Pseudozygopleuridae Knight, 1930 gen.

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 : 867-868

Family Pseudozygopleuridae Knight, 1930 gen. et sp. indet.

( Fig. 12 )

Material. Three larval shells from LD-04 ( NHMUK PI MG 1532–1534).

Description. Teleoconch not observed. Protoconch is conical, elongate and composed of five whorls. The initial whorl is smooth and has a diameter of 0.08 mm. Collabral ornamentation is initiated on the second whorl, continues to the base of the protoconch, and consists of narrow costellae that intersect at or just below the mid-whorl. Costellae are sigmoidal: on the upper part of the whorls they are slightly curved and oriented at 40 Ǫ to the shell axis, and on the lower part of the whorls they are oriented at 200 Ǫ to the shell axis. Growth lines are visible as faint collabral ribs between, and perpendicular to, the costellae. Aperture is circular, with a small columellar fold. Four whorls are present in these specimens.

Remarks. Hoare & Sturgeon (1978) showed that protoconchs of species of Pseudozygopleuridae are very similar, but can be readily differentiated from those of the Zygopleuridae . Diagnostic characters of pseudozygopleurid protoconchs that are present in these specimens include an elongate, conical shape with 3–5 whorls; smooth initial whorl with collabral ornamentation from the second whorl; and narrow opisthocline to sigmoidal transverse costellae that are equally spaced, extend up and below from the suture and curve uniformly to the mid-whorl ( Hoare & Sturgeon 1978). In contrast, protoconchs of the Zygopleuridae have smooth whorls with fine riblets at the sutures (Nutzel & Mapes 2001; Kaim 2004) or straight ribs in the Ampezzopleurinae (Nutzel 1998, 2005). Protoconchs of the Ladinulidae are similar to those of the Pseudozygopleuridae but can be differentiated due to their vertical costellae ( Bandel 2006). These specimens are, therefore, representatives of the Pseudozygopleuridae . No teleoconch is apparently present in these specimens as the sculpture of the prodissoconch is uninterrupted.

Pseudozygopleuridae is primarily a Palaeozoic family. Apart from the Early Triassic specimens described herein, the only other Mesozoic pseudozygopleurid is Plocezyga from the Jurassic of Poland ( Kaim 2004). The specimens attributed to Plocezyga by Kaim (2004), however, lack a protoconch with the diagnostic sigmoidal pseudozygopleurid ornamentation described by Hoare & Sturgeon (1978, 1980). Instead they have a reticulate pattern, shouldered whorls and transverse costae on the protoconch, indicating that they belong to a different genus and family.

Nutzel (1998) used the diameter, ornamentation and whorl number of the protoconch to separate planktotrophic from non-planktotrophic pseudozygopleurid species. The specimens recorded in this study have small protoconch I diameters (0.12–0.14 mm), and the same ornamentation and number of whorls as pseudozgopleurid larval shells interpreted as planktotrophic by Mapes & Nutzel (2009), suggesting that they too had a planktotrophic larval lifestyle. A teleoconch was not developed in any of the specimens, which may indicate that the adults were unable to live in this environment.

Figures

Figure 12. Pseudozygopleuridae gen et sp. indet., larval shell. A, B, NHMUK PI MG 1532; A, apertural view; B, lateral view; C, NHMUK PI MG 1533, apical view; D, side view of initial whorl, NHMUK PI MG 1532. Scale bars = 100 µm.