Cliftonia psittacina ( Wahlenberg, 1821 )

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 : 405-407

Cliftonia psittacina ( Wahlenberg, 1821)

Fig. 9A–F, H–K; Table 1.

1821 Anomites psittacinus Wahlenberg ; Wahlenberg 1821: 65.

1965 Cliftonia psittacina ( Wahlenberg, 1821) ; Bergström 1968: 11–12 (see synonyms therein), pl. 4: 7, 8; pl. 5: 1, 2.

Material.—122 differently preserved specimens. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); western Latvia, southern Estonia. Drill core and depth (in meters) of sample intervals with brachiopods. Depth of fragmentary preserved specimens is marked by “?”. Collection GIT 542:Adze-6, 844.2–?844.9;Aizpute-41, 994.65–?1001.3; Blīdene-5, 815.75; Engure, 882.15–884.4; Ikla,?538.5; Kandava-26,?962.7–962.8; Kardla-570, 175.0; Mežmaļi-16, 911.8–?915.0; Priekule-20, 1369.6–1369.7; Riekstini-15, 857.5–?861.3; Ruhnu-500,?615.3–?617.2; Stirnas-18,?901.7–?911.8; Sturi-8,?940.5; Vilcini-19, 904.9–914.8; collections LDM G: Adze, 844.1; Anši,?919.45–?921.5; Blīdene-5, 815.75–820.7.0; Dižrungi-17,?894.4–?896.2; Cicere,?899.4; Ēdole-60, 837.5–838.78; Kuili,?948.0; Mežvagari-13,?875.0–?877.6; Pāvilosta,?1082.6–?1099.0; Piltene-32, 939.0; Pliekalni-14, 884.0–?888.0; Priekule-23,?1368.6; Remte-3,?965-966; Talsi-55,?866.7

Description.—Dorsi-biconvex shell with subcircular to oval outline, maximum width up to 30 mm at about mid valve length, ratio of shell length/width is 0.8 (variation 0.6 to 0.9), cardinal angles obtuse ( Table 1). Ventral sulcus shallow with six to ten costae; dorsal fold with four to nine, or occasionally more.

The valve surface around the umbo is covered by fine filae prior to the first concentric lamellae with about 13 costae. Up to 7 costae and costellae occur per 5 mm at 5 mm in front of umbo, and up to 30 along the commissure. Anteriorly, the distance between growth lines decreases. Ventral interarea up to 2 mm high, weakly apsacline, concave under the beak. Interarea smooth with numerous parallel growth lines. Pseudodeltidium with short monticulus with length of one third to a half of the height of the interarea. Foramen small, circular, apical. Dorsal interarea low, slightly concave.

Interior of the ventral valve is represented by only a few specimens. Teeth supported by thin subvertical dental plates which join with slightly divergent thin septa, bounding the muscle field laterally. The cavity under the pseudodeltidium is filled by secondary shell material, centrally penetrated by the foramen opening or tube, which is exposed on two fragments of small, silicified ventral valves with platelike teeth rising from the valve floor and extending at right angles; dental plates not clearly developed (Fig. 9 J, K).

Dorsal valve has slightly keeled cardinal process lacking a cowel. Brachiophores triangular, divergent postero-laterally. Short wedge-shaped septa is separated from cardinalia by callosity on shell floor (Fig. 9 C). Muscle fields obscure in both valves. The lamellae on the costae are displayed as small depressions on the interior surface.

Remarks.—The development of a monticulus has proved to be a controversial character at genus-level taxonomy in the triplesiids. All ventral valves from the East Baltic sections have a pseudodeltidium with a short fold or monticulus developed only during early growth stages. The development and taxonomic value of the monticulus has been discussed by Amsden (1973, 1974), Wright (1993, 1971) and Wright and Jaanusson (1993). The presence of a short monticulus, only early in ontogeny, separates the Baltic specimens from typical representatives of the genus Cliftonia , which had a monticulate pseudodeltidium throughout life ( Wright 2000). Nevertheless the East Baltic specimens are similar to the North American Cliftonia tubulistriata ( Savage, 1913) from the Edgewood Group ( Amsden 1974), that also has a short monticulus on the pseudodeltidium. The American species differs from both Hirnantian species, C. psittacina and C. oxoplecioides , in having a circular rather than transverse outline. The Scandinavian ( Bergström 1968; Cocks 1982) and Baltic specimens of Cliftonia clearly differ from the types of C. oxoplecioides from the Kildare Limestone in Ireland ( Wright 1963) in having a stronger external ornament, stronger concentric growth lamellae and a less obvious fold and sulcus. C. oxoplecioides plicata Benedetto, 1990 , from the Don Braulio Formation (Hirnantian) in the Argentian Precordillera, is similar to the nominate subspecies but in contrast possesses an unusually high fold and deep sulcus together with a striking reticulate ornament. The reduction of monticulus on C. psittacina is one of the features which potentially differentiates it from the type species.

The monticulus described here has some similarity with those of the related genus Oxoplecia , which has variably developed monticulus, that may be lost towards the hingeline. Oxoplecia , however differs from the Cliftonia by lacking a ventral tube ( Wright 1963), which is present in the East Baltic specimens.

Stratigraphic and geographic range.—This widespread species occurs in the Hirnantian Stage, Upper Ordovician. It is a key element of the Hirnantia brachiopod fauna in Argentina, Canada, China, the Czech Republic, England, Ireland, Norway, Sweden, Thailand, and Wales (see Rong and Harper 1988; Rong et al. 2002; Benedetto et al. 1990). It also occurs in Estonia and Latvia (this paper).