Acrothyra bonnia, Skovsted & Knight & Balthasar & Boyce, 2017

Skovsted, C. B., Knight, I., Balthasar, U. & Boyce, W. D., 2017, Depth related brachiopod faunas from the lower Cambrian Forteau Formation of southern Labrador and western Newfoundland, Canada, Palaeontologia Electronica (Oxford, England: 1990) 17 (15), pp. 1-52 : 39

Acrothyra bonnia n. sp.

Figure 20

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Holotype. Ventral valve NFM F-2562 from the Forteau Formation of western Newfoundland, Deer Arm Limestone, sample GM07-5-1 ( Figure 20.7-9 ).

Material. 114 ventral and 52 dorsal valves from the Forteau Formation of the Bonne Bay area, western Newfoundland (GM07-2-1, GM07-4-1, GM07-5-1, GM11-5-E, GM11-8A-1, GM11-14 G-C, ICS 1518, ICS 1519, JSP 1982−01).

Diagnosis. Species of Acrothyra with short apsacline ventral pseudointerarea with a triangular intertrough. Ventral valve interior with elongate apical process reaching to about 1/3 of valve length and with a triangular depression but without apical pits. Differ from A. proavia ( Matthew, 1899) and A. signata Matthew, 1902 by the much shorter ventral pseudointerarea, from A. sera Matthew, 1902 and A. comleyensis Cobbold, 1921 by the apsacline ventral pseudointerarea and from A. minor Walcott, 1905 by the straight and slightly longer ventral pseudointerarea and the more rounded valve outline.

Etymology. From the occurrence of the species in the vicinity of Bonne Bay in western Newfoundland.

Description. Ventribiconvex acrotretid shells. Ventral valve sub-triangular in outline and low cone-shaped ( Figure 20.1-2, 20.7 ). Greatest width at about mid-valve. Ventral pseudointerarea apsacline, relatively short and inclined by about 75° with a triangular intertrough expanding toward the posterior margin ( Figure 20.2, 20.4, 20.6, 20.12-13 ). Pedicle foramen situated just below apex and not completely enclosed within larval shell ( Figure 20.3 ). Dorsal valve rounded sub-rectangular in outline with marginal umbo and greatest width anterior of mid-valve ( Figure 20.5-6 ). Dorsal pseudointerarea with a wide, triangular median groove, and narrow propareas ( Figure 20.14 ). Exterior surface of both valves ornamented by fine concentric fila ( Figure 20.1, 20.3, 20.5 ).

Ventral valve interior with low, tongue-like apical process reaching to about 1/3 of valve length and expanding anteriorly ( Figure 20.7-12 ). The crest of the process has a shallow elongated triangular depression connected to pedicle foramen by a narrow channel ( Figure 20.8 ). A small arcuate muscle scar is present at the anterior termination of the apical process, and two rounded umbonal muscle scars are situated at the middle of its flanks ( Figure 20.9 ). No apical pits are present. Posterolateral muscle scars are present close to the lateral terminations of the pseudointerarea ( Figure 20.12 ). Dorsal valve interior poorly known but with a long median septum increasing in height slightly anterior of mid valve and rounded cardinal muscle scars close to the posterolateral margin ( Figure 20.14 ).

Remarks. This species is referred to Acrothyra based on the short, apsacline ventral pseudointerarea. It differs from Aphelotreta Rowell, 1980 by the long, tongue-like apical process of the ventral valve and the long dorsal median septum. Acrothyra bonnia n. sp. differs from all other species of the genus by the rounded triangular ventral valve morphology with a relatively short and apparently straight ventral pseudointerarea. The most obvious similarities are to A. minor from the middle Cambrian of Idaho ( Walcott, 1905; Rowell, 1966), but the new species differ both in the shape of the valves and in the development of the pseudointerarea. Acrothyra bonnia is the currently oldest known of the established species of the genus.

Distribution. Assemblage 3. Cambrian Series 2, Stage 4, Forteau Formation of the Bonne Bay area, western Newfoundland, Canada.

Figures

FIGURE 20. Acrothyra bonnia n. sp. from the Forteau Formation of western Newfoundland, Deer Arm Limestone, sample GM07-5-1. 1-3, Ventral valve NFM F-2559; 1, external view; 2, oblique lateral view; 3, detail of larval shell and pedicle foramen. 4, Ventral vave NFM F-2560, oblique lateral view. 5-6, Dorsal valve NFM F-2561; 5, external view; 6, oblique posterior view. 7-9, Ventral valve NFM F-2562 (Holotype), 7, internal view; 8, oblique lateral view of posterior margin and apical process; 9, detail of oblique anterior view of shell interior showing internal opening of pedicle foramen and muscle scars lateral to apical process. 10-11, Ventral valve NFM F-2563, 10, internal view; 11, oblique anterior view. 12-13, Ventral valve NFM F-2564, 12, internal view; 13, detail of ventral pseudointerarea in oblique posterior view showing triangular intertrough. 14, Dorsal view NFM F-2565, oblique lateral view showing low median ridge. All scalebars equal 250 µm except in figures 3, 9 and 13 that equals 100 µm.