Plectothyrella crassicostis ( Dalman, 1828 )

Hints, Linda & Harper, David A. T., 2015, The Hirnantian (Late Ordovician) brachiopod fauna of the East Baltic: Taxonomy of the key species, Acta Palaeontologica Polonica 60 (2), pp. 395-420 : 413-415

publication ID	https://doi.org/ 10.4202/app.2013.0010
persistent identifier	https://treatment.plazi.org/id/81298792-FFF2-A25C-FCE0-FF0DFB9CD0A5
treatment provided by	Felipe
scientific name	Plectothyrella crassicostis ( Dalman, 1828 )
status

Treatment

Plectothyrella crassicostis ( Dalman, 1828)

Fig. 14 View Fig ; Table 2.

1828 Atrypa? crassicostis Dalman ; Dalman 1828: 47–48.

1968 Plectothyrella platystrophoides Temple, 1965 ; Wright 1968: 357, fig. 4.

2002 Plectothyrella crassicosta ( Dalman, 1828) ; Bergström 1968: 19, pl. 7: 5–8.

2008 Plectothyrella crassicostis ( Dalman, 1828) ; Cocks 2008: 187 (see synonyms therein).

Material.— 98 specimens, partly fragmentary embedded in rock. Porkuni Regional Stage, Kuldiga Formation, Hirnantian ( Upper Ordovician ); western Latvia, southwestern Estonia, and Kaliningrad Region ( Russia). Drill core and depth (in meters) of sample intervals with brachiopods. Depth of fragmentary preserved specimens is marked by “?”. Collection GIT 542 View Materials : Adze-6, 838.5–?844.4; Aispute-41,?993.55–996.35; Blīdene-5, 815.75; Engure 882.1–?884.4; Ikla ,?584; Malinovskaya ( Russia, Kaliningrad Region; depth is unknown); Mežmaļi-16,?905.9–914.55; Riekstini-15, 846.0–855.7; Ruhnu-500, 609.8–?611.8; Stirnas-18,?899.0–?910.0; Sturi-8,?942.6; Vilcini-19, 901.0–907.9. Collections LDM G: Anši-12, 914.6–914.8; Blīdene-5, 816.6–819.3; Cicere-10,?897.8–898.0; Dizrungi-17,?881.6; Dreimaņi-11, 954.65–?955.5; Ēdole-60,?836.8–848.0; Kronauce-20, 1050.9; Mežvagari-13, 869.5–878.3; Pāvilosta,?1098.6; Piltene-31, 970.7; Pliekalni-14,?882.0–?884.8; Puikule, 1398.3 .

Description. —Ventribiconvex rostrate, costate shell with subcircular to anteriorly elongate outline, anterior commissure sulcate, lateral undulating. Ratio of ventral valve length/ width varies from 0.77 to 0.90; thickness up to 90% of shell width. Maximum convexity at about mid length in dorsal valve, and in the posterior half of ventral valve. Ventral beak incurved or overhanging the umbonal part of dorsal valve. Palintrope variably developed. Dorsal fold and ventral sulcus originating 2–3 mm from beaks ( Table 2).

Ornament consists of high, angular costae, 17 to 23 around the beak. Dorsal fold consists of two anteriorly-broadening costae bifurcating at 3–6 mm from umbo and separated from each by a deep median interspace. The fold is flanked laterally on both sides by one thin and 10 to 13–14 stronger costae, which become thinner towards the posterior edge. Ventral sulcus with strong median and two or more thinner costae; sulcus is limited on both sides by one or two strong costae. In some cases additional costellae appear on fold and sulcus. The growth lines are variably developed, very strong on some specimens. The palintrope and shell surface are filated.

Ventral valve ( Fig. 14A, F View Fig ) has wide open delthyrium, about twice as wide as high with somewhat elevated (thickened?) edges ( Fig. 14A View Fig 2 View Fig , A 3 View Fig ); the permesothyrid pedicle opening, circular in diameter, about 0.7 mm; stout cyrtomatodont teeth have postero-laterally, divergent thickenings. Delthyrial chamber short thickened in umbonal part.

Dorsal interior is represented by one valve. Sockets form oblique grooves below the posterior edge of the valve, inner socket ridges are connected to the thick outer hinge plates. Crura strong, curved antero-ventrally, rod-like, rising from thickened shell floor ( Fig. 14B View Fig ). Trilobated thickening of shell floor occurs in front of the cardinalia. Muscle field not impressed. Ribs impressed on interior surface of valve.

Remarks.—The Baltic specimens are similar to P. crassicostis from Sweden ( Bergström 1968) and also to specimens previously described as P. platystrophoides (a junior synonym of P. crassicostis ) in Ireland ( Wright 1968), England and North Wales ( Temple 1965). The preservation of specimens (mostly moulds) from the latter and several other areas (e.g., China, Chen et al. 2006; Sardinia, Leone et al. 2009) hinders accurate comparisons with the Baltoscandian specimens. The Baltic specimens are of medium size (width about 20 mm), and very similar to those from Québec in posterior view (ventral beak covers the dorsal umbo) and by the arrangement of costae ( Lespérance and Sheehan 1976: pl. 110: 7–9, 12). The thick inner socket ridges mentioned by Lespérance and Sheehan (1976: pl. 110: 16) are also similar to the Baltic specimen ( Fig. 14B View Fig ).

The lateral branches of the median septum are similar to those in P. crassicostis from Kildare ( Wright 1968: fig. 4B) and Québec ( Lespérance and Sheehan 1976).

The dorsal interiors of the Chinese specimens figured by Sheng (1982: pl. 3: 5, 6) seems to differ from the Baltic and other specimens of P. crassicostis by a much greater number of costae, revealed by the ribbing impressions on the interior surface of the valve.

Stratigraphic and geographic range. —The Hirnantian Stage, Upper Ordovician. A similar form is reported from the lowest Llandovery in England. This characteristic taxon of the Hirnantia fauna occurs on a range of different continents (Africa, Asia, Europe, North and South America), across the Bani and Kosov provinces (see Rong and Harper 1988; Rong et al. 2002; Benedetto et al. 1990). In Baltos-

Tentaculites sp. in the lower part of sample (figured in Kaljo et al. 2008) (E 1). Polished cross section of the shell ( GIT 542-3/3) cut by drilling of the upper edge on E 1 showing due compression broken anterior part and long crura (E 2). F. Shell, GIT 542-202, Engure, 882.1 m, dorsal (F 1) and posteri- or (F 2) views. G. Shell, LDM G 272-1, Piltene- 31, 970.7 m, postero-lateral view. H. Shell, LDM G 328-126, Blīdene- 5, 818.3 m, ventral (H 1) and posterior (H 2) views. Scale bars 5 mm.

candia it occurs in Sweden ( Bergström 1968), Estonia, and Latvia (this paper).