Patelliconus HORNÝ, 1961

Horný, Radvan J., 2009, Patelliconus Horný, 1961 And Mytoconula Gen. N. (Mollusca, Tergomya) From The Ordovician Of Perunica, Acta Musei Nationalis Pragae Series B 65 (1 - 2), pp. 25-36 : 26-28

Genus Patelliconus HORNÝ, 1961

partim 1903 Palaeacmaea HALL et WHITFIELD ; J. Perner, p. 27. 1961 Patelliconus HORNÝ : R. Horný, p. 301.

1963 Patelliconus HORNÝ : R. Horný, p. 29.

1977 Patelliconus HORNÝ : E. L. Yochelson, p. 310, 311.

1994 Patelliconus HORNÝ : G. Geyer, p. 81.

1998 Patelliconus HORNÝ : J. Frýda, p. 46.

2002 Patelliconus HORNÝ : R. Horný, p. 76.

2005 Patelliconus HORNÝ : P. Bouchet et al., p. 126.

T y p e s p e c i e s: Palaeacmaea primula BARRANDE in PERNER, 1903. Middle Ordovician, lower Darriwilian, Šárka Formation.

D i a g n o s i s: Genus of the family Protoconchoididae with low, conical, thin-shelled conch; early shell narrower, followed by a change in slope between the early shell and teleoconch; aperture planar, widely ovate in outline, l/w ratio 1.10–1.25; apex subcentral, slightly shifted in anterior direction, projecting; muscle scar circular, narrow, ring-shaped; outer sculpture, concentric growth lines and shallow periodical rugae, corresponding to position of the muscle ring.

D i s c u s s i o n: Patelliconus HORNÝ, 1961 was bas- ed on Palaeacmaea primula BARRANDE in PERNER, 1903 from the early Middle Ordovician (lower Darriwilian) Šárka Formation of the Barrandian Area (Perunica). A very similar middle Cambrian mollusc is Protoconchoides established by Shaw 1962, originally as a subgenus of Scenella BILLINGS, 1872 (type species, Scenella hermitensis RESSER, 1945 ). Geyer (1994), emending Protoconchoides (spelled as Protoconchioides ) as a separate genus, noted its morphological similarity with various Cambrian and Lower Ordovician genera of China and the Siberian Platform, and also with Patelliconus HORNÝ, 1961 (erroneously dated by Geyer as 1963). As the main differences between Patelliconus Horny and this genus, Geyer pointed out its larger size, strongly rugose outer shell surface, and location of the muscle scar, lying in a more central position. These features, alone however, can hardly be recognized as generic ones. The largest specimen of Patelliconus primulus is 11.5 mm long, the length of the largest specimen of P. douli GEYER, 1994 is 9.4 mm. Rugosity of the outer shell surface in Patelliconus is connected with the extremely thin shell, reflecting positions of repeated location of the ring-shaped muscle zone, and may be a better specific character. Thus the position of the muscle ring varies according to the age of the animal.

The representative of Patelliconus from the Upper Ordovician of Norway, P. osloensis YOCHELSON, 1977 , shows radial structures on the internal shell surface which may indicate a closer relationship with the new genus Mytoconula ,.

A close morphologic similarity of the Cambrian Protoconchoides and Ordovician Patelliconus may lead to a synonomy of these genera; nevertheless, this similarity could be a result of a morphologic convergence (see also Frýda, 1998). Patelliconus and Protoconchoides display an important feature, unfortunately rarely completely fossilized: a narrower early shell, followed by wider teleoconch (according to Geyer: “Apex, if preserved, projects as a nipple”; 1994, p. 82; plate I, figs P, Q). This character may be an important diagnostic feature of a higher systematic category.

I n c l u d e d s p e c i e s: Patelliconus primulus (BARRANDE in PERNER, 1903), Middle Ordovician, lower Darriwilian, Šárka Formation; Barrandian Area, Bohemia.

Patelliconus primulus (BARRANDE in PERNER, 1903)

Text-figs 1–4

1903 Palaeacmaea primula Barr. sp.; J. Perner, p. 27.

1903 Palaeacmaea immigrans Barr. sp.; J. Perner, p. 28.

1961 Patelliconus primulus ( Perner, 1903) ; R. Horný, p. 301.

1963 Patelliconus primulus ( Perner, 1903) ; R. Horný, p. 29–31.

1977 Patelliconus primulus (Perner) ; E. L. Yochelson, p. 311.

2002 Patelliconus primulus (Barrande in Perner, 1903); R. Horný, p. 76.

H o l o t y p e: By monotypy, specimen NM L 8425, figured here in Text-fig. 1.

S t r a t u m t y p i c u m: Middle Ordovician, lower

Darriwilian, Šárka Formation.

T y p e l o c a l i t y: Osek near Rokycany.

M a t e r i a l: 6 specimens, incl. the holotypes of

P. primula and P. immigrans .

D i a g n o s i s: Species of Patelliconus HORNÝ, 1961 with a subcentral apex; shell interior almost smooth; a concentric band-like muscle zone periodically migrating towards the apertural margin. L/w ratio 1.10–1.25.

D e s c r i p t i o n: The description published by Horný (1963) is completed by observation based on two new specimens, MBHR 2418 and NM L 39616.

Specimen MBHR 2418, collected by Jaroslav and Petr Kraft at Osek near Rokycany ( Text-fig. 4 ) is a counterpart of an incomplete, immature specimen with preserved cavity left after the apical part of the juvenile shell. The early shell with ovate aperture bears fine, dense comarginal threads and initially a distinct groove then rapidly expanding into the wider teleoconch. The apical angle of the early shell is 70° (l), 90° (w).

Specimen NM L 39616, collected by Štěpán Rak, Jr. at Volduchy – Díly near Rokycany ( Text-fig. 2 ) is a mature specimen, 10.5 mm long and 9.5 mm wide (l/w ratio 1.10), an internal mould with weathered apical part but relatively well-preserved internal shell surface and the apertural margin. It bears three raised concentric rugae corresponding to grooves in the shell interior. There are no visible traces of migration of muscle insertions; the continuous muscle zone probably displaced by saltation as in Pygmaeoconus HORNÝ, 1961 (see Horný 2006a) and in the majority of other cyrtonellids. The adapertural muscle scar is adorally bordered with a thin line which probably represents either an adapertural margin of the band-like muscle scar or, less likely, the pallial line. (Location of the pallial line close to muscle insertions figured by Runnegar and Pojeta, Jr. [1964], fig. 2 in a reconstruction of the Rasetti´s specimen of Scenella sp. ). The posterior slope of the shell bears fine, irregular, radial structures normal to the apertural margin. The ring-shaped muscle zone, located on a raised wall or ridge, is about 0.3 mm wide, flat, probably continuous, without isolated scars of muscle bundles ( Text-fig. 1c ). The aperture is planar, its outline almost circular, the l/w ratio 1.03. The apical angle of adult shell varies between 90–100°. The longitudinal shell profile is asymmetric as in Pygmaeoconus : the slightly shorter side interpreted as anterior is almost straight, the posterior side is gently arched.

Figures

Text-fig. 1. Patelliconus primulus (BARRANDE in PERNER, 1903). The holotype, NM L 8425. A – apical view, B – left lateral view, ×......; C – fragment of shell with weak unequal and irregular collabral growth structures near the posterior margin, ×...... Šárka F., Osek near Roky- cany.

Text-fig. 2. Patelliconus primulus (BARRANDE in PERNER, 1903). Speci- men NM L 5891. A – apical view, B – right lateral view, × ........ Šárka F., Osek near Rokycany.

Text-fig. 3. Patelliconus primulus (BARRANDE in PERNER, 1903). Specimen NM L 39616, internal mould. A – api- cal view, B – oblique left posterolateral view, both ×......; C – right posterolateral view showing the second and third successive positions of muscle band (arrowed), located on top of each elevation (= in the deepest place of the wide depression of shell interior at the periphery of a flat ramp), ×....... Šárka F., Volduchy – Díly near Rokycany.

Text-fig. 4. Patelliconus primulus (BARRANDE in PERNER, 1903). Specimen MHBR 2418, revultex impression. A – apical view showing the early shell and earliest stages of teleoconch. Note patches of shell with irregular incremental structures; B – left lateral view; C – oblique left apicolateral view showing finely ribbed early shell and patches of shell in earliest stages of teleoconch, all ×....... Šárka F., Osek near Rokycany.