Plagionephrodes laqueus praelaqueus, Casier & Olempska, 2008

Casier, Jean-Georges & Olempska, Ewa, 2008, Early Frasnian ostracods from the Arche quarry (Dinant Synclinorium, Belgium) and the Palmatolepis punctata Isotopic Event, Acta Palaeontologica Polonica 53 (4), pp. 635-646 : 638-640

Plagionephrodes laqueus praelaqueus ssp. nov.

Fig. 3L–N .

?1971 Plagionephrodes laqueus ( Matern, 1929) ; Becker, 1971: table 1.

Etymology: From the Latin prae —before and laqueus = lace referring to the ornamentation of a species of Plagionephrodes described by Matern (1929). Referring to the fact that Plagionephrodes laqueus praelaqueus ssp. nov. and P. laqueus laqueus ( Matern, 1929) belong to the same phylogenetic succession.

Holotype: IRScNB n° b5066. Carapace. AR−06−27. Fig. 3L 1 , L 2. L = 0.78 mm; H = 0.49 mm; W = 0.22 mm.

Type locality: Access path to the Arche quarry.

Type horizon: Chalon Member of the Moulin Liénaux Formation, Early Frasnian, Devonian.

Diagnosis.—A small subspecies of Plagionephrodes laqueus characterised by an ornamentation composed of an anterior vertical ridge and a posterior spur on each valve separated by some other irregular ridges in the middle part of the carapace.

Material. —126 valves and carapaces (samples AR−06−17, 18, 20−23, 25−28).

Description.—Small preplete carapace with a slightly curved dorsal border extending from the anterior two−seventh of the greatest length almost to the posterior extremity. The hinge line is straight. The ventral margin is straight or gently concave. Anterior margin largely rounded sometimes forming an obtuse angle with the dorsal border, or exceptionally slightly angular at the extremity. Posterior margin more rounded than the anterior, and curvature more accentuate ventrally. The greatest length is at mid−height, and the greatest height is at two−seventh of the carapace length. The left valve is larger than the right, and projects greatly all along the free and dorsal margins in right lateral view. The overlap is important along the ventral border. The contour of the left valve is quite different. Its greatest height is slightly shifted posteriorly, and the anterior extremity is acuminate. The ornamentation is more developed on the right valve, and is composed of a vertical ridge located at the anterior quarter of the valves and of a spur in the posterior quarter of the length at mid−height. Some other irregular ridges are present in the middle part of the carapace, sometimes extending from the upper part of the anterior ridge to the postero−ventral sector. A slightly crenulated marginal rim is visible all along the free border, but it is more developed close to the extremities. In dorsal view, the contours of valves are nearly straight between the anterior ridge and the posterior spur, which are prominent. The position of the posterior spurs corresponds to the greatest width. The anterior and the posterior borders are well rounded but the surface before the anterior ridge and behind the dorsal spur is concave.

Remarks.—The new subspecies and Plagionephrodes laqueus laqueus ( Matern, 1929) belong to the same phylogenetic succession. P. laqueus laqueus is known from the Middle Frasnian of Belgium and Boulonnais ( France) ( Becker 1971). This species is larger and possesses a strongly developed ornamentation composed of ridges forming a “ω” letter of the Greek alphabet, surmounted by a ridge parallel to the dorsal border. Plagionephrodes ineptus Becker, 1971 , also from the Frasnian of the Dinant Synclinorium ( Becker 1971), possesses a spur in the dorsal sector only. In Belgium, P. laqueus praelaqueus is also known from the Chalon Member, from the Sourd’Ave Member and from the Pont d’Avignon Member in the Nismes section (study in progress). The P. laqueus ( Matern, 1929) specimen mentioned but not figured by Becker (1971) from the access path to the Arche quarry belongs probably to the new subspecies.

Figures

Fig. 3. Early Frasnian ostracods from the access path to the Arche quarry, Dinant Synclinorium, Belgium. A, B. Refrathella incompta Becker, 1971. A. Carapace in right (A) and dorsal (A) views. Sample AR−06−28. IRScNB n° b5055. B. Left valve. Sample AR−06−26. IRScNB n° b5056. C–E. Uchtovia 1 2 materni Becker, 1971. C. Heteromorph carapace in left lateral view. Sample AR−06−26. IRScNB n° b5057. D. Heteromorph carapace in dorsal view. Sample AR−06−28. IRScNB n° b5058. E. Tecnomorph carapace in dorsal view. Sample AR−06−28. IRScNB n° b5059. F, G. Hypotetragona tremula Becker, 1971. F. Heteromorph carapace in left lateral view. Sample AR−06−23. IRScNB n° b5060. G. Tecnomorph carapace in dorsal view. Sample AR−06−28. IRScNB n° b5061. H–J. Scobicula gracilis sp. nov. H. Holotype. Carapace in left lateral (H) and dorsal (H) views. Sample AR−06−20. IRScNB n° b5062. 1 2 I. Carapace in left lateral view. Sample NI−07−136 from the Sourd d’Ave Member in the Nismes section. IRScNB n° b5063. J. Carapace in left lateral view. Sample NI−07−12 from the Chalon Member in the Nismes section. IRScNB n° b5064.K. Cavellina? sp. A. Carapace in left (K) and dorsal (K) views. Sam1 2 ple AR−06−26. IRScNB n° b5065. L–N. Plagionephrodes laqueus praelaqueus ssp. nov. L. Holotype. Carapace in right lateral (L) and dorsal (L) views. 1 2 Sample AR−06−27. IRScNB n° b5066. M. Internal view of right valve. Sample AR−06−17. IRScNB n° b5068. N. Internal view of left valve. Sample AR−06−18. IRScNB n° b5069. O. Plagionephrodes ineptus Becker, 1971. Carapace in right (O) and dorsal (O) views. Sample AR−06−28. IRScNB n° 1 2 b5070. Scale bar 200 µm.

Fig. 1. Locality map of the Arche quarry, the Lion quarry and the Frasnes railway section; the access path to the Arche quarry indicated by arrow.

Fig. 2. Lithologic sections of the Chalon Member and of the extreme base of the Arche Member exposed on the north−eastern side and on the south−eastern side of the access path to the Arche quarry.