Neoschizodus laevigatus ( Goldfuss, 1837 )

Foster, William J., Danise, Silvia & Twitchett, Richard J., 2017, A silicified Early Triassic marine assemblage from Svalbard, Journal of Systematic Palaeontology 15 (10), pp. 851-877 : 864-865

Neoschizodus laevigatus ( Goldfuss, 1837)

( Fig. 9 )

Material. Five specimens from LD-04 ( NHMUK PI MB 1251 ; NHMUK PE PEI 5489; NHMUK PE PEI 5519; NHMUK PE PEI 5522), and eight specimens from LD-05 ( NHMUK PE PEI 5476–5477 ; NHMUK PE PEI 5499; NHMUK PE PEI 5501; NHMUK PE PEI 5518; NHMUK PE PEI 5521). Fifty-nine prodissoconch valves from LD-04 ( NHMUK PI MB 1252 ; NHMUK PI MB 1254 ; NHMUK PI MB 1255–1260 ; NHMUK PE PEI 5480; NHMUK PE PEI 5484; NHMUK PE PEI 5486; NHMUK PE PEI 5511; NHMUK PE PEI 5513; NHMUK PE PEI 5520), and 14 prodissoconch valves from LD-05 ( NHMUK PI MB 1253 ; NHMUK PE PEI 5514) .

Description. Shell is trigonally subovate, equivalve, inflated below the umbo, inequilateral, slightly higher than long. Umbo is small and orthogyrate, with an elevated, subangular to rounded, umbonal ridge. Indistinct posterior ridge. Anterodorsal margin recurvate, passing to widely arched ventral margin, posterodorsal margin straight, entire inner margin smooth. Smooth sculpture except for faint concentric growth lines.

The larval shells have a similar external morphology to the adult shells, with a short and subumbonal hinge and short, narrow nymphs running down the anterior and posterior margins. The prodissoconch is smooth with fine concentric growth lines.

Remarks. Neoschizodus laevigatus is a cosmopolitan Early and Middle Triassic species with high variability in its morphological characters. The shell shape and indistinct posterior ridge mean that the present specimens agree with the characters of N. laevigatus , and similar specimens identified from the Early Triassic (e.g. Kumagae & Nakazawa 2009; Hautmann et al. 2011).

One of the adult specimens preserves a prodissoconch ( Fig. 9I ) and its morphology is identical to that of the other prodissoconchs that were found as isolated specimens. The hinge plate of the prodissoconchs was not observed to be myophorian – instead the hinge dentition is typical of a taxodont – but it is not known how a myophorian hinge plate develops through ontogeny ( Newell & Boyd 1975). Hautmann & Nutzel (2005) suggested that, in bivalves, the presence of a small prodissoconch I and a relatively large prodissoconch II indicates a planktotrophic larval stage. Therefore, a planktotrophic larval stage is interpreted for these specimens.

Mode of life. Shallow infaunal, facultatively motile, unattached, suspension feeder ( Hautmann et al. 2013).

Figures

Figure 9. Neoschizodus laevigatus (Goldfuss, 1837). A, NHMUK PI MB 1251, external view, left valve; B–G, J, larval shells; B, NHMUK PI MB 1256, external view, right valve; C, NHMUK PI MB 1259, external view, left valve; D, NHMUK PI MB 1257, external view, right valve; E, NHMUK PI MB 1260, internal view, left valve; F, NHMUK PI MB 1258, internal view, left valve; G, NHMUK PI MB 1255, internal view, right valve; H, I, beak of adult specimen, NHMUK PI MB 1251; J, hinge plate of a larval shell, NHMUK PI MB 1260. Scale bars: A = 2 mm; B–G, I = 200 µm; H = 1 mm; J = 50 µm.