Pampoecilorhynchus geniculatus, Baliński, 2002

Baliński, Andrzej, 2002, Frasnian-Famennian brachiopod extinction and recovery in southern Poland, Acta Palaeontologica Polonica 47 (2), pp. 289-305 : 295-297

Genus Pampoecilorhynchus Sartenaer, 1968 Pampoecilorhynchus geniculatus sp. nov.

Figs. 4 , 5D–I .

Holotype: ZPALBp 46/52d, complete shell illustrated in Fig. 5G .

Type locality: Bed 12 of the set II, trench Z−17 near the village of Dębnik, Cracow Region, southern Poland.

Type horizon: Platy Limestone Unit, Lower Famennian, Early Palmatolepis triangularis Zone.

Etymology: After geniculation of the anterolateral shell margin.

Diagnosis.—Shell medium sized, rarely exceeds 21 mm in width, wider than long, dorsibiconvex, anterior commissure strongly uniplicate; costae rounded to slightly sharp, usually two to three on flanks, two in sulcus and three on fold; first two lateral costae bounding the ventral sulcus have a tendency to form spurs anteriorly; anterior and lateral margins of both valves always geniculate in large specimens. Differs from all other species of the genus by the character of the shell costation that is thicker and with costae confined to the anterior half of the shell only.

Material.—21 complete or slightly damaged and 53 fragmentary shells. Most specimens exfoliated to some degree. Large specimens make up most of the collection with the smallest 6.7 mm in length.

Description.—Shell medium sized, usually not exceeding 21 mm in width, wider than long, transversely elliptical to transversely subpentagonal in outline, dorsibiconvex, uniplicate with well defined sulcus and fold; hinge line angular, lateral margins arched, anterior margin truncated. In large shells (above 12–14 mm in length) the anterior and lateral margins are geniculate with a serrated commissure between the valves laying in the middle of the geniculation. Medial sector of geniculate part of the shell flat, slightly concave in adjacent lateral sectors. Postero−lateral commissure sticking out and sharp.

Ventral valve with moderately convex postero−median region and almost flat to even slightly concave flanks; beak small erect, deltidial plates small, disjunct. Sulcus clearly bounded by strong lateral costae that arise some distance from the umbo; the bottom of the sulcus usually slightly convex.

Brachial valve about twice as convex as the ventral one; in lateral profile the valve is gently arched to straight or even slightly concave in anterior half. Fold distinct, begins some distance from the umbo, separated laterally by deep troughs; when fold bears three or more costae the lateral costae are slightly higher than the medial costae.

Posterior region of shell with very delicate, low and rounded radial ribs; the median ribs are better marked and anteriorly they become very strong costae. The lateral ribs disappear in the middle of the shell, or, some of them continue anteriorly to give rise of strong lateral costae. This gives a general impression that both posterior and anterior regions of the shell display a different kind of ornamentation.

The majority of shells (64%) bear two costae in sulcus and three costae on the fold, 20% of shells have three and four costae, respectively. In 8% of shells there is a single plication in sulcus and two on the fold, and 4% of shells have four and five costae, respectively. There are two to three lateral costae; parietal costae usually absent, developed in a few specimens only. In cross−section costae are rounded to slightly angular, with tendency to being more angular anteriorly. Micro−ornament not observed.

Ventral valve interior with distinct and subparallel to slightly convergent dental plates. Dorsal valve interior with wide outer hinge plates and narrow crural plates supported by a long median septum forming Y−shaped septalium ( Fig. 4 ). Connectivum not observed in all seven sectioned specimens.

Remarks.—The strongly costate shell with the interior dominated by the dental plates and strong uncovered septalium suggest that the specimens described here represent the Early Famennian genus Pampoecilorhynchus Sartenaer, 1968 . The new species differs from other representatives of the genus by having fewer costae, with costae strongly developed on the anterior half of the shell only. From the type species Pampoecilorhynchus nux ( Gosselet, 1887) the new species differs by its smaller and wider shell with distinctly geniculate antero−lateral shell margins. From P. abellanus Sartenaer, 1968 the new species differs by its more strongly globose shell. Shells of P. lecomptei ( Sartenaer, 1954) and P. arianus Sartenaer, 1968 are also geniculated but differ from P. geniculatus sp. nov. by their strong costae originating at their umbones.

Externally the new species is closest to Pampoecilorhynchus praenux Sartenaer, 1958 described from the Early Famennian of the Dinant Basin and Vesdre Massif ( Sartenaer 1958, 1968, 1985). The present species differs from the Belgian form by its fewer medial and lateral costae that are smaller with rounded cross−sectional profiles in the posterior region of the shell and its distinct geniculation of the shell margins. Pampoecilorhynchus praenux is the index fossil for the brachiopod zone in Belgium, that there is associated with conodonts of the Middle Pa. triangularis Zone ( Sartenaer 1985) . Pampoecilorhynchus cf. praenux described by Matyja and Żbikowska 1974) from the Pa. crepida Zone of the Minkowice 1 borehole (Lublin Basin, Poland) was rejected from the species by Sartenaer (1985: 321). It is probable that P. geniculatus is the oldest representative of the genus.

Occurrence.—According to Sartenaer (1968: 31–33; 1985: 321) the genus Pampoecilorhynchus occurs in the Early Famennian (mainly Pa. triangularis conodont Zone) of USA, Germany, Belgium, France, Iran, and Afghanistan. P. geniculatus sp. nov. occurs in dark−grey micrites representing the Early and basal part of the Middle Pa. triangularis Zone in trench Z−17 near the village of Dębnik, Cracow Region, southern Poland.

Figures

Fig. 4. Transverse serial sections of two shells Bp 46/52g (A) and Bp 46/52h (B) of Pampoecilorhynchus geniculatus sp. nov. Numbers refer to distances in mm from the ventral apex.

Fig. 5. Early Famennian brachiopods, trench Z−17, Dębnik anticline, Poland. A, B. Chapinella striata sp. nov. A. Holotype Bp 46/33a in dorsal, ventral, lateral, and anterior views; A5 shows surface microornamentation. B. Shell Bp 46/33b in ventral view. C. Orbiculatisinurostrum laeve (Gürich, 1903); shell Bp 46/28a in dorsal and anterior views. D–J. Pampoecilorhynchus geniculatus sp. nov. D. Juvenile shell Bp 46/52a in dorsal and ventral views. E. Shell Bp 46/52b in dorsal and anterior views. F, G. Two shells Bp 46/52c and Bp 46/52d in dorsal, ventral, lateral, posterior, and anterior views (G, holotype). H. Expanded laterally shell Bp 46/52e in dorsal and posterior views.I. Elongated shell Bp 46/52f in dorsal, posterior, and anterior views. All × 1.5 except A5 × 4.