kyrgyz Range, Kazakhstan.

Type horizon: Upper Member of Almaly Formation, Caradocian (Upper Ordovician).

Paratypes: A small specimen ( NMW 98.66 G.947, Fig. 5I) with nearly complete spire; a nearly complete specimens where the outline of the outer lip is preserved ( NMW 98.66 G.948, Fig. 5J–L), a partial spire with initial bulbous part of infilling preserved ( NMW 98.66 G.949, Fig. 5M, N), a specimen with partially preserved aperture ( NMW 98.66 G.950, Fig. 6A ), base of a specimen ( NMW 98.66 G.951, Fig. 6B ) with shell preserved; a partial last whorl with shell intact ( NMW 98.66 G.952, Fig. 6C ); two cross sections ( NMW 98.66 G.953, Fig. 6D ; NMW 98.66 G.954, Fig. 6E ). All specimens were prepared out of the slab from where the holotype came .

Other material.—The shell is extremely common in the type area and forms coquinas of different sized specimens, together with the Tunisiglossa almalensis (see Fig. 2 for distribution).

Diagnosis.—A species of Ptychonema with four whorls that differs from other species of the genus by having a strongly rounded, parabolic whorl profile. It shares a small size and deep sinus placed mostly above the periphery with P. marocanum Horný, 1997 , but differs from other species of the genus in this respect. It differs from P. marocanum in having more strongly curved growth lines on the upper whorl surface, and by having a less inclined and thicker inner lip. The nature of the inner lip also differ from that of the other species of the genus.

Description.—Shell turbiniform with 4 whorls of moderate whorl translation (W = 0.5). The earliest whorl(s) are abandoned and the apex is filled, giving a sunken and tapering infilling of the first open part of the conch. The conch is slightly wider than it is high, with an apical angle of about 90 °. Whorls are evenly and strongly rounded, parabolic in profile, nearly flat at the incised sutures and with a corresponding low profile of the evenly rounded base. Successive whorls overlap at lower 1/3 of whorl height. The shell is anomphalous. Ornamentation consists of fine growth lines only; they are prosocline on the upper whorl surface, sweeping strongly back abaperturally into a wide, symmetrical, or nearly symmetrical, U−shaped sinus. Most of the sinus is placed slightly above the periphery, and is about as wide (axial direction) as it is deep (abaperturally). In some specimens a slightly raised zone (spirally) is associated with the position of sinus. On the lower whorl surface the growth lines sweep strongly forward, with the most anterior point (adaperturally) of the projection at lower 2/3 of whorl height. The growth lines turn more orthocline toward the base, but they are slightly excavated (curved) at the inner−margin base (viewed ventrally). The aperture is tangential, with a low apertural angle (E = 10 °). The outer apertural lip is thin with only slight thickening at the basal part, and a thicker inner margin. Inner margin slightly curved, but essentially parallel to axis of coiling. A single pedal muscle scar is found at the base of the whorl at the umbilical margin, about half a whorl back (the abapertural boundary of the scar is concealed by preserved shell), and curving adaperturally for a short distance. The scar is elongated, widening adaperturally where the margin forms a weak asymmetric arch projecting furthest adaperturally closest to the aperture. Four marginal lobes are developed at the adapertural part, the broadest near the umbilicus with three lobes of about equal width away from the umbilicus.

Remarks.—With its rounded, tangential aperture Ptychonema agyris should in life position have had the apertural plane parallel to the substrate, accompanied by a high degree of inclination of the axis of coiling ( Vermeij 1971; Linsley 1977). However, the inclination of the axis of coiling in P. agyris is very low, as indicated by the low angle (E) of the apertural plane, and this is contrary to geometric expectations ( Fig. 7A, B ). This arrangement requires some amount of regulatory detorsion to achieve balance of the shell. Regulatory detorsion effectively ensures that the centre of gravity of the shell is over the mid−line of the foot by reducing the angle between the mid−axis of the foot and the axis of coiling ( Linsley 1977; law of shell balance). Inevitably, with such a configuration, the emargination (i.e., the sinus) will not have an anterior position since it is positioned largely above the periphery of the whorl ( Fig. 7C, D ). This is as well contrary to geometric expectations for emarginations in shells of anisostrophic coiled gastropods ( Linsley 1977; law of re−entrants). A steep apertural angle in Palaeozoic gastropods is usually associated with a radial aperture, held normal to the substrate ( Linsley 1977). However, Isakar et al. (1991) recognized a pseudo−tangential aperture in the large Silurian gastropod Horologium , essentially giving the same morphological arrangement as in a shell with a radial aperture.

The abandoned apex is characteristic of Ptychonema agyris , but also seems to be present in P. marocanum (personal observation by JORE, November 2005); the character may be shared among all species of the genus (see discussion of P. bussacense below). Filled apices are as well seen in other distantly related genera such as Palliseria , Lytospira , and Hormotoma , but these differ in that the infilling (internal mould) of the initial open shell area is bulbous. In Ptychonema agyris this part is depressed and tapering ( Fig. 5I, M, N). Given that the shell in this species is thin, the internal mould may reflect the original shape of the initial whorl thus indicating the presence of a heterostrophic submerged protoconch. Heterostrophy is characterized by a change from hyperstrophic growth of the protoconch to orthostrophic growth at the onset of the teleoconch, typical for members of the subclass Heterostropha (see Bandel 1994). The oldest undoubted heterostrophic gastropod is known from the Lower Ordovician ( Frýda and Rohr 2006). Previously, the oldest form was described from the Devonian ( Frýda and Blodgett 2001; Bandel and Heidelberger 2002), but an unnamed specimen was also depicted from the Late Ordovician Boda Limestone of Sweden by Gubanov et al. (1999).Without the actual protoconch preserved in P. agyris a heterotrophic affiliation is not unequivocal, but because of the suggestive direction of the initial coiling the protoconch is unlikely to compare to a large bulbous archaeogastropod−type. It is, however, prudent to keep in mind that variation of protoconch morphology is evident within the Archaegastropoda, for instance exemplified by the Cirroidea, and that the range and variation of assorted open coiled protoconch morphologies are still largely unexplored (see Frýda 1999; Frýda and Rohr 2004).

The species from Kyrgyz Range is similar to Ptychonema marocanum Horný, 1997 , from Morocco in the small size of the shell, though maximum size is about 30% larger (height about 9 mm vs. 6 mm in the Moroccan species). Other similarities are in number of whorls, the anomphalous shell, and the conch being wider than high. Ptychonema agyris differs noticeably in the more strongly rounded whorls, the growth lines that sweep more strongly back on the upper whorl face, a less inclined and thicker inner lip. The sinus may also be deeper in the species from Kyrgyz Range. Horný (1997), on the other hand, described considerable variability of this character in Ptychonema , ranging from a short slit making a bordered selenizone, a sinus with a weakly defined pseudo−selenizone, or a simple, sometimes raised, band−like spiral zone. In Ptychonema marocanum the variation is manifested by a weakly defined pseudo−selenizone that may be associated with a raised zone that is rarely bounded by spiral lines and with highly irregular lunulae where such can be distinguished. Even with the limited material prepared for description of the species from Kyrgyz Range, a raised zone associated with the sinus is sometimes evident ( Fig. 6C ), and there is also variability in the strength of the growth lines (for example Fig. 5G). Anything approaching a pseudo−selenizone, however, is not recognised in Ptychonema agyris .

Differences in width and depth of the sinuses and size differences set both Ptychonema agyris and P. marocanum apart from the other recognised species of Ptychonema . Similarities exist between the species from Kyrgyz Range and the Cincinnatian Ptychonema sp. in the whorl profile, but with a more pronounced shoulder−like upper whorl surface in the American species. Some of the variation though may be owing to larger size, continuous ontogenetic change, and taphonomy. The sinuses in both species are placed high on the whorl, though it is narrower and less deep in the American Ptychonema sp. In this it compares to the type species Ptychonema nigrum Barrande in Perner, 1903, as well as P. bussacense ( Sharpe, 1853) and P. desideratum (Barrande in Perner, 1903). These three species are all considerably larger than both Ptychonema agyris and P. marocanum , and seem to develop a better defined pseudo−selenizone, with a less deep sinus.

Similarities among the three large species are many, and Horný (1997) considered that P. desideratum may be a junior synonym of P. bussacense . In the Natural History Museum, London, three specimens (BMNH Pg3983, Pg3985, Pg3987) labelled Pleurotomaria bussacensis Sharpe from Portelo de Leredo in the Serra de Bussaco were studied, which lend support to this suggestion (assessment of the type material is still needed to judge this matter). These partially preserved specimens are from the type area of the species and seem to conform to the description of the species by Sharpe (1853), as well as the description and figure of P. desideratum given by Horný (1997). All specimens are large (between 20 and 40 mm across), with five(?) whorls, a wide and not very deep sinus that is raised on a spiral zone in one specimen (BMNH Pg3983). The shells seem to be narrowly phaneromphalous or even anomphalous. In BMNH Pg3985 the apex was clearly abandoned, with preservation similar to that seen in Ptychonema agyris ( Fig. 5M, N).

Vostokova (1956) described gastropods from the comparable Anderken Formation and the Otar “Horizon” of the Chu−Ili Range in Kazakhstan. Most diverse where a lophospirid assemblage preserved in sandstones and siltstones, and a Maclurina assemblage found in limestone. In the argillaceous sediments Latitaenia kasachstanica Vostokova, 1956 dominates numerically, but neither this nor any of the other species described by Vostokova (1956) can be compared to Ptychonema . Part of the reason may be heterogeneous dispersal of the assemblages, and the more shallow nature of the deposits containing Ptychonema .

Figures

Fig. 6. The holopeid gastropod Ptychonema agyris Ebbestad, sp. nov. from the Almaly Formation, Caradocian (Upper Ordovician) of Kyrgyz Range. A. NMW 98.66G.950, specimen with partially preserved aperture. B. NMW 98.66G.951, base of a specimen with shell preserved. C. NMW 98.66G.952, partial last whorl with shell intact. D. NMW 98.66G.953, cross section of large specimen. E. NMW 98.66G.954, cross section of initial whorls of small specimen. Scale bars 2.5 mm.

Fig. 7. The holopeid gastropod Ptychonema agyris Ebbestad, sp. nov. from the Almaly Formation, Caradocian (Upper Ordovician) of Kyrgystan. A. Reconstruction of the shell in dorsal view. B. Reconstruction of the shell in lateral view. C. Reconstruction of the shell in presumed life position with the apertural plane horizontal and with slight regulatory detorsion to balance the shell. Note how the emargination is not in an anterior position. D. Reconstruction of the shell in presumed life position with the apertural plane horizontal, but without regulatory detorsion, so that the emargination is in an anterior position.