Kyrshabaktella diabola, 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 : 26-28

Kyrshabaktella diabola n. sp.

Figure 14

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Holotype. Ventral valve NFM F-2517 from Devils Cove member, Mount St. Margaret, sample MSM- 3.

Etymology. After Latin Diabolus in reference to the derivation of the majority of specimens from the Devils Cove member in western Newfoundland.

Diagnosis. Species of Kyrshabaktella with biconvex, sub-triangular to oval shell. Ventral valve with narrow pseudointerarea with parallel-sided pedicle groove and high, raised propareas without flexurelines. Dorsal valve with wide, triangular pseudointerarea dominated by slightly raised diamond-shaped median groove separated from raised propareas by sharp folds. Dorsal valve interior with low, tongue-like median ridge expanding into a low platform anterior of mid-valve.

Differ from all other species of Kyrshabaktella by the narrow ventral pseudointerarea with paralellsided pedicle groove and elongated, curved propareas, and by the wide dorsal pseudointerarea with raised, diamond-shaped median groove.

Material. Three ventral and one dorsal valve from the Devils Cove member of the Forteau Formation of western Newfoundland (MSM-3, MSM-5, ECP- 3, ICS 1520) and a single dorsal valve from the lower Forteau Formation of southern Labrador ( ICS 1575).

Description. Slightly biconvex brachiopod shell with sub-circular to oval outline ( Figure 14.1-9 ); shell length and width about 2-3 mm. Apex of both valves marginal, ventral valve with hemicircular, posteriorly open emarginature, about 0.2-0.3 mm wide ( Figure 14.2-4 ). Larval shells poorly differentiated but appear to be sub-circular. Valve exterior otherwise ornamented with fine concentric ridges ( Figure 14.1-2 ). Orthocline ventral pseudointerarea narrow and elongated along a substantial portion of the posterolateral shell margins, up to about a third of valve length ( Figure 14.3 ). Pedicle groove narrow, almost parallel-sided, and continuous with valve floor. Propareas raised strongly above valve floor but without flexure lines. Dorsal pseudointerarea orthocline and sub-triangular, representing about 50% of valve width and 20% of valve length ( Figure 14.9-10 ). Median groove wide, concave, and slightly raised above valve floor. The median groove is extended slightly anteriorly over the median ridge, resulting in a diamond-shape ( Figure 14.9 ). The dorsal propareas are narrow but strongly raised above valve floor and differentiated from the median groove by sharp folds.

Ventral valve interior poorly preserved in available specimens. Dorsal valve interior with slightly raised median ridge which expands anteriorly to slightly anterior of mid-valve, into a tongue-like platform ( Figure 14.6-10 ). The anterior termination of the median tongue is cleft by a small indentation and laterally bounded by elongated anterior-lateral musclescars ( Figures 14.8, 14.10 ). Paired, triangular umbonal musclescars are inserted just below the pseudointerarea close to the junction of propareas and median groove ( Figure 14.10 ). Short dorsal vascula lateralia are present between the umbonal musclescars and median tongue ( Figure 14.8 ).

Remarks. Kyrshabaktella diabola is currently only known from a small number of specimens, but is quite distinct from all other species of Kyrshabaktella by the narrow ventral pseudointerarea with elongated, curved propareas, and the dorsal pseudointerarea with wide, slightly raised, and diamond-shaped median groove. The species is probably closely related to K. davidii Holmer and Ushatinskaya in Gravestock et al., 2001 from similarly aged rocks in South Australia ( Betts et al., 2016, 2017) and K. tatjanae Ushatinskaya in Pelman et al., 1992 from the slightly younger Amgan Stage of the Altai-Sayan Foldbelt. Poorly preserved specimens referred to Kyrshabaktella sp. from the lower-mid Dyeran Stage of southern Nevada ( Skovsted and Holmer, 2006) show a similar, diamond-shaped dorsal median groove, but the propareas of both ventral and dorsal valves appear to be shorter than in K. diabola , and the Nevada specimens probably represent a distinct species. Skovsted and Holmer (2006) demonstrated a columnar shell structure in Kyrshabaktella sp. from Nevada, but no details of shell structure could be observed in K. diabola . A single specimen described as Fossuliella ? sp. from the Löndal Formation of North Greenland is similar to K. diabola in shape of the ventral valve and the presence of a ventral emarginature ( Peel et al., 2016). However, nothing is known about the dorsal valve morphology or ventral valve interior of Fossuliella ? sp. and the Greenland specimen also differ from K. diabola in the fine pitting of both larval and adult shell.

Kyrshabaktella diabola appears to be restricted to relatively shallow water facies of the Devils Cove member and Middle shale in western Newfoundland and in the lower Forteau Formation at Fox Cove in southern Labrador; the small number of specimens precludes far reaching statements about its distribution. However, it appears to be present in strata that records the early momentum of shelf deepening and transgression (TST).

Distribution. Assemblage Dyeran Stage (unnamed Cambrian Series 2, stage 4), Devils Cove member and Middle shale, Forteau Formation of southern Labrador and western Newfoundland, Canada.

Figures

FIGURE 14. Kyrshabaktella diabola n. sp. from the Forteau Formation of southern Labrador and western Newfoundland. 1-2, Ventral valve NFM F-2516 from Devils Cove member, East Castor Pond, sample ECP-01; 1, external view; 2, detail of larval shell and resorbed pedicle groove. 3-4, Ventral valve NFM F-2517 (Holotype) from Devils Cove member, Mount St. Margaret, sample MSM-03; 3, internal view; 4, oblique posterior view showing pedicle groove. 5, Ventral valve exterior NFM F-2518 from Devils Cove member, Mount St. Margaret, sample ICS1520. 6-8, Dorsal valve NFM F-2519 from Lower Shale member, Mount St. Margaret, sample MSM-5; 6, internal view; 7, oblique internal view; 8, detail of internal surface showing vascula lateralia and median tongue with muscle scars. 9-10, Dorsal valve NFM F-2520 from the Lower shale, Fox Cove, sample ICS1575; 9, internal view; 10, detail of dorsal interior showing dorsal pseudointerarea and median tongue. All scalebars equal 500 µm except in figure 2 where the bar equals 125 µm