Pomerantsoceras pollux (Barrande, 1866)

Manda, Štěpán & Turek, Vojtěch, 2009, Minute Silurian oncocerid nautiloids with unusual colour patterns, Acta Palaeontologica Polonica 54 (3), pp. 503-512 : 509-511

Pomerantsoceras pollux (Barrande, 1866)

Figs. 4–10 .

1866 Cyrtoceras pollux Barr. ; Barrande 1866: pl. 148: 16–19.

1866 Cyrtoceras pollux var. castor Barr. ; Barrande 1866: pl. 148: 20–23.

1874 Cyrtoceras pollux var. castor Barrande 1847 ; Barrande: 487.

1874 Cyrtoceras pollux Barrande ; Barrande 1847: 526.

Holotype: By monotypy, specimen NM−L 570 figured by Barrande

Type locality: Praha−Malá Chuchle, Vyskočilka e2.

Type horizon: Holotype is a slightly flattened internal mould preserved in dark grey argillite wackestones. Fragments of graptolites and small shell fragments (juvenile molluscs?) covered by pyrite are visible in the aperture. This mode of preservation is characteristic for the Vyskočilka area in the late Wenlock (Homerian, T. testis Zone ; unpublished data) and early Ludlow (Gorstian, C. colonus and early L. scanicus zones; Manda and Kříž 2007) strata.

Material.— Besides the holotype from Barrande (1866), six incomplete specimens with missing apex (NM−L 571, CGU SM 318–323 ), a body chamber (CGU SM 317 ), and a fragment of a body chamber (CGU SM 316 ) are available .

Descriptions.—Shell very slightly curved, exogastric. Angle of expansion low, decreasing with shell growth; in early shell it is about 8 °, at fully−grown shell about 4–5 °. Cross section elliptical, laterally compressed, height/width ratio varies between 1.2–1.5. Siphuncle ventral, thin with diameter about 0.2 mm at shell height 2.5 mm. Septal necks very short, cyrtochoanitic, connecting rings very thin and very weakly expanding within chambers; on the ventral side they are in contact with shell wall. Suture oblique to the shell axis, with distinct lateral lobes and ventral and dorsal saddles; ventral saddle is deeper than dorsal. Septa moderately concave, with maximum depth is in shell axis; depth of the septa is about 1/5–1/6 of shell height. Phragmocone chambers very low, distance of septa varies only slightly; ratio of shell height and phragmocone chamber length varies between 4.7–6.2. Shell with fine growth lines or smooth. Body chamber relatively short, length of the body chamber is less than 1/4 of shell length. Aperture open, at fully−grown shell very slightly contracted, hyponomic sinus absent. The shell thickness increases up to 0.25 mm (at height 6.8 mm). Maximum measured shell height 7 mm, estimated total shell length 45–50 mm.

Discussion.—Barrande (1866) figured another shell on pl. 148 as figs. 20–23, which he described as “ Cyrtoceras pollux var. castor ” (holotype by monotypy; NM−L 571; type locality Butovitz e1, i.e., Praha−Butovice, Na břekvici Section, see Kříž 1992; Ludlow, Gorstian, early C. colonus Zone ; see Fig. 10A ). According to Barrande (1874), “ Cyrtoceras pollux var. castor ” differs from Cyrtoceras pollux by having a less compressed cross section and slightly deeper ventral and lateral saddles at the suture. The holotype of Cyrtoceras pollux is

2 mm 2 mm

slightly flattened by diagenetic compaction and has slightly deformed original cross section dimensions ( Fig. 10B 1 ). Despite the malformation of the sutures in the adoral part of the phragmocone in the holotype of “ Cyrtoceras pollux var. castor ” the sutures and spacing between them are similar in dimension to those in Cyrtoceras pollux ( Fig. 10A 1 ). Consequently, suture and cross section differences are the result of shell malformation and diagenesis and thus, “ Cyrtoceras pollux var. castor ” is considered synonymous with C. pollux .

Silurian species described by Barrande (1866, 1867) as Cyrtoceras pollux and Cyrtoceras pollux “ var. castor ” exhibit a very similar morphology to the Ordovician species Pomerantsoceras tibia . These species shared the small slightly curved gently expanding exogastric shell, thin marginal siphuncle, very weakly vaulted connecting rings, laterally compressed shell, relatively short body chamber, very short phragmocone chambers, and sutures with a wide lateral lobe. The Silurian Pomerantsoceras pollux differs from P. tibia in having cyrtochoanitic rather than achoanitic (see Kröger 2007) septal necks. Despite difference in the shape of septal the necks both of these species are added to genus Pomerantsoceras .

Occurrence.—Silurian of the Prague Basin, Bohemia ( Czech Republic). Wenlock, Homerian, T. testis Zone ; Motol Formation; Praha−Butovice, Kační Quarry ( Fig. 1 , for description see Kříž 1999). Specimen CGU SM 319 was found in a nodule of dark grey cephalopod−graptolite packstone, together with bivalves Isiola lyra , Cardiola agna ; graptolite Monograptus flemingi ; and cephalopods Arionoceras sp. , Michelinoceras sp. , Parakionoceras cf. originale , and Pseudocycloceras duponti .

Ludlow, Gorstian, Colonograptus colonus Zone (i.e., Neodiversograptus nilsonni Zone ); Kopanina Formation; Praha−Butovice, Na břekvici Section (see Kříž 1961, 1992, 1999; Kříž et al. 1993). Specimens CGU SM 316–318. Cephalopod fauna from this locality was described by Barrande (1865– 1977) and perhaps represents the most diverse cephalopod assemblage in the Prague Basin. It includes among others Sphooceras truncatum, Disjunstoceras disjunctum, Parakionoceras originale , Ophioceras rudens , Peismoceras pulchrum , Uranoceras uranus , Rizoceras robustum , Phragmoceras imbricatum , and Pseudocycloceras duponti . Cephalopods occur in nodules of cephalopod wacke−packstone.

Ludlow, Ludfordian, Monograptus latilobus Zone ; Kopanina Formation; Praha−Lochkov, Nad ubikacemi Section. Specimens of P. pollux were found in a 50 cm thick bed of grey cephalopod packsone corresponding with the upper part of “ Ananaspis fecunda Horizon ”. Rare bivalves are present which belong to the Cardiola conformis Community ( Kříž 1998). Among associated cephalopods, Arionoceras sp. , Michelinoceras michelini , Parakionoceras originale , Peismoceras optatum , Dawsonocerina dulce , Sphooceras sp. , Pseudocycloceras agassizi , Ophioceras simplex , and Oonoceras imperiale are characteristic.

Figures

Fig. 4. Pomerantsoceras pollux (Barrande, 1866). Specimen CGU SM 319, the body chamber, in lateral (A), ventral (B), and dorsal (C) views. Kační Quarry, Testograptus testis Zone, Homerian, Wenlock. Photographed in alcohol.

Fig. 5. Pomerantsoceras pollux (Barrande, 1866). Specimen CGU SM 322, in lateral (A, C), dorsal (B), and ventral (D) views. Lochkov, Nad ubikacemi Section, Monograptus latilobus Zone, Ludfordian, Ludlow. Photographed in alcohol.

Fig. 6. Pomerantsoceras pollux (Barrande, 1866). Specimen CGU SM 323, the body chamber, in dorsal (A), lateral (B), ventral (C), and lateral (D) views. Lochkov, Nad ubikacemi Section, Monograptus latilobus Zone, Ludfordian, Ludlow. Photographed in alcohol.

Fig. 7. Pomerantsoceras pollux (Barrande, 1866). Specimen CGU SM 321, in lateral (A, C), ventral (B), and dorsal (D) views. Lochkov, Nad ubikacemi Section, Monograptus latilobus Zone, Ludfordian, Ludlow. Photographed in alcohol.

Fig. 8. Sketches of the colour pattern in Pomerantsoceras pollux (Barrande, 1866). A. Specimen SM 319, Kační Quarry. B. Specimen CGU SM 322, Nad ubikacemi Section. C. Specimen CGU SM 323, Nad ubikacemi Section. D. Specimen CGU SM 321, Nad ubikacemi Section.

Fig. 9. Shell malformations in Pomerantsoceras pollux (Barrande, 1866). A. Specimen CGU SM 318 with a false rib at the base of body chamber, in lateral (A1) and ventral (A2) views. B. Specimen CGU SM 316, body chamber with conchal furrow−like malformation, in ventral view. C. Specimen NM−L 571, anomalous septal growth, in lateral view (C1), and detail of youngest phragmocone chamber with a pair of pits, in lateral view (C2). All figured specimens come from Butovice Na Břekvici Section, Colonograptus colonus Zone, Gorstian, Ludlow. Each specimen is coated with ammonium chloride.

Fig. 10. A. Holotype of Pomerantsoceras pollux “var. castor” (Barrande, 1866). Specimen NM−L 571, in lateral (A1) and ventral (A2) views, and detail of the siphuncle (A3); Butovice Na Břekvici Section, Colonograptus colonus Zone, Gorstian, Ludlow. B. Holotype of Pomerantsoceras pollux (Barrande, 1866). Specimen NM−L 570, in lateral view showing diagenetic rupture of the shell (B1); and lateral (B2, B4) and ventral (B3) views. Malá Chuchle, Vyskočilka locality, Testograptus testis, Homerian, Wenlock. Specimens A1, A2, and B1 are coated with ammonium chloride. Scale bars 5 mm.

Fig. 1. A. Distribution of Silurian rocks in the eastern part of the Prague Synform and position of named localities (see Kříž 1992, Röhlich 2007). B. Butovice, Kační Quarry−Na břekvici Section, stratigraphy, graptolite zones, lithology (adopted from Kříž 1992, Kříž et al. 1993) and range of Pomerantsoceras pollux. C. Lochkov, Nad ubikacemi Section, stratigraphy, graptolite zones, lithology and range of Pomerantsoceras pollux (Barrande, 1866). Abbreviations: C. lundgreni, Cyrtograptus lundgreni; C. colonus, Colonograptus colonus; N. kozlow., Neocuculograptus kozlowskii, M., Monograptus; P., Pristiograptus.