Eomiodon securiformis ( Sharpe, 1850 )

Schneider, Simon, Fürsich, Franz T., Schulz-Mirbach, Tanja & Werner, Winfried, 2010, Ecophenotypic plasticity versus evolutionary trends-morphological variability in Upper Jurassic bivalve shells from Portugal, Acta Palaeontologica Polonica 55 (4), pp. 701-732 : 726-727

Eomiodon securiformis ( Sharpe, 1850)

Fig. 5A–H .

1850 Cyprina securiformis n. s.; Sharpe 1850: 182–183, pl. 22: 1–3. non 1860 Cyprina securifomis Contej. ; Contejean 1860: pl. 26: 10, 11. 1986 Eomiodon securiformis ; Fürsich and Werner 1986: 324, fig. 24. 1994 Eomiodon securiformis ; Fürsich 1994: 343, fig. 24A.

Type locality: Not designated; “ between Sobral and Torres Vedras and also about three miles south−west of Alenquer ” ( Sharpe 1850); NW and NE parts of Arruda subbasin .

Type horizon: “Subcretaceous limestone“ ( Sharpe 1850); presumably Sobral member of the Farta Pao formation (Upper Kimmeridgian / Lower Tithonian) .

Original diagnosis.—“Cordato−triangular, with the dorsal margin continued in a regular curve from the umbo to the posterior angle; ventral margin rounded; anterior margin with a deep depression below the beaks, which are placed very forward. The dorsal side is nearly perpendicular to the rest of the valve and marked with two slight longitudinal carinae. Surface nearly smooth, with some concentric wrinkles which are stronger near the ventral margin.” ( Sharpe 1850).

Description.—Shell thick and rounded; slightly elongate to equally−sided sub−triangular in outline; slightly longer than high (average length/height ratio 1.2) and moderately inflated; relatively variable in shape; beaks moderately to strongly prosogyrous. Lunula medium−sized in young individuals, becoming relatively smaller in adulthood. Two distinct radial ridges descending from beak in posterior part of shell. First ridge slightly rounded, bordering main disc towards a small posterior band. Pronounced second ridge separating entire disc from markedly impressed acute−oval area, extending almost to ventral margin. Escutcheon deeply incised, stretching over almost half of the posterior dorsal shell margin. Disc ornamented with more or less distinct, slightly irregular commarginal growth lines. Commarginal lamellae prominent in young individuals at anterior shell portion, fading in central part of disc; reappearing at posterior part, running across first radial ridge up to summit of second ridge ( Fig. 5E, G ). Lamellae lacking in adulthood. Pallial line integripalliat. Adductor muscle scars relatively small, round−elliptic, almost equally sized. Hinge plate broad, triangular. Right valve: 3a relatively small, knob−like triangular, almost fused with hinge plate at lower end. 3b broad and prominent, elongated−triangular. 5b weakly developed, slen− der. AI broadened and more prominent in upper part, clearly separated from antero−dorsal shell margin by distinct furrow; distinctly set off from 3a. P1 sharp and prominent, distinctly elevated in lower part. Left valve: 2b well−developed. 4b elongated, distinct, almost fused with P2. A2 slender and distinct, clearly set off from 2b. Nymphs long and prominent, extending considerably above the escutcheon margin.

Remarks.—Although size increase in Eomiodon securiformis is obvious and stratigraphically younger specimens tend to display less elongated shell shapes, a separation of two stratigraphically and/or morphologically distinct species is impossible. From most other species, E. securiformis is easily distinguished by its much larger size. Probably most closely related to E. securiformis is Eomiodon cuneatus (Sowerby, 1816) , occurring in Upper Jurassic strata of Great Britain, France, and northern Germany (de Loriol and Pellat 1866; de Loriol et al. 1872; Huckriede 1967). This species also displays a certain size increase during the Late Jurassic in northern Germany, which was, however, not recorded in detail, but seems to have occurred rather gradually ( Huckriede 1967). However, even the “giant forms” mentioned by Huckriede (1967) hardly reach 45 mm in length, while E. securiformis frequently attains lengths of more than 60 mm, with a maximum size of 85 mm. Moreover, E. cuneatus is generally more triangular in outline, with a more elongated and pointed postero−ventral shell portion. Additionally, the radial ridges in the posterior shell part are less prominent. To date, E. securiformis has never been recorded from outside the Lusitanian Basin. The specimen figured by Contejean (1860) as “ Cyprina securifomis Contej. ” is an internal mold, and cannot be determined to genus or species; the presence of only one extremely sharp, radial ridge and the large posterior shell part, which is almost rectangular to the disc clearly indicate that this is not E. securiformis . However, a single, beautifully preserved articulated specimen from the Lower Kimmeridgian of the Algarve region, housed at the BSPG, is clearly E. securiformis and therefore prevents us from considering the species a form endemic to the Lusitanian Basin.

A second, previously undescribed species of Eomiodon ( Eomiodon sp. A of Werner 1986) occurs in the Alcobaça and Farta Pao formations (Arranhó II member) ( Fig. 16 ). These specimens hardly reach a length of 15 mm. However, it is obvious that they do not represent juvenile E. securiformis , as they are nearly circular in outline, while E. securiformis is markedly triangular even in early growth stages. Moreover, Eomiodon sp. A is ornamented with prominent regular commarginal lamellae, which are continuously well−developed from the anterior to the posterior margin, while the lamellae in young E. securiformis are more widely spaced and fade on the posterior flank just to reappear behind a weakly developed radial carina that is not present in E. sp. A. The escutcheon area in E. securiformis is much larger than in E. sp. A, and the nymph commonly rises above the inner shell margin. Additionally, the anterior lateral tooth (A2) in E. sp. A is much stronger developed than in E. securiformis .

Figures

Fig. 5. Specimens of neomiodontid bivalve Eomiodon securiformis (Sharpe, 1850) from the Upper Jurassic of Portugal. A–C. Sobral member, Late Kimmeridgian, E Arranhó. A. Hinge plates of left and right valve. GML 25915. B. Interior of right valve, showing hinge arrangement and parts of the muscle scars. GML 25916. C. Left valve view of articulated specimen. GML 25917. D. Articulated, strongly elongated, gerontic specimen. Sobral member, Late Kimmeridgian, Santa Cruz. GML 25918. E. Small articulated specimen with clearly visible commarginal lamellae. Alcobaça formation, Early Kimmeridgian, Vestiaria. GML 25919. F. Articulated gerontic specimen with ventrally elongated shell. Alcobaça formation, Early Kimmeridgian, Salgados. GML 25920. G. Short, rounded, articulated specimen. Sobral member, Late Kimmeridgian, E Arranhó. GML 25921. H. Large, high, and short specimen. Sobral member, Late Kimmeridgian, Porto das Barcas. GML 25922.

Fig. 16. Left valve of neomiodontid bivalve Eomiodon sp. from Early Tithonian, Arranhó II member, Santa Cruz (GML 25929) in internal (A) and external (B) views.