SILESIACA, Skawina & Dzik, 2011

Skawina, Aleksandra & Dzik, Jerzy, 2011, Umbonal musculature and relationships of the Late Triassic filibranch unionoid bivalves, Zoological Journal of the Linnean Society 163 (3), pp. 863-883 : 878

TIHKIA (?) SILESIACA SP. NOV.

( FIGS 10 , 11 )

Holotype: ZPAL V.33/286 ( Fig. 10H, I ).

Type locality: Lipie Śląskie-Lisowice, Opole Silesia, Poland .

Type horizon: Dark-grey marly claystone lens within fluviatile fine sandstone unit of early Rhaetian age.

Etymology: After the region of Silesia.

Diagnosis: Elongated shell of medium size, reaching 80 mm, and generalized morphology, with juvenile stage bearing concentric ribs parallel with the mantle margin. A set of about four small muscle scars tightly arranged in a line near the top of the umbo.

Material: About 100 specimens in the ZPAL collection; two of them with preserved gills .

Description: Shell length almost three times its height. Its largest inflation is at 1/3–1/4 of its length from the anterior end. The shell wall is relatively thin, except for the region of cardinal teeth. The umbones are rather prominent, situated approximately one-third of the width from the anterior end. In at least one specimen, ZPAL V.33/305, dark mineralized gills of morphology similar to those in S. parvus sp. nov. are visible, although not well enough preserved to prove that they are filibranch. The presumably juvenile specimen from Marciszów ( Fig. 10A–D ) shows juvenile ornamentation that is similar to that in S. parvus sp. nov., although less prominent. Its umbonal musculature is indistinguishable from that of the Lisowice specimens.

Remarks: Specimens are preserved mainly as internal moulds or internal moulds of closed shells in marly limestone concretions, with the shell preserved as palimpsest on their surface or, rarely, as impressions of opened shells in claystone. The species differs from S. parvus sp. nov. in a proportionally much thicker shell wall, larger mature size of a less swollen shell, not so anteriorly situated umbo, compact distribution of the umbonal muscle scars, and rather indistinctly impressed pallial line, with no detectable mantle muscle scars (this may have resulted from a less perfect preservation). Tihkia silesiaca sp. nov. resembles the type species of the genus T. corrugata Sahni & Tewari, 1958 ( Sahni & Tewari, 1958) from the Late Triassic Maleri Formation in size, shape, and shell surface ornamentation, but differs in slightly less swollen shell and a thinner wall. The differences in known aspects of the shell morphology are not great, but no data on umbonal musculature are available for the Indian species. It seems rather unlikely that they are similar in this respect because of the significant time span separating them (probably about 20 Myr). The same applies to T. karooensis ( Cox, 1932) from the Manda Beds at Gingama, Songea District of Tanzania. The hinge structure remains unknown, although one of the type specimens shows slightly displaced valves partially exposing the hinge region that seems similar to that of the Indian species. Some juvenile specimens in the type series are only slightly corroded at their apices, but no juvenile shell ornamentation is visible. Even less corroded specimens in the same NMNH collection from Ntawere Village, 5 km north of Katumbi, upper Luangwa River valley, Zambia, also seem to have smooth apices. C. B. Cox (1969) suggested a Mid Triassic age for both the Ntawere Formation (early Anisian) and the Manda Formation (late Anisian) based on vertebrates unknown outside Gondwana, where all the marine stratotypes of the Triassic units are located. This date has been accepted by Nesbitt et al. (2010), although their identification of Silesaurus , shared with the Krasiejów fauna, which can be directly correlated with the Alpine Carnian, instead suggests a Late Triassic age of the Manda Formation bivalves.

Figures

Figure 10. Tihkia silesiaca sp. nov. from the latest Triassic of Silesia (A–D) and Lisowice (E–H; external views of isolated valves and the natural mould). A–D, right valve with wall transformed into sparitic calcite ZPAL V33/310 from Marciszów: A, magnification of umbo showing concentric juvenile ribs; B–D, external, inner, and dorsal views of the specimen. E–F, moulds of right valve ZPAL V33/299 and left valve ZPAL V33/301 from Lisowice. G–H, holotype, mould of articulated valves ZPAL V.33/286 in lateral and dorsal views, respectively.

Figure 11. Tihkia silesiaca sp. nov. from the latest Triassic of Lisowice: restoration of the external view of right valve (A) and dorsal muscle attachments (B).