Marquez-Aliagaetal.(2005)presentedinthisjournalaninterestingpaperontheearlyphylogenyofoysters,inwhich they provided clues for ambivalent attachment(i.e.attachmentbytheleftorrightvalveindifferently)oftwo Middle Triassic cementing bivalve species from theGermanMuschelkalk.Chieflybasedonthisassumption,the species in question were transferred from theprospondylid? genus Enantiostreon to the ostreidgenus Umbrostrea, and a descent of the Ostreidaefrom prospondylid bivalves was postulated. Because ofthe far-reaching consequences of these conclusions forthe taxonomy of cementing bivalves, a re-discussion ofthe shell morphology of oysters in the light of theirenigmatic phylogenetic origin seems useful.Marquez-Aliaga et al. (2005, p. 129) stated that“there is a broad consensus that the most importantcharacter defining oysters is attachment to the substrateby the left valve”. However, this introductory statementof Marquez-Aliaga et al. (2005) is in contrast to thecentral find of their study, namely the occurrence ofambivalent attachment in some oyster-like species fromthe Middle Triassic Muschelkalk of the Germanic basin.If this observation was true, transferring the species inquestion (‘Ostracites’ cristadifformis Schlotheim, 1823and ‘O.’ spondyloides Schlotheim, 1823)totheOstreidae, as suggested by Marquez-Aliaga et al.(2005), would require to drop left-sided (sinistral)attachment as a key character of oysters. Even if thiswas accepted, the phylogenetic relationship of ambiva-lently cemented pteriomorphs would still be unclear.Marquez-Aliagaetal.(2005)proposedthatantimarginalribs were an additional shell morphological character ofthe Ostreidae, and that the presence of such ribs in theirspecimens would support its classification as ‘true’oysters.However,antimarginalribsalsooccurinseveralunrelated bivalve families (e.g. Checa and Jimenez-Jimenez, 2003, p. 147), which means that they haveeither developed convergently in different lineages, orthat they represent a plesiomorphic character state. Inorder to give an unambiguous definition of oysterswithout referring to sinistral attachment, it is necessaryto identify additional autapomorphic characters. As Ihave suggested earlier (Hautmann, 2001a, 2004),oysters have two other shell morphological peculiaritiesthat separate them from similar cementing bivalves:(1) Oysters lack a pallial line (Fig. 1), because theirpallial muscles begin and end within the mantlelobes. The only exception is Recent Saccostrea,where the mantle retractor muscles are directlyattached to the valves and a disjunct pallial line isdeveloped (e.g. Stenzel, 1971,p.N968).(2) The attachment surface of the ligament is usuallyarched, with the resilifer of the right valve beingelevated and the flanking attachment area of thelateral ligament impressed (Fig. 2). The shape of