Fordilla Barrande, 1881

Kouchinsky, Artem, Bengtson, Stefan, Clausen, Sébastien & Vendrasco, Michael J., 2015, An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia, Acta Palaeontologica Polonica 60 (2), pp. 421-512 : 439

Genus Fordilla Barrande, 1881

Type species: Fordilla troyensis Barrande, 1881 ; lower Cambrian , New York State, USA .

Remarks.— Elicki and Gürsu (2009) noted a high degree of intraspecific and intrageneric variation in the very similar Cambrian bivalves Fordilla Barrande, 1881 and Pojetaia Jell, 1980 . In addition to the extensive variation within lower-level taxa, many of the proposed distinguishing characteristics such as muscle scar placement and dentition are not preserved in many fossils. Thus the most useful diagnostic character between these two genera appear to be shell shape, with the umbo typically more anteriorly placed in Fordilla Jell 1980 ; Runnegar and Bentley 1983). Elicki and Gürsu 2009) argued that Pojetaia and Fordilla can be reliably distinguished by total size ( Fordilla is larger) and the ratio of anterior length to total length. In the fossil described herein Fig. 21A ), the umbo is distinctly shifted towards the anterior margin in a way more reminiscent of Fordilla troyensis , the type species of that genus. The smaller specimen ( Fig. 21B ) may be a poorly preserved juvenile, but its more centrally placed umbo, ration of anterior length to total length (ca. 45%) and total length (ca. 1 mm) are similar to Pojetaia Jell, 1980 (see Elicki and Gürsu 2009).

Figures

Fig. 21. Internal moulds of bivalved molluscs from lower Cambrian Emyaksin Formation, Bol’shaya Kuonamka River, Anabar Uplift, Siberia; sample 7/70. A. Fordilla cf. F. sibirica Krasilova, 1977, SMNH Mo 167690; right lateral (A1), dorsal (A2), and oblique dorsal (A3) views; A4, close-up of A1. B. Undetermined fordillid bivalve, SMNH Mo 167691, left lateral view. C. Fragment of a larger specimen, interpreted herein as Fordilla cf. F. sibirica, SMNH Mo 167692, lateral view. Scale bar 1000 μm, except A4, 100 μm.