Acrotreta dissimilis ( 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 : 149-150

Acrotreta dissimilis ( Biernat, 1973)

Fig. 7.

Spondylotreta dissimilis sp. n.; Biernat 1973: 78, pl. 11: 2–9, 12: 1–2. Holotype: ZPAL Bp.XV /10d, ventral valve (W = 1.3, L = 1.1)

Type locality: Wysoczki. Holy Cross Mountains , Poland .

Type horizon: Tremadoc chalcedonites.

Material.—Total of 10 dorsal and 13 ventral valves. Figured ventral valves: RM Br133801 (W = 0.59, L = 0.52, H = 0.38), RM Br133802, RM Br133803 (W = 1.21, L = 0.89, H = 0.6); RM Br133799 (W = 1.03, L = 0.97), RM Br133800 (W = 0.63, L = 0.56, WI = 0.30, LI = 0.05, LS = 0.36, BS = 0.18), RM Br133133804, RM Br133805 (W = 1.06, L = 0.88, WI = 0.40, LI = 0.09, LS = 0.58, BS = 0.36)).

Diagnosis.—Ventral valve procline to catacline, less than 90% as high as long. Ventral pseudointerarea well defined with interridge. Ventral interior with short median septum and apical process enclosing interior pedicle tube. Muscle fields and mantle canals of both valves poorly defined. Dorsal pseudointerarea occupying less than half of valve width. Median buttress poorly defined to absent. Dorsal median septum very low and narrow.

Fig. 7. A–O. Acrotreta dissimilis (Biernat) .

(A A,, K ×. 34 Dorsal), detail valve of RM larval Br shell 133799 (K,, exterior × 160). B. Dorsal valve RM Br133800, interior, × 55. C, I, J, L, M. Ventral valve RM Br133801, exterior (C, × 54), detail of larval shell (I, × 169), oblique posterior view (J, × 85), detail of larval shell (L, × 138), larval pitting (M, × 460). D, G, O. Ventral valve RM Br133802, oblique lateral view (D, × 41), oblique posterior view (G, × 19), detail of pustulose ornamentation (O, × 92). E. Ventral valve RM Br133803, ventral interior, × 28. F. Dorsal valve RM Br133804, interior, × 28. H. Dorsal valve RM Br133805, interior, × 33. N. Ventral valve RM Br133806, detail of larval pitting, × 350. All specimens from the Tremadoc chalcedonites, Wysoczki.

Description.—Shell somewhat transversely oval, around 75–95% as long as wide (N = 5). Ventral valve usually slightly procline to catacline, less than percent 80% as high as long (N = 2). Ventral pseudointerarea well defined with interridge. Pedicle foramen elongate oval, around 50 µm across, lacking exterior tube. Ventral interior with short median septum and apical process enclosing interior pedicle tube, occupying somewhat less than 50% of valve height (N = 1). Muscle fields and mantle canals of both valves poorly defined.

Dorsal valve flattened to slightly concave. Dorsal pseudointerarea occupying less than 50 percent of valve width (N = 2), with wide median groove and apsacline propareas. Median buttress poorly defined to absent. Dorsal median septum very low and narrow, extending for around 60–70% of total length (N = 2).

Larval shell of both valves close to circular, about 0.20–0.25 mm across. Ornamentation of both valves with strongly developed fila with fine pustules (less than 10 µm across), forming irregularly spaced radiating lines.

Discussion.—This species was referred to Acrotreta by Holmer and Popov (1994), and the new material described here supports this view. A dissimilis is the earliest known species of the genus, and differs from all other species in having a less conical ventral valve, with a much shorter and lower ventral median septum and apical process (see also Biernat 1973: pl. 11: 3). Moreover, the mantle canals and muscle imprints of both valves are poorly impressed by comparison with all other species of Acrotreta .