Silesunio parvus, Skawina & Dzik, 2011, 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 : 874-877

SILESUNIO PARVUS GEN. ET SP. NOV.

( FIGS 1 , 2A , 7–9 )

Holotype: ZPAL Ab/ III 2210 ( Fig. 2A ).

876 A. SKAWINA and J. DZIK

Type locality: Krasiejów, Opole Silesia, southern Poland .

Type horizon: Lacustrine grey claystone and red finely grained mudstone bed within red-coloured fluviatile series of Late Carnian calcareous mudstones ( Dzik & Sulej, 2007).

Etymology: The species name refers to the small size of specimens (latin parvus , meaning little, tiny).

Diagnosis: Elongated shell of small size does not exceed 50 mm and generalized morphology, with juvenile stage bearing concentric ribs parallel with the mantle margin. Umbonal muscles tend to disperse over the anterior slope of the beaks.

Material: Several hundred specimens of various preservation in the ZPAL collection; five of them with preserved gills.

Description: Shell length usually more than two times its height (ratio ranging from 1.9 to 2.9). Its largest inflation is approximately at the midlength of the valve. Shell wall moderately thick, with greatest thickness in the region of the cardinal teeth. The umbones are prominent, located at about 1/4–1/5 of the length of the shell from its anterior end. Umbonal musculature is clearly recognizable on both valves. Separate small scars are distributed on the top, anterior slope, and over the interior side of each umbo, varying in number from 2 to 16 on each valve.

Remarks: Although the length of imprints of bivalve shells in the Krasiejów lacustrine bed vary from 10 to 50 mm, most specimens are open valves of lengths below 20 mm. Apparently this is a result of high juvenile mortality in the time-averaged fossil assemblage, to which dead shells were steadily contributed. Moulds of mostly closed shells forming calcareous concretions are generally larger than their imprints in clay, and their length vary from over 20 to 50 mm. These fossils probably originated as a result of a catastrophic covering of a living population with suspended sediment. Mature shells can be distinguished from juveniles by the thickening of the shell margin and condensation of growth increments.

Specimens of this species are similar to Tihkia silesiaca sp. nov. in shape, position of the main muscle scars, and in distribution of concentric growth lines on the shell, but differ in much larger mature size and dispersed, highly variable umbonal muscle scars, in a more swollen shell, more anteriorly located umbo, as well as better developed pallial line, and sometimes visible mantle muscle scars on the surface of the shell. Some of these differences result from the relatively thicker shell wall. This species differs from species of Tihkia in small size of mature specimens. It is almost coeval with the Maleri Formation fauna, which is generally similar to that of Krasiejów, despite the large geographic distance between them ( Dzik & Sulej, 2007).

Antediplodon lewisi Richards, 1948 from the Norian Stockton Formation of Pennsylvania at Montclare near Phoenixville, Montgomery County, Pennsylvania ( Richards, 1948), shows shell shape and size (44 ¥ 17 mm) similar to S. parvus sp. nov. With a lack of information on beak ornamentation or musculature, the generalized shell outline cannot be used as the only basis for species identification.

Figures

Figure 1. Phosphatized gill supports in Silesunio parvus gen. et sp. nov. from the early Late Triassic lacustrine deposits at Krasiejów, Southern Poland. A, general view of specimen ZPAL AbIII/2208 with valves in ‘butterfly position’; gills visible as darker striated structures owing to staining with dark iron minerals. B, displaced sets of gill supports show bending independent of each other owing to the lack of interconnections (which proves a filibranch organization). C, D, specimen ZPAL AbIII/2168 with a more typical preservation of gill blades.

Figure 2. Beak sculpture on the shell of various unionoids. A, juvenile holotype specimen ZPAL AbIII/2210 of Silesunio parvus gen. et sp. nov. from the early Late Triassic of Krasiejów. B, juvenile recent Margaritifera margaritifera from a stream in Germany (specimen collected and supplied by Reinhard Altmüller). C, recent Unio tumidus from Krutynia River, Poland. D, Anodonta anatina from the same site.

Figure 7. Silesunio parvus gen. et sp. nov. from the early Late Triassic of Krasiejów: ‘butterfly style’ opened valves. A, ZPAL AbIII/2164; B, ZPAL AbIII/2167.

Figure 8. Silesunio parvus gen. et sp. nov. from the early Late Triassic of Krasiejów: restoration of the interior of left valve (A), external view of right valve (B), and ventral view of the hinge and dorsal muscle attachments (C).

Figure 9. Diagrammatic presentation of variability in disposition of the umbonal musculature in Silesunio parvus gen. et sp. nov. from the early Late Triassic of Krasiejów; camera lucida drawings. Specimens ZPAL AbIII: A, B, 2161–2162; C, D, 2165–2166; E, 2176, F, 2185; G, 2199; H–O, 2199–2205.