Aramazdospirifer orbelianus ( Abich, 1858 ), 2022

Serobyan, Vahram, Danelian, Taniel, Crônier, Catherine, Grigoryan, Araik & Mottequin, Bernard, 2022, Aramazdospirifer orbelianus (Abich, 1858) n. comb., a new cyrtospiriferid brachiopod genus and a biostratigraphically important species from the lower Famennian (Upper Devonian) of Armenia, Comptes Rendus Palevol 21 (6), pp. 145-156 : 150-154

Aramazdospirifer orbelianus ( Abich, 1858) n. comb.

( Figs 3-7; Table 1 )

Spirifer orbelianus Abich, 1858: 438 , 440, 524-525, pl. 1, figs 2, 3; pl. 2, figs 4, 5.

Cyrtospirifer orbelianus — Rzhonsnitskaya 1948: 1480. — Arakelyan 1952: 40, 42. — Abrahamyan 1957: 70, pl. 8, fig. 3. — Arakelyan 1964: 75, 77, 82, 94. — Abrahamyan et al. 1973: 218. — Abrahamyan 1974: 60, pl. 22, figs 2, 3.

Spirifer orbelianus — Sartenaer 1974: 10 (only the Armenian specimens).

Cyrtiopsis orbelianus — Arakelyan et al. 1975: 24. — Rzhonsnitskaya & Mamedov 2000: 331, table 1.

Uchtospirifer orbelianus — Afanasjeva in Alekseeva et al. 2018a: 41, pl. 30, fig. 4; text-fig. 103. — Grechishnikova in Alekseeva et al. 2018b: 832, 855.

NEOTYPE. — Abich (1858) did not designate a type specimen for his new species among the four ones he illustrated. Abich’s collection is stored in the Saint Petersburg Mining Institute Museum, but all the spiriferides collected by Abich are lost (D. Bezgodova, personal communication, 2019). Therefore, the single specimen illustrated by Abrahamyan (1957: pl. 8; fig. 3) is hereby designated as the neotype and figured in Figure 3 I-N. It is stored at the Geological Museum in Yerevan under the collection number IGSNASRAGM 3897/AB97/48 .

TYPE LOCALITY AND HORIZON. — Marly/sandy limestone layers ( Abrahamyan 1957, 10th limestone layer of the Amaghu section, appendix 1) of the Ertych horizon, Noravank section (formerly Amaghu), Central Armenia.

OCCURRENCE AND AGE. — This species is one of the most biostratigraphically valuable species in Armenia for the recognition of the lower Famennian and is used for the definition of a brachiopod zone, namely the ‘ Cyrtospirifer ’ orbelianus Zone of Abrahamyan (1957), as well as the ‘ Cyrtiopsis ’ orbelianus -‘ Cyrtiopsis ’ armenicus Zone of Rzhonsnitskaya & Mamedov (2000) ( Fig. 2). Initially, Abrahamyan (1957) reported this species from sequences considered as constituting the lower part of the upper Famennian. Following the extensive stratigraphic study of Abrahamyan (1964) and Arakelyan (1964), it appeared that the previously reported orbelianus bearing sequences are actually early Famennian in age. Therefore, Abrahamyan (1974) specified that A. orbelianus n. comb. is restricted to the lower Famennian (possibly corresponding to the equivalent of the Palmatolepis crepida - P. glabra pectinata conodont zones of Spalletta et al. (2017)). Previously this species has been observed in Armenia in the Argichi, Lanjanist (Kadrlu), Chanakchi (Zangakatun), Ertych and Noravank sections ( Fig. 1) ( Abrahamyan 1957; Arakelyan 1964) and doubtfully in the Gyumushlug section of Nakhichevan ( Fig. 1; Abrahamyan 1957). It is worth noting that only two poorly preserved specimens of possible A. orbelianus n. comb. have been found so far in Nakhichevan by Abrahamyan (1957) (IGSNASRAGM collections). Nevertheless, both specimens lack longitudinal and median elevations, and display an erect beak, which are not characters present in A. orbelianus n. comb. Moreover, further sampling in several sections exposing the lower Famennian strata in Nakhichevan, including the Gyumushlug section, did not reveal the presence of Abich’s species (e.g. Arakelyan 1964; Aristov et al. 1979; Feliks et al. 1980; Grechishnikova 1986).

MATERIAL EXAMINED. — Eighty articulated specimens, ten ventral and five dorsal valves from the marly/sandy limestones of the Ertych horizon of the Djravank (sixty articulated specimens and ten ventral valves), Noravank (type locality; twelve articulated specimens and four dorsal valves) and Ertych (eight articulated specimens and one dorsal valve) sections. Three articulated specimens in Abrahamyan’s collection ( IGSNASRAGM) from the Noravank section and two ventral valves from the Gyumushlug section (Nakhichevan).

DESCRIPTION

Shell medium-sized (up to 39 mm in width and 39.1 mm in length), wider than long to longer than wide, generally ventribiconvex, rounded subpentagonal in outline, widest at hinge line, highest at about midlength; cardinal extremities acute to mucronate, but strongly extended in some specimens; anterior margin strongly excavated by sulcus, emarginate; anterior commissure uniplicate.

Ventral valve strongly inflated, rounded pentagonal in outline, with convex flanks sloping steeply towards lateral commissures; highest at about midlength or slightly posteriorly; umbo strongly inflated, large and prominent; beak erect (85-90°), interarea apsacline, triangular, moderately high, well-defined, concave; delthyrium wide, with most of its height covered by pseudodeltidium, the latter formed by several distinct plates; foramen minute, rounded to ovate; sulcus wide, shallow to moderately deep, originating from beak, widening and becoming deeper anteriorly, sulcus margins gentle. Sulcus bears a longitudinal and median elevation which inconspicuously originates in the posterior part of the valve; it widens, thickens and becomes more conspicuous anteriorly (highest and widest at the anterior margin); tongue high, perpendicular to commissural plane with its distal part sometimes bent dorsally, subogival in outline.

Dorsal valve wider than long, inflated with flanks sloping moderately to strongly towards lateral commissures, subquadrangular to subtrapezoidal in outline; highest in the anterior third of the valve, but gradually decreasing towards the anterior margin; interarea linear (up to 3 mm high), slightly concave, orthocline; fold well-defined, wide, moderately high, inconspicuously originating from beak, widening and becoming higher anteriorly, often bearing a longitudinal and median elevation starting from dorsal beak. Ornamentation of up to 35 rounded (generally 28-30, 5-6 ribs per 5 mm at anterior margin near sulcus and fold), simple, flattened, low ribs on each flank, becoming fainter towards posterolateral margins; in sulcus and on fold, up to 25 ribs, increasing by bifurcations, much narrower than those present on flanks; interspaces as wide as ribs on flanks, but wider than ribs in sulcus and on fold; micro-ornament of capillae both on ribs and in interspaces with concentric growth lines sometimes thickened as growth varices.

Ventral valve interior with thin, long, intrasinal and divergent dental plates, becoming much less divergent to almost parallel more anteriorly, converging dorsally in umbonal region (as seen in transverse section); delthyrial plate well-developed, thick (particularly in large specimens); umbonal callus well-developed, central and lateral apical cavities large and filled in by callus; teeth relatively small, subrectangular.Dorsal valve interior with unsupported ctenophoridium composed of up to 33 well developed, relatively long lamellae; hinge plate divided; outer hinge plates slightly concave, crural bases dorsally convergent; spiral cones not preserved in the sectioned specimens.

Variability

The shell shape varies from almost globular forms with thickness exceeding as width and length, having narrower, shallow sulcus to transversely elongated wider forms with broad sulcus and relatively less inflated valves. The beak angle, the height of the longitudinal elevation and the number of ribs are also shifting.

Ontogeny

Juvenile forms differ from adults in having less inflated valves, straight beaks, shallower sulcus, weak fold, fainter ribs, inconspicuous elevations developed only anteriorly and the less convex delthyrial plate. The size distribution during growth represented by the length/width, thickness/width, width of sulcus/width and length of dorsal valve/width plots ( Fig. 6) shows a continuous and progressive growth with no distinct grouping. The relative proportions of Aramazdospirifer orbelianus n. comb. represented by sufficient material remain constant (linear regression: y = ax + b; significant probability value: p <0.001*** whatever the degree of development of individuals [ Fig. 6]). Moreover, the correlation is positive with width varying proportionally with length, thickness, width of sulcus and the length of dorsal valve. To complete the scatter plots, the measurements (in mm) of numerous individuals of Aramazdospirifer orbelianus n. comb. are also presented in Table 1 and Figure 7. The length of the dorsal valve shows less dispersed values ( Fig. 7).