Eodicellomus elkaniiformis Holmer and Ushatinskaya

Betts, Marissa J., Claybourn, Thomas M., Brock, Glenn A., Jago, James B., Skovsted, Christian B. & Paterson, John R., 2019, Shelly fossils from the lower Cambrian White Point Conglomerate, Kangaroo Island, South Australia, Acta Palaeontologica Polonica 64 (3), pp. 489-522 : 497-498

Eodicellomus elkaniiformis Holmer and Ushatinskaya in Gravestock et al., 2001

Fig. 6 .

1986 Edreja aff. distincta ; Laurie 1986: 451, figs. 5F, 12C–H.

?1988 “Elkaniid-like lingulide”; Rowell et al. 1998: 14, pl. 1: G.

2001 Eodicellomus elkaniformis ; Holmer and Ushatinskaya in Gravestock et al. 2001: 126, pl. 19: 1–4, 6–11 (non 5a–c).

2006 Eodicellomus elkaniformis ; Jago et al. 2006: 414, fig. 4K, L.

2016 Eodicellomus elkaniformis ; Betts et al. 2016: 195, fig. 17I, K–M.

Material.—One dorsal valve from Clast 1, six dorsal valves from Clast 4,40 dorsal valves and ten ventral valves from Clast 5; eleven figured ( SAM P57235–57241). From the Dailyatia odyssei Zone, WPC, Kangaroo Island, South Australia.

Description.—Shells are biconvex to weakly ventribiconvex in profile, but variable in outline, ranging from slightly longer than wide ( Fig. 6B, C ) to equidimensional in juvenile shells to slightly transversely oval in larger shells. Valves of Eodicellomus elkaniiformis from the WPC clasts are relatively large (up to ~ 5 mm width; Fig. 6B, C ), biconvex shells with strongly thickened visceral platforms. Mature shells (normally greater than 2 mm width) are on average 92% as long as wide; maximum shell width at, or just posterior of, mid-length.

Ventral valve with a distinctly acuminate beak ( Fig. 6E 2 , G). Pseudointerarea apsacline, wide, taking up on average 75% valve width. Propareas are well-developed, narrow and subtriangular with curved anterior edges. Propareas have flattened to gently concave surfaces, are thickened distally and are raised above the valve floor ( Fig. 6E–G ). Pedicle groove is deeply set below the level of the pseudointerarea and is defined within the umbo by a distinctive concave triangular plate that does not reach, or is only slightly adpressed to the valve floor ( Fig. 6F ). Flexure lines are well-developed.

Interior of the ventral valve in mature specimens is greatly thickened, developed as a high, raised, visceral platform. Posterior slope of the platform hosts a pair of elongate central muscle scars ( Fig. 6E, F ). Postero-lateral muscle scars narrowly sub-elliptical to kidney-shaped, occurring on variably elevated muscle pads, that in mature specimens form distinctive platforms, raised high above the valve floor ( Fig. 6E, F ). Vascula lateralia gently curved distally and may be deeply impressed ( Fig. 6E ).

Dorsal valve with a rounded posterior margin. Pseudointerarea is anacline, flattened with a broad, triangular median plate ( Fig. 6B–D ). Propareas are long, curved, sometimes enrolled, and very narrow with well-developed flexure lines ( Fig. 6A, D ). Dorsal visceral area is deeply recessed and concave in the posterior half of the valve, gradually thickening and increasing in elevation to form a platform hosting a pair of central muscle scars ( Fig. 6A–D ). Median ridge develops just posterior of mid-valve, directly between the raised central muscle scars, and extends and widens anteriorly of central muscle scars ( Fig. 6A, D ). Postero-lateral muscle scars are elongate, widely divergent and elevated on the postero-lateral slopes of the valve on distinctive muscle pads. In mature and gerontic dorsal valves the postero-lateral muscle pads form into blunt-ended “brachiophore-like” projections that merge posteriorly with the pseudointerarea and are supported anteriorly by distinctive short ridges Fig. 6C ). Vascula lateralia are straight, deeply incised, very widely divergent and extend to the anterior margin Fig. 6A ). Vascula media arise just anterior of mid valve on either side of the median ridge as deeply incised, relatively broad, straight, weakly divergent grooves ( Fig. 6A ).

Remarks. —Valves of Eodicellomus elkaniiformis from the WPC clasts have biconvex shells with strongly thickened visceral platforms. Recent SEM and microCT work on E. elkaniiformis from the Arrowie Basin has resolved details of internal morphology not previously attainable with traditional SEM techniques ( Jacquet et al. 2018). Exfoliated shells from the WPC reveal a characteristic, layered microstructure, which is comparable with their findings ( Fig. 6E, F ). This shows that within the secondary layer, rhythmic compact laminae are separated by either apatite infills or void spaces, once likely filled with organic-rich (chitinous) matrix. This microstructure occurs in both valves, though are more prevalent in dorsal valves which exhibit greatly thickened platforms. Jacquet et al. (2018) showed that these raised platforms tend to exhibit more secondary loss of the organic-rich material, often leaving obvious void spaces in the shells in these areas.

Eodicellomus is comparable to members of the recently reintroduced family Neobolidae Walcott and Schuchert in Walcott, 1908, as they have the diagnostic trilobate thickened visceral platform on the ventral valve, and platforms developed on the dorsal valve interior ( Popov et al. 2015: 23). However, Eodicellomus has well-developed flexure lines on both the dorsal and ventral propareas and wide pseudointerarea, which are not present in other members of the Neobolidae ( Fig. 8C 3 , D, E; Popov et al. 2015).

HolmerandUshatinskayainGravestocketal.(2001:125– 126) recognised a strong similarity between Eodicellomus and late Cambrian–Ordovician elkaniids, which have raised muscle platforms on valve interiors ( Holmer 1993). However, they did not place Eodicellomus in the Elkaniidae because it lacks pitted microornamentation on the adult shell (Holmer and Ushatinskaya in Gravestock et al. 2001). Like the Eoobolidae , the Elkaniidae has broad variability in the pitted post-metamorphic ornamentation, from regularly spaced and shaped rhomboid pits in the Lower Ordovician Lamanskya splendens Moberg and Segerberg, 1906 ( Holmer 1993: fig. 6a–e) to the irregular, rounded pits of the middle Cambrian Broeggeria salteri ( Holl 1865; Popov and Holmer 1994: fig. 58c, l). However, a pitted metamorphic shell is often lost to abrasion, and the development of pustulose post-metamorphic ornament is often highly variable. Hence, these are unreliable characters upon which to diagnose family-level classifications ( Balthasar 2009: 416).

Stratigraphic and geographic range.— Eodicellomus elkaniformiis is an East Gondwanan endemic. It occurs in the Arrowie Basin ( Micrina etheridgei – Dailyatia odyssei zones): Ajax Limestone, Mt. Scott Range , northern Flinders Ranges; Mernmerna Formation and Wirrapowie Limestone, south and east Arrowie Syncline, northeast Flinders Ranges; Winnitinny Creek Member and Second Plain Creek Member, Wilkawillina Limestone, Bunkers Graben, southern-central Flinders Ranges; Moorowie Formation, Chambers Gorge area, eastern Flinders Ranges. Stansbury Basin ( D. odyssei Zone ): Parara Limestone, Horse Gully, Curramulka Quarry and SYC-101 borehole, Yorke Peninsula; WPC clasts, Kangaroo Island. Amadeus Basin: Todd River Dolomite, Phillipson No. 1 Borehole, Northern Territory.

Figures

Fig. 6. The lingulid brachiopod Eodicellomus elkaniiformis Holmer and Ushatinskaya in Gravestock et al., 2001 from the lower Cambrian White Point Conglomerate, Kangaroo Island, South Australia. Dorsal (A–D) and ventral (E–G) valves. A. SAM P57235 in interior view. B. SAM P57236 in interior view. C. SAM P57237 in interior view. D. SAM P57238 in interior view. E. SAM P57240 in interior (E1) and oblique (E2) views. F. SAM P57241 in oblique view. G. SAM P57239 in oblique view.

Fig. 2. Correlation chart showing shelly fossil biozones against the stratigraphic succession in the Flinders Ranges, Fleurieu Peninsula, and Kangaroo Island (adapted from Betts et al. 2018: fig. 27). Note: The succession in the NE Kangaroo Island is based on the Investigator-1 drill core in which the Mt. McDonnell Formation is not apparent (Gehling et al. 2011). Lower boundary of White Point Conglomerate (WPC) is likely faulted.Abundant and diverse shelly fauna (SSF) from the bioclastic limestone clasts in the WPC have an upper Dailyatia odyssei Zone age. Dashed lines indicate uncertain stratigraphic relationship. Abbreviations: K., Kulparina; P., Parabadiella; S., Sunnaginia.

Fig.8. The lingulid brachiopod Curdus pararaensis Holmer and Ushatinskaya in Gravestock et al., 2001 from the lower Cambrian White Point Conglomerate, Kangaroo Island, South Australia. Dorsal (A, B, E, G, H) and ventral (C, D, F) valves. A. SAM P57262 in external view. B. SAM P57263 in external view. C. SAM P57264 in external (C1) and oblique (posterior) (C2) views; C3 shows external ornament of faint concentric growth lines. D. SAM P57265 in internal view. E. SAM P57266 in internal view. F. SAM P57267 in internal view. G. SAM P57268 in internal view. H. SAM P57269 in internal view.

Fig. 3. The solenopleurid trilobite Trachoparia? sp. from the lower Cambrian White Point Conglomerate, Kangaroo Island, South Australia. Latex cast of partial cranidium (SAM P57221) showing pustulose ornament and strongly tapered glabella in dorsal (A1), anterior (A3), and oblique anterolateral (A4) views; testate cranidium in dorsal view (A2).