LUCINOIDEA, J. Fleming, 1828

Taylor, John D. & Glover, Emily A., 2006, Lucinidae (Bivalvia) - the most diverse group of chemosymbiotic molluscs, Zoological Journal of the Linnean Society 148 (3), pp. 421-438 : 423

STATUS OF LUCINOIDEA

Traditionally, the Lucinidae have been grouped together with the families Fimbriidae , Thyasiridae and Ungulinidae within the superfamily Lucinoidea ( Dall, 1901; Thiele, 1934; Chavan, 1969), largely on the basis of similarity of shells and some anatomical features. Chemosymbiosis in Lucinidae was first reported in the early 1980s ( Berg & Alatalo, 1984; Reid & Brand, 1986) and subsequently Thyasiridae and Fimbriidae were shown to have some chemosymbiotic species ( Dando & Southward, 1986; Southward, 1986; Janssen, 1992), but no chemosymbionts have been reported from any Ungulinidae . Also, often included within the Lucinoidea are the poorly known brackish water family Cyrenoididae and the Mesozoic and Palaeozoic fossil groups Mactromyidae , Paracyclidae , and Babinkidae ( Chavan, 1969; Boss, 1982; Johnston, 1993; Skelton & Benton, 1993; Amler, 1999). Species of the Recent genus Bathycorbis have been claimed as living representatives of the Mactromyidae (otherwise with no post-Cretaceous records) on the basis of supposed similarities of hinge teeth ( Chavan, 1959, 1969). However, the affinities of these small (c. 5 mm) offshore bivalves from Australia, known only from shells, are uncertain.

Most discussions of relationships within the Lucinoidea have concerned only the families Lucinidae , Thyasiridae , Ungulinidae and Fimbriidae , with the tacit assumption that they form a monophyletic group. Several scenarios have been proposed that attempt to place the Lucinidae , Thyasiridae , Ungulinidae and sometimes Fimbriidae into an evolutionary sequence. There have been major differences between each of these, relating to either anatomical characters, or time of first appearance in the fossil record ( Allen, 1958; McAlester, 1966; Boss, 1970; Reid & Brand, 1986; Hickman, 1994). A particular problem has concerned the position of the Ungulinidae , a family with apparently underived anatomical features, but with a relatively late appearance (Cretaceous) in the fossil record. Allen (1958) regarded the Ungulinidae as basal in his phylogenetic scenario, but McAlester (1966) and Boss (1970) thought it the most derived, the latter suggesting that the outer ctenidial demibranchs, absent in Lucinidae , had been reacquired in Thyasiridae & Ungulinidae . Boss (1970) considered the Fimbriidae more closely related to the Lucinidae than to Ungulinidae or Thyasiridae . Subsequent to the discovery of chemosymbiosis in lucinoids, Reid & Brand (1986), Reid (1990), and Hickman (1994) considered that the trait was probably plesiomorphic for the superfamily, but partially lost in the Thyasiridae and totally lost in the Ungulinidae .