Arcomytilus morrisii ( 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 : 723-724

Arcomytilus morrisii ( Sharpe, 1850)

Fig. 3A–F .

1850 Mytilus morrisii , n. s.; Sharpe 1850: 187, pl. 22: 5a, b.

1866 Mytilus morrisii Sharpe ; de Loriol and Pellat 1866: 89, pl. 9: 1, 2.

1868 Mytilus morrisii Sharpe ; de Loriol and Cotteau 1868: 187, pl. 13: 9.

1872 Mytilus morrisii Sharpe ; de Loriol et al. 1872: 335, pl. 19: 2.

1988 Arcomytilus morrisii ( Sharpe, 1850) ; Fürsich and Werner 1988: 123–125, text−figs. 10–12, pl. 4: 5–10.

For extensive synonymy and description see Fürsich and Werner (1988).

Type locality: Not designated; “ between Sobral and Torres Vedras ”

( Sharpe 1850); western Arruda subbasin.

Type horizon: “limestone of the subcretaceous series “; presumably doi:10.4202/app.2009.0062

Arranhó I member of Farta Pao formation (Upper Kimmeridgian/ Lower Tithonian).

Original diagnosis.—“Shell falciform, with an elevated keel dividing the valves into two very unequal portions, the posterior gibbose and rounded, the anterior excavated, and nearly perpendicular to the plane which separates the valves; with the exception of a small smooth space near the beak on the anterior side, the valves are covered with numerous rounded bifurcating ribs, crossed by concentric lines, which latter are most strongly marked on the anterior side; the ribs on the posterior side are coarse, and radiate from the beak; those on the anterior side rise from the keel, and are much finer than the others: beaks pointed and terminal.” ( Sharpe 1850).

Remarks.—The holotype of Arcomytilus morrisii figured by Sharpe (1850) represents a large, fully grown, typical specimen of this species. Consequently, the comprehensive original diagnosis matches the appearance of the species quite accurately. However, as discussed above, a large variability of shell shapes exists. As noted by Fürsich and Werner (1988), some specimens of A. morrisii , especially from Oxfordian and Lower Kimmeridgian strata in Portugal, closely resemble Arcomytilus pectinatus (Sowerby, 1821) ( Fig. 3I ). Additionally, morphometric analyses revealed affinities to Arcomytilus bathonicus ( Fig. 3H ) and Arcomytilus asper ( Fig. 3G ) (which are, however, distinctly smaller) of several specimens of A. morrisii . However, all of these species are quite variable in shape and several other nominal species of Jurassic Arcomytilus closely resemble certain specimens from Portugal. On one hand, these problems may indicate that the genus is in need of a comprehensive revision. On the other hand, morphological species concepts in extant Mytilidae have been proven to be insufficient in smooth ( Mytilus : Gosling 1992a, b; Gardner 2004; Bathymodiolus : Maas et al. 1999) and ribbed ( Brachidontes : Lee and Ó Foighil 2004, 2005; Aguirre et al. 2006) representatives, suggesting that a separation of fossil species may also be difficult in many cases. Similar results were achieved by a morphometric study using complex measurements for outline description of Devonian mytiliform bivalves ( Hajkr et al. 1960). Of course, a typically−shaped individual of Arcomytilus morrisii can be distinguished easily from a typically−shaped A. pectinatus shell. Knowledge of typical shape and shape variability of a species, however, requires the possibility to study large populations, preferably from several localities. Based on the more than 300 specimens from Portugal included in this study, A. morrisii is regarded a valid species that can, in most cases, be clearly distinguished from A. pectinatus or other similar species. Typical A. morrisii specimens are characterised by a relatively slender elongated shape, tapering, often curved beaks, a rounded posterio−ventral margin (in A. pectinatus , it is angularly truncated or even incurved in gerontic specimens), and a coarser and more irregular ribbing pattern than in other species.

Most likely, A. pectinatus invaded the Lusitanian Basin soon after the establishment of marginal marine environments in the Middle Oxfordian. Possibly due to geographical isolation, A. morrisii evolved from the former species in the basin, while A. pectinatus persisted in other shallow areas of the Western European epicontinental seas. Presumably, A. morrisii spread from central Portugal into northern and central France during the Tithonian, as it has been recorded in these regions with largely typically−shaped specimens from strata of that age (de Loriol and Pellat 1866, de Loriol and Cotteau 1868; de Loriol et al. 1872).

Figures

Fig. 3. Specimens of mytilid bivalve Arcomytilus from the Middle and Upper Jurassic of Portugal and France. A–F. Arcomytilus morrisii (Sharpe, 1850). A. Large articulated specimen with pointed umbones and slightly imprinted anterior part. Arranhó II member, earliest Tithonian, Serra de Alrota. GML 25900. B. Small adult specimen, left valve with fine ribbing pattern and relatively straight anterior margin. Alcobaça formation, Late Kimmeridgian, Consolação. GML 25901. C. Young adult specimen, left valve with coarse ribbing pattern and large, elevated disc. Arranhó II member, Early Tithonian, Santa Cruz. GML 25902. D. Small adult, articulated specimen. Alcobaça formation, Early Kimmeridgian, Salir do Porto. GML 25903. E. Large articulated specimen with extremely triangular outline and wide−spaced, strong ribs. Arranhó II member, Early Tithonian, Lameiro das Antas. GML 25904. F. Adult specimen, left valve with bi− and trifurcation and simultaneous insertion of ribs. Arranhó II member, Early Tithonian, Santa Cruz. GML 25905. G. Arcomytilus asper. Right valve. Late Bathonian, Luc−sur−Mer, Calvados, France. MNHN J 08224. H. Arcomytilus bathonicus. Right valve. Late Bathonian, Luc−sur−Mer, Calvados, France. MNHN, coll. Deshayes 1876−8. I. Arcomytilus pectinatus. Right valve. “Corallien”, La Rochelle, Charente−Maritime, France. MNHN, coll. d’Orbigny 4247.