Diencobolus subovalis ( Biernat, 1973 )

Holmer, Lars E. & Biernat, Gertruda, 2002, Urban and Peri-urban small and medium-size Enterprise Development for sustainable Vegetable Production and Marketing Systems, Acta Palaeontologica Polonica 47 (1), pp. 141-156 : 144-145

Diencobolus subovalis ( Biernat, 1973)

Fig. 3.

Orbiculoidea subovalis sp. n.; Biernat 1973: 103, pl. 3: 8–14, fig. 38. Holotype: ZPAL Bp.XV / 65m, dorsal (?) valve (W = 1.80, L = 2.00) Type locality: Wysoczki. Holy Cross Mountains , Poland.

Type horizon: Tremadoc chalcedonites.

Material.—Total of 3 ventral and 4 dorsal valves in addition to 4 indeterminate valves. Figured ventral valve: RM Br133776; dorsal valves: RM Br133774, RM Br133775, RM Br133777 (W = 1.36, L = 1.59), RM Br133778.

Diagnosis.—Shell moderately biconvex, equivalved, elongately suboval in outline, holoperipheral growth in both valves; both valves externally very similar, maximum height at submarginal umbo. Ventral interior with short elevated visceral area.

Description.—Shell biconvex, equivalved, elongately suboval in outline, somewhat more than 110% as long as wide (N = 2), holoperipheral growth in both valves. Both valves externally very similar, maximum height at the submarginal, slightly swollen umbo, about 8–10% of the valve length from the posterior margin (N = 2). Concentric ornamentation lamellose. Larval and post−larval shell of both valves finely pitted, with closely packed rounded pits about equal size up to about 4 µm across. Ventral interior with short, elevated visceral area, extending to about 1/3 of valve length (N = 1), and bearing two muscle scars. Dorsal interior characters poorly defined. Well−defined marginal limbus, and strong concentric ornament

Remarks.—This species was referred questionably to the discinid genus Orbiculoidea by Biernat (1973), and all valves were assumed to be dorsal by comparison with similarities in shell shape and ornamentation of the dorsal valves of many Palaeozoic discinids. However, the new material includes two different valves, with virtually the same external morphology but different interior characters. One type of valve has an elevated visceral platform with two muscle scars, quite similar to the ventral visceral platform of many Palaeozoic linguliformeans. Holmer et al. (2001) described Diencobolus simplex from the Upper Cambrian of Kazakhstan, and this species is clearly similar to the Polish form in having dorsal and ventral valves that are almost indistinguishable, lacking both pseudointerareas and a pedicle opening, and possessing a pitted micro−ornamentation. D. naukatensis (Holmer et al. 2000) from the early Arenig of Kirgizia, is characterized also by this very simple morphology. D. subovalis differs from both these species in having a more biconvex shell with a submarginal apex in both valves, as well as an elevated ventral visceral area and lamellose ornamentation. The shell structure of D. subovalis is not well preserved, but appears to be baculate ( Fig. 3N).