Actinostreon Bayle, 1878

Koppka, Jens, 2015, Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland — Ostreoidea, Zootaxa 3927 (1), pp. 1-117 : 49-50

Genus Actinostreon Bayle, 1878

Type species. Ostrea solitaria J. de C. Sowerby, 1824 (p. 105, pl. 468, fig. 1); subsequent designation by Douvillé, 1879, p. 92. Weymouth (Dorset, England), „ Trigonia clavellata ” Beds ( Arkell 1933 in Arkell 1929–1937, p. 186), Middle Oxfordian, Upper Jurassic. The whereabouts of the type is unknown.

Diagnosis (modified from Stenzel 1971, p. N1158). Subequivalve, outline elongate oval and curved (crescentic); strongly plicate; adductor scar roundish, dorsally flattened or with slight sinus, position dorsocentral to posterodorsal; small straight chomata at least in some species; shell chambers numerous, small to medium-sized (about 6–10 cm), lenticular, predominantly (empty) filled with sparry cement.

Remarks. Malchus (1990) assumed that Actinostreon lacks chomata but later corrected that view (Malchus, 1998) based on Polish material from M. Machalski. It is still unknown, however, when species of that genus began to develop chomata.

There exists some confusion concerning the taxonomic position of Actinostreon and concept of the genus that warrants clarification. Bayle (1878) introduced the genus name on his legends to plate 132 [“ Actinostreon solitarium Sowerby , sp.”, figs. 2–6] and plate 143 [“ Actinostreon syphax ( Coquand, 1854) ”, figs. 1–3] without any diagnosis or further explications.

When Malchus (1990, p. 102, table 17) placed Actinostreon in the newly established Oscillophini (Palaeolophinae, Palaeolophidae) [now Arctostreidae Vialov, 1983 (Carter et al. 2011, p. 8)] he was unaware of the existence of Actinostreon syphax , as the corresponding plate 143 was missing in his xerox copy of Bayle (1878) (Malchus, pers. comm., 2014).

Malchus did, however, study a Morrocan specimen of “ O. ” syphax which showed compact foliated shell microstructure, in strong contrast to the multi-chambered Actinostreon gregareum and A. solitarium specimens studied by others and by him. He considered syphax as a member of highly plicate compact foliated oysters of the genus Ambigostrea, Liostreinae (Ostreidae) , today Flemingostreidae (Carter et al., 2011).

Unfortunately, Malchus (1990) inadvertently listed the palaeolophid (now arctostreid) genus Actinostreon also under the liostreine tribe Ambigostreini ( Malchus, 1990, p. 68–70, p. 176). In addition, he erroneously used Bayle’s original combination “ Actinostreon syphax ” on page 86 and in the figure legend to plate 25, figure 6. These lapsus calami (Malchus, pers. comm., 2014) have lead at least once to an ambiguous taxonomic assignment ( Dhondt et al. 1999, p. 69–70, 74) and has contributed to a contentious argument about the validity of the phylogenetic distinction between fossil and recent plicate oysters ( Hautmann 2001, 2006; Malchus 2008). However, independent of these errors, it seems that current molecular phylogenetic results favor Malchus’ view of convergent development of Lopha -like oysters (see introduction to the systematics chapter).

In addition to this problem, there also exist doubts concerning the first valid introduction of the species syphax and of its valid spelling. Coquand (1854) spelled his species scyphax (p. 143, 144, 153, pl. 4) as well as syphax (p.150) but later used syphax ( Coquand 1862) . It appears, therefore, that Coquand (1862) is the first revisor of his own species choosing syphax as the valid spelling (see IZCN 2000 online, Art. 24.2.3. and 24.2.4.).