Eucalathis methanophila Bitner, 2010

Kaim, Andrzej, Bitner, Maria Aleksandra, Jenkins, Robert G. & Hikida, Yoshinori, 2010, A monospecific assemblage of terebratulide brachiopods in the Upper Cretaceous seep deposits of Omagari, Hokkaido, Japan, Acta Palaeontologica Polonica 55 (1), pp. 73-84 : 79-80

Eucalathis methanophila Bitner sp. nov.

Figs. 2–5.

Etymology: From methane and Latin suffix, phila, meaning to like. Refers to its occurrence at the methane seep.

Type material: Holotype: UMUT MB30198 ( Fig. 2A 1 –A 5 ), articulated shell ; Paratypes: UMUT MB30199−30202 ( Fig. 2B–E), articulated shells .

Type locality: Omagari site, Nakagawa area , Northern Hokkaido, Japan. Coordinates 44 ° 39’ 26’’N, 144 ° 2’ 25’’E .

Type horizon: Hydrocarbon seep deposits of Omagari Formation , Campanian, Upper Cretaceous .

Diagnosis.— Eucalathis of medium size with surface ornamented by numerous beaded ribs, high beak with large, oval foramen, anterior commissure rectimarginate, loop wide with subparallel descending branches narrowed anteriorly.

Measurements (in mm).—See Table 2.

Description.—External morphology: Shell small (maximum length 5.7 mm), auriculate, variable in outline from widely subtriangular to elongate oval, with maximum width usually at two thirds of shell length. Shell biconvex with ventral valve slightly more convex. Shell surface covered with numerous beaded, tuberculate ribs (20–30) which increase in number by intercalations and bifurcations; growth lines distinct ( Fig. 2B 4 , F 1 , F 2). Lateral commissures nearly straight; anterior commissure rectimarginate to incipiently uniplicate. Hinge line wide, slightly curved. Beak relatively high, erect to suberect with sharp, distinct ridges. Foramen large, oval; disjunct deltidial plates form narrow ridges.

doi:10.4202/app.2009.0068

Internal morphology: Two specimens were sectioned of which the better preserved is figured ( Fig. 3 ). Pedicle collar present. Teeth long, smooth, nearly horizontally inserted into large dental sockets. Inner socket ridges massive, long. No hinge plates observed. Crura short, massive. Loop short and wide with subparallel to slightly divergent descending branches that narrow anteriorly ( Fig. 4 ). Transverse band not preserved in either of the sectioned specimens.

Shell microstructure: Two specimens were studied. The shell is composed of two layers: a primary microgranular layer and a secondary fibrous layer ( Fig. 5 ). In both cases the shell is diagenetically modified, fragmentary fibres of the secondary layer are completely fused ( Fig. 5A 1 ). The primary layer is 27–53 µm thick in the ribs and 12–25 µm in sulci. The secondary layer is 144–205 µm thick in the ribs and 100–114 µm in sulci. The total thickness of the shell is 127 to 241 µm. The punctae are more densely distributed in the rib regions ( Fig. 5 ).

Remarks.—The investigated specimens display, both externally and in a loop character, typical features of the genus Eucalathis . This genus was erected for the specimens similar to Terebratulina but having disjunct crural processes (Fischer and Oehler 1890); in Terebratulina crural processes are united to form a ring. In the ornamentation of the numerous fine ribs the specimens from Japan resemble three living species, Indo−Pacific E. murrayi ( Davidson, 1878) , eastern Atlantic E. tuberata (Jeffreys, 1874) , and Caribbean E. cubensis Cooper, 1977 but differ in having higher beak and wider loop ( Davidson 1878, 1886; Cooper 1977, 1981b; Zezina 1987). Additionally the studied specimens are nearly twice as large as E. tuberata .

Stratigraphic and geographic range.—Campanian (Upper Cretaceous) seep carbonate at Omagari (Hokkaido, Japan).

Figures

Fig. 1. Geological map of the Nakagawa area, Hokkaido, showing the location of the Omagari site. Modified from Takahashi et al. (2003).

Fig. 5. Terebratulide brachiopod Eucalathis methanophila Bitner sp. nov., Campanian, Omagari, Japan. A. UMUT MB30197, transverse section of ventral valve, visible fused fibres (A1), visible boundary of primary and secondary layers (A2). B. UMUT MB30203, ventral valve, transverse sections of the whole shell (B1), visible boundary between primary and secondary layers, and punctae (B2). All SEM.

Fig. 4. Recontruction of internal structures of Eucalathis methanophila Bitner sp. nov. based on transverse serial sections (Fig. 3) and 3D digital reconstruction (see at http://app.pan.pl/SOM/app55-Kaim_etal_SOM.pdf). Drawing by Ewa Widłak−Kaim.

Fig. 3. Transverse serial sections of terebratulide brachiopod Eucalathis methanophila Bitner sp. nov. through specimen UMUT MB30203, Campanian, Japan.Original dimensions of the specimens: L = 4.9 mm, W = 4.4. mm, T = 2.0 mm. Numbers indicate distance in mm from the tip of the ventral umbo.