Isognomon Solander

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 : 724-726

Subgenus Isognomon Solander in Lightfoot, 1786 Type species: Ostrea perna Linné, 1767 “in Indiis”, Indian Ocean;Recent.

Remarks.—Four species of Isognomon (Isognomon) from Upper Jurassic strata in the Lusitanian Basin can be distinguished. Two of these species, i.e., I. (I.) promytiloides Arkell, 1933 and I. (I.) rectangularis Fürsich and Werner, 1989 , represent epibyssate forms with relatively flat, laterally compressed valves and only weakly prosogyrous beaks. These taxa are not further discussed herein, as they have been described adequately by Fürsich and Werner (1989a).

Two other Isognomon (I.) species from the Lusitanian Basin that were held to be synonymous by Fürsich and Werner (1989a) are adapted to an endobyssate, semi−infaunal life style. The multivincular ligament of these taxa is usually situated in 14 to 19 resilifers in adult shells. Specimens with 12 to 23 resilifers may occur. Initial rapid shell and ligament growth decreases gradually with the bivalve attaining maturity. In gerontic specimens, total shell growth had nearly ceased, while the ligament area still increased in height. Nevertheless, the angle between the ligament surfaces in left and right valves is constant during ontogeny.

Isognomon (Isognomon) rugosus (Münster, 1835) Figs. 4A, F, H, 19A, B, E–G.

1835 Perna rugosa sp. nov.; Münster in Goldfuss 1835: 105, pl. 108: 2b only.

1952 Isognomon (Isognomon) bergeroni (Bigot Mss) sp. nov.; Chavan 1952: 23, pl. 1: 39–42.

1986 Isognomon rugosus ; Fürsich and Werner 1986: fig. 24.

1989a Isognomon (Isognomon) rugosus (Münster, 1835) ; Fürsich and Werner 1989a: 123–125, text−fig. 12, pl. 8: 2 only.

?2002 Isognomon (Isognomon) rugosum (Münster in Goldfuss, 1835); Delvene and Fürsich 2002: 200, pl. 1: 3.

?2007 Isognomon (Isognomon) rugosum (Münster, 1835) ; Delvene 2007: 14, pl. 1: 7.

non 2007 Isognomon (Isognomon) rugosus (Münster, 1835) ; Schneider and Werner 2007: p. 135, fig. 8A–C.

For extensive synonymy and description see Fürsich and Werner (1989a). Type locality: Not designated; “Weserkette”, Lower Saxony / Westphalia, NW Germany .

Type horizon: “In dark Jurassic limestone and the underlying ferruginous sandstone” (translation of Münster in Goldfuss 1835);? Heersumer Schichten,? Lower Oxfordian .

SCHNEIDER ET AL.—ECOPHENOTYPIC PLASTICITY AND EVOLUTIONARY TRENDS IN JURASSIC BIVALVES 725

10 mm

Original diagnosis.—“ Perna testa subrhomboidea dorso convexa anguloso−rugosa in alam depressam producta, umbonibus prominantibus, margine cardinali abliquo, canaliculis (18) subarcuatis.” (Münster in Goldfuss 1835).

doi:10.4202/app.2009.0062

Emended diagnosis.—Thick−shelled, anterio−ventrally inflated, small− to medium−sized Isognomon (maximum ligament length ~ 70 mm) with subtrigonal, subrhomboidal, or subrectangular shell outline, strongly prosogyrous, tapering beaks, and an average of 3.3 resilifers per centimetre ligament length.

Remarks.— Fürsich and Werner (1989a) provided a detailed description of this species, which, besides the indication of size, accurately matches the displayed shell characters. The designated lectotype ( Fürsich and Werner 1989a) figured by Münster in Goldfuss (1835: pl. 108: 2b), could not be detected either at the Goldfuss Museum, Bonn or at the BSPG, and must therefore be considered lost. The figure shows the inner side of a fragment of a left valve, including the ligamental area and a large portion of the anterior margin ( Fig. 15G). Completeness of the ligamental area cannot be clearly inferred from the figure. The posterior and ventral part of the shell (more than 50% of the original size) is missing. The lectotype plots well with Group I, when included in the resilifer density plot ( Fig. 9 ). This largely holds also true of several specimens of “ Isognomon bergeroni ” from the Sables de Glos ( Fig. 15F; Late Oxfordian in age; Buffetaut et al. 1985) in western France investigated at the MNHN in Paris. However, the specimens of Isognomon rugosus from the Tithonian of northern and central France figured by de Loriol and Pellat (1866, 1874–75), also largely plot with I. rugosus from Portugal ( Fig. 9 ), suggesting that this species may have survived in France, and Isognomon lusitanicus (see below) may represent a form endemic to the Lusitanian Basin. Thus, measurements based on figures from old monographs should be taken with care, as, often due to printing demands, the magnification of figured specimens may deviate distinctly from that given in the figure captions.

Isognomon (Isognomon) lusitanicus ( Sharpe, 1850) Figs. 4B–E, G, I, 19C, D, H, I.

1850 Perna lusitanica , n.s.; Sharpe 1850: 189–190, pl. 23: 7, 8.

1986 Isognomon rugosus ; Fürsich and Werner 1986: fig. 24.

1989a Isognomon (Isognomon) rugosus (Münster, 1835) ; Fürsich and Werner 1989a: 123–125, text−figs. 12, 13, pl. 8: 1, 6, 7, pl. 12: 7, pl. 18: 2, pl. 20: 3.

2007 Isognomon (Isognomon) rugosus (Münster, 1835) ; Schneider and Werner 2007: 135, fig. 8A–C.

Type locality: Not designated; “between Enxarra dos Cavalheiros and San Sebastian, and between Sobral and Torres Vedras ” ( Sharpe 1850), western Arruda subbasin .

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

Original diagnosis.—“Subrhomboidal, very thick and heavy, marked irregularly by the lines of growth: anterior side deeply depressed, and sloping inwards; posterior side slightly rounded away towards the palleal margin: cardinal margin arched. Hinge−areas very broad, receding one from another, and projecting anteriorly: ligamental hollows long and numerous, broader than the intervening spaces, and slightly increasing in width posteriorly, about twenty−four in number in a large specimen. Valves very thick, especially towards the hinge; space occupied by the animal nearly rectangular and very small in proportion to the whole shell. Right valve larger and thicker, and with a deeper hinge−area than the left.” ( Sharpe 1850).

Emended diagnosis.—Thick−shelled, anterio−ventrally inflated, medium− to large−sized Isognomon (ligament length usually 80–125 mm in adults) with subtrigonal, subrhomboidal, or subrectangular shell outline, strongly prosogyrous, tapering beaks, and an average of 1.8 resilifers per centimetre ligament length.

Remarks.—The original diagnosis of Sharpe (1850) characterises the species quite well. Hence, none of the studied specimens exhibits 24 resilifers. Only 10 out of 189 valves with completely preserved ligament area yield more than 20 resilifers, with a maximum of 23 resilifers in a single specimen. Moreover, a marked asymmetry between left and right valves as proposed by Sharpe (1850) could not be detected. The type specimen of Sharpe (1850) ( Fig. 15I) plots well with Group II, when included in the resilifer density plot ( Fig. 9 ).

Except with respect to size, the description of the shell of Isognomon rugosus in Fürsich and Werner (1989a) matches exactly with the characters found also in Isognomon lusitanicus . Generally, I. lusitanicus can be easily distinguished from I. rugosus by its substantially larger size in adulthood, and, most strikingly, by the average number of resilifers per centimetre, which is by 1.5 lower than in I. rugosus . Consequently, I. lusitanicus must be considered a valid species. Additionally, the two species occur in different time intervals ( I. rugosus : Late Oxfordian–Early Kimmeridgian; I. lusitanicus : Late Kimmeridgian–Early Tithonian). We infer that I. lusitanicus evolved in the Lusitanian Basin from I. rugosus during the Kimmeridgian, approximately 152 Ma ago. To date, I. lusitanicus has not been recorded from outside the Lusitanian Basin, and is considered as endemic.

Figures

Fig. 9. Scatter plot of resilifer number over ligament length in Isognomon. The two groups that correspond to lithostratigraphy are clearly visible. Numbers in squared brackets refer to Fig. 2.