Mytoconula, Horný, 2009

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 : 30-34

Genus Mytoconula gen. n.

T y p e s p e c i e s: Mytoconula vonkai sp. n., Middle Ordovician, upper Darriwilian, Dobrotivá Formation; Bohemia, Barrandian Area.

D e r i v a t i o n o m i n i s: Ater the village of Mýto near the type locality, with suffix -conula (Lat.), a small cone. Feminine.

D i a g n o s i s: Genus of the family Protoconchoididae with low, conical, thin-shelled conch, with apex shifted about a half way between the shell centre and the anterior apertural margin; early shell is narrower, sharply separated by a groove from the wider teleoconch; aperture planar, widely ovate in outline, l/w ratio 1.12–1.39; fine radial striation of shell interior; concentric muscle zones disintegrat- ed into a series of small scars.

D i s c u s s i o n: Dense radial striation of the internal shell surface may represent an ancestral type of musculature of tergomyan molluscs, traceable in several early Palaeozoic genera – e. g. Pilina KOKEN in KOKEN et PERNER, 1925 ( P. liaoningensis YU WEN et YOCHELSON, 1999 ); Bipulvina YOCHELSON, 1958 ( B. croftsae YOCHELSON, 1958 ); Drahomira BARRANDE in PERNER, 1903 ( D. kriziana HORNÝ, 2005 ), and even the cyrtonellids – Sinuitopsis PERNER, 1903 ( S. neglecta BARRANDE in PERNER, 1903). In this connection Cambridium HORNÝ, 1957 should also be mentioned. This middle Cambrian genus, originaly a representative of the order Cambridiida HORNÝ in KNIGHT et YOCHELSON, 1960, may be classified as an ancient tergomyan with strongly emphasized radial structure.

Patelliconus osloensis YOCHELSON, 1977 from Upper

Ordovician of Norway (Grimsöy Formation, upper Kat-

ian) has a radially striated internal shell surface. Yochelson (p. 311) describes this structure as closely spaced, irregular ridges near the margin, which may have been areas of blood vessels leading to the mantle margin or may have been connected with attachment of the mantle to the shell. According to his fig. 2K, these ridges bifurcate or trifurcate adaper-

turally, being similar to structures in Pilina liaoningensis

YU WEN et YOCHELSON, 1999 or e. g. Cambridium nikiforovae HORNÝ, 1957 . For these reasons, in spite of the subcentral apex, I prefer to classify this species as Mytoconula osloensis ( YOCHELSON, 1977) .

S p e c i e s i n c l u d e d: Mytoconula vonkai sp. n.,

Middle Ordovician, upper Darriwilian, Dobrotivá Formation; Bohemia, Barrandian Area.

Mytoconula vonkai sp. n.

Text-figs 5–8

H o l o t y p e: specimen NM L 31983, figured here in

Text-fig. 5.

P a r a t y p e: specimen NM L 31984, figured here in

Text-fig. 6.

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

Darriwilian, Dobrotivá Formation.

T y p e l o c a l i t y: Mýto near Holoubkov.

D e r i v a t i o n o m i n i s: Named after ing.

Vladimír Vonka, collector and palaeontology enthusiast ,

who found important specimens for our understanding of morphology.

M a t e r i a l: 22 specimens, including the type specimens, deposited in collections of the National Museum , Prague .

D i a g n o s i s: see the genus.

D e s c r i p t i o n: The low cone-like shell is widely ovoid in outline with a subcentral, anteriorly shifted apex. The average length:width ratio is 1.25 (14 specimens measured). The smallest specimen, NM L 38765, is 4.20 mm long, 3.5 mm wide, and 0.6 mm high, the largest, NM L 38761a, is 12.10 mm long and 10.8 mm wide. The growth structures indicate that the early shell aperture had an almost rounded outline (NM L 38764b). The apex is shifted towards the apertural margin, interpreted as anterior. The early shell has steeper sides, about 90° (NM L 38768); apical angle of mature shells reaches 120°. Both the anterior and posterior apertural margins are rounded. The anterior side is steeper than the posterior, which is shallowly convex between the apex and posterior margin in mature specimens. The apertural margin is planar, sharp, periodically slightly flaring in mature specimens (NM L 38914). The shell wall is very thin, maximum thickness being about 0.05 mm. Its composition and structure are unknown, being weathered and substituted by limonite. External sculpture consists of simple, somewhat irregular and unequal growth lines (NM L 38914, Text-fig. 5d ). Two or three periodical rugae corresponding to the location of concentric muscle zones are developed predominantly in the posterior area of shell (NM L 38914); these structures are weak or absent in the majority of specimens studied.

As the shells are small and the shell wall is thin, the muscle insertions are rather small and indistinct. Furthermore, the weathered structure of the surface of internal moulds is not smooth enough to reflect such fine details. Several specimens preserved as internal moulds showed numerous fine radial ridges (grooves on the internal shell surface) running from the periphery of the early shell and almost reaching the apertural margin (NM L 38914, NM L 38767, NM L 31983; Text-fig. 5 ). When crossing the periodical rugae of the shell, these weak ridges increase in strength and breadth and have the character of short muscle scars (NM L 31893, NM L 38764c, Text-fig. 7 ). Two slightly stronger diverging ribs were developed in the posterolateral area (NM L 31893, NM L 31894, NM L 38765; Text-fig. 6 ). The pallial line and associated structures have not been ascertained.

Figures

Text-fig. 5. Mytoconula vonkai sp. n. The holotype NM L 31983, internal mould. A – apical, B – right apical and right antero-lateral views, both with vis- ible series of muscle scars in two raised zones, ×......;. Note the two long divergent scar zones in A. Dobro- tivá F., Mýto near Holoubkov.

Text-fig. 6. Mytoconula vonkai sp. n. Specimen NM L 31984, internal mould. A – apical, B – right lateral views, ×...... and ×....... Note the out- line of an indeterminate object on which the larva settled. This specimen, embedded in a siliceous concretion, lied closely near the holo- type NM L 38914. Dobrotivá F., Mýto near Holoubkov.

Text-fig. 7. Mytoconula vonkai sp. n. Counter- part of specimen NM L 3876c, apical view showing weathered radial structures; A – ×......, B – ×....... Dobrotivá F., Malé Přílepy.

Text-fig. 8. Mytoconula vonkai sp. n. Specimen NM L 38914, internal mould with patches of shell. A – apical view, ×......; B – oblique posterior view showing radial ribs on internal mould, ×......; C – anteri- or part with a large patch of shell with irregular incremental structures, ×......; D – left lateral view, ×......;. Dobrotivá F., Malé Přílepy.