Antulanella pancici ( Antula, 1903 )

Radulović, Barbara, Motchurova-Dekova, Neda & Radulović, Vladan, 2007, New Barremian rhynchonellide brachiopod genus from Serbia and the shell microstructure of Tetrarhynchiidae, Acta Palaeontologica Polonica 52 (4), pp. 761-782 : 766-770

publication ID	https://doi.org/ 10.5281/zenodo.13741110
persistent identifier	https://treatment.plazi.org/id/039987AB-FF89-283B-FFF0-FDE9D3216C6D
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scientific name	Antulanella pancici ( Antula, 1903 )
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Antulanella pancici ( Antula, 1903)

Figs. 2–12 View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig .

1903 Rhynchonella Pančići sp. nov.; Antula 1903: 34, pl. 1: 1–4.

1911 Rhynchonella Pančići Antula ; Petković 1911: 7.

1930 Rhynchonella Pančići Antula ; Petković 1930: 103, pl. 2: 7D (only).

1953 Rhynchonella pančići Antula ; Sučić 1953: 86, 87, 108, pl. 2: 5–7.

1961 Rhynchonella pančići Ant. ; Sučić 1961: 51, 82.

1978 Rhynchonella pančići Antula ; Jankičević 1978: 126, 129, 149, 154.

2000 Cyclothyris pancici (Antula) : Radulović 2000: 122, 124, pl. 1: 4, 5.

2005 Cyclothyris? pancici (Antula) ; Polavder and Radulović 2005: 57, figs. 2A–D, 3A, B.

Lectotype (designated herein).—Specimen NHM 484/3, illustrated in Fig. 3A View Fig . In Antula’s collection housed in NHM under the No. M 484, there were 19 syntype specimens, not singly numbered. In the inventory book it is stated that there should be 27 specimens, collected by Antula in 1893 and 1894. Four specimens figured by Antula (1903: pl. 1: 1–4) are drawings, possibly partly modifying the outline of the originals (see Fig. 12 View Fig ). Unfortunately, we can not recognize any of Antula’s figured specimens among the present 19 specimens. We propose herein the best preserved specimen as a lectotype, the remaining 18 specimens now becoming paralectotypes. Recently, we collected also additional topotype material from Crnoljevica.

Diagnosis.—Same for the genus.

Material.—The lectotype, 18 paralectotypes and 40 topotype specimens from Crnoljevica.

Measurements (in mm; see also Fig. 2 View Fig ): Description.— External morphology: Shell small to very rarely medium−sized, subglobose, outline variable, from subcircular to roundly pentagonal, or slightly transversely elliptical. In juvenile specimens, valves equally biconvex, in adults strongly biconvex with dorsal valve somewhat more convex. Length slightly surpassing width in most specimens, very rarely as long as wide, or wider than long. Maximum width and thickness situated at about mid−length. Beak strong, pointed and suberect. Beak ridges very distinct, delimiting a moderately wide concave interarea. Hypothyrid foramen slightly auriculate, minute, circular, rarely oval. Deltidial plates disjunct. Squama and glotta present, but not well expressed. Anterior commissure highly and roundly uniplicate. Each valve ornamented with 26–32 simple subtriangular costae, 6–8 on fold, 5–7 in sulcus. Fold and sulcus poorly developed anteriorly, not sharply separated from lateral flanks. Apical angle ranges from 90 to 95 degrees.

Internal morphology: Seven specimens were sectioned of which four are figured ( Figs. 5–8 View Fig View Fig View Fig View Fig ). Deltidial plates disjunct but very close together, relatively thick, inwardly curved. Dental plates ventrally divergent to subparallel, slender, largely confined to ventral umbo. Hinge−teeth subquadrate, or spherical, crenulated, with distinct denticulae, nearly vertically inserted in large well−developed sockets. Pedicle collar absent. Well defined inner and outer socket−ridges. Hinge plates slightly ventrally deflected in early stages, becoming anteriorly subhorizontal to horizontal, slender and wide, straight or gently arched ventrally. Septalium not present. Euseptoidum reduced to a short and low ridge ( Fig. 11A View Fig 1 View Fig , A 2 View Fig ). Crural bases crescentic, not clearly separated from hinge plates ( Fig. 11A View Fig 3 View Fig , B 1 View Fig ). Crura with dorso−ventrally widened distal ends (giving rise to a diabolo appearance) ( Figs. 9A–C View Fig , 10A View Fig 5 View Fig , B), rarely raduliform, or canaliform ( Fig. 9D View Fig ), all these types belonging to the raducal group sensu Manceñido (2000).

Shell microstructure: Four specimens were studied ( Figs. 10 View Fig , 11 View Fig ). The impunctate shell of Antulanella pancici is composed of two calcitic layers, primary microgranular and secondary fibrous. Calcite prisms perpendicular or slightly inclined to the internal shell surface were also observed ( Figs. 10A View Fig 2 View Fig , 11A View Fig 1 View Fig ). They are similar to those described by Motchurova−Dekova (2001) and are considered to be the result of secondary diagenetic calcite formation and should not be confused with tertiary prismatic layer.

The primary layer is 20 ̊m thick in the sulci and 30 ̊m in the ribs and preserved only in the shell parts covered with sediment. It is composed of elongate microgranular calcite crystals, perpendicular to the secondary layer ( Figs. 10A View Fig 1 –A View Fig 3 View Fig , 11A View Fig 1 View Fig ).

The secondary layer is homogeneous (not differentiated in several packages), variable in thickness, 210–450 ̊m in costae and 130–250 ̊m in sulci. It is built up of anisometric fibres, finer close to the exterior shell surface, 12–15 ̊m wide and 3–5 ̊m thick, which gradually become larger in the central and inner part of the shell, 15–30 ̊m wide and 5–10 ̊m thick. The majority of the fibres have anvil−like cross−

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sections, but some of them tend to have elongate rhombic sections ( Figs. 10A View Fig 1 –A View Fig 4 View Fig , 11A View Fig 1 View Fig ). Diagenetic modifications of the fibres including fused fibrous elements were also observed ( Fig. 10A View Fig 3 View Fig ). Relatively thin myotest (not illustrated herein) is developed in the muscle fields.

The internal skeletal structures consist of modified fibres smaller than those building the secondary layer. The fibres forming the inner hinge plates are the largest, 12–15 ̊m wide and 5–8 ̊m thick. Fibres bounding the inner socket ridges are 8–12 ̊m wide and 4–6 ̊m thick, while fibres in the hinge teeth ( Fig. 11B View Fig 2 View Fig , B 3 View Fig ) are smaller, 6–8 ̊m wide and 4–6 ̊m thick.

Remarks.— Antula (1903) appropriately described the variability of the external morphology of this species ( Fig. 12 View Fig ). He classified specimens from the type locality into three morphological groups: (i) equally long and wide, strongly convex, almost globose with no sulcus in the anterior; (ii) width somewhat greater than length, moderately convex; ventral valve with sulcus in the anterior part; (iii) length almost equal to the width, convexity smaller than in others; sulcus slightly pronounced.

The average length of the specimens from Crnoljevica is 12.0 mm and only one relatively large specimen (17.5 mm in length) was found ( Fig. 4G View Fig ).

On the basis of external characters, such as transversely elliptical outline and the type of ribs, the described species resembles Valanginian–Hauterivian Lamellaerhynchia picteti Burri, 1953 (ex. Rhynchonella lata d’Orbigny, 1847 ; partim. Pictet, 1872) known from the Jura region. For this reason, Petković (1930) and Sučić (1953) assigned the transversely elliptical forms of this species to Rhynchonella lata d’Orbigny, 1847 . Apart from being stratigraphically younger, Antulanella pancici differs from L. picteti in having much smaller dimensions and generally less numerous ribs.

Orbirhynchia asymmetrica Smirnova 1972 (32–33, pl. 1: 10; the same specimen was later refigured by the same author in 1990: 8, pl. 1: 8) from the Late Barremian of North Caucasus is a rhynchonellide brachiopod which externally has a subglobose shell and outline very similar to Antulanella pancici , judging from the published illustrations. However, O. asymmetrica has a somewhat larger size, greater number of ribs (34–36) and “asymmetric anterior end”, as stated in the description of this species. It is curious that Smirnova (1972, 1990) wrote that the anterior commissure of her species is asymmetrical yet each of her figured specimens displays a symmetrical anterior commissure. A. pancici and O. asymmetrica are clearly distinguished by their internal morphology, especially by the development of two different types of crura: with widened distal ends, rarely raduliform or canaliform in Antulanella and falciform in Orbirhynchia , which places them in two different superfamilies.

Stratigraphic and geographic range.— A. pancici is known only from a few Barremian localities of the east Serbian Carpatho−Balkanides. The specimens from the type locality could not be precisely dated using the associated macrofossils and microfossils. Based on the associated orbitolinid species Paracoskinolina View in CoL ? jourdanensis, the specimens of A. pancici from the Prekonozi locality are dated as Early Barremian ( Polavder and Radulović 2005).