CARDIINAE, Lamarck, 1809

Schneider, Jay A., 2002, Phylogeny of cardiid bivalves (cockles and giant clams): revision of the Cardiinae and the importance of fossils in explaining disjunct biogeographical distributions, Zoological Journal of the Linnean Society 136 (3), pp. 321-369 : 359-361

CARDIINAE

All the most parsimonious trees support a clade uniting Cardium , Bucardium , Vepricardium and Dinocardium . A close relationship amongst the first three of these taxa has long been recognized, often with Bucardium considered a subgenus of Cardium ( Keen, 1951, 1969a, 1980; Voskuil & Onverwagt, 1989) or Vepricardium a subgenus of Bucardium ( Popov, 1977) . However, the position of Dinocardium has not been well understood. Based on its lack of ornament, weak umbonal ridge and weakened posterior ribs, Keen (1951, 1969a, 1980) considered Dinocardium a subgenus of the noneucardiid Laevicardium . However, Popov (1977), Kafanov & Popov (1977), Kafanov (1980) and Schneider & Carter (2001) have found that the shell microstructure, shell morphology and anatomy clearly indicate that Dinocardium is a eucardiid, and specifically that it belongs in the Cardiinae . Dinocardium ’s radial ribs are wide and grooved underneath, as in virtually all Cenozoic eucardiids and unlike all profragines and noneucardiids ( Schneider, 1998a; Schneider & Carter, 2001). Five synapomorphies unite Dinocardium with Vepricardium , Bucardium and Cardium (see Results).

Acanthocardia s.l. as understood by Keen (1951, 1969a, 1980) and Popov (1977) is not monophyletic, for Schedocardia is not most closely related to Acanthocardia s.s. or Acanthocardia s.s. + Rudicardium. Schedocardia is the first cardiine to appear in the fossil record, in the Late Palaeocene ( Keen, 1969a, 1980; Schneider, unpubl.), and is the most basal cardiine in a slim majority (56%) of the most parsimonious trees. Otherwise, Schedocardia is one branch of a four-way polytomy with Dinocardium + Vepricardium + Bucardium + Cardium , Acanthocardia s.s. + Rudicardium and Chesacardium + Planicardium .

Planicardium and Chesacardium are the most problematic taxa in the analysis. Olsson (1967) erected Planicardium as a monotypic genus with resemblances to Clinocardium and Dinocardium . The only species placed in Planicardium by Olsson (1967) was Cardium virginianum Conrad, 1839 , Miocene-

Conrad (1863): Cardium (Cerastoderma) : acutilaqueatum Conrad, 1839 carolinensis Conrad, 1863 craticuloides Conrad, 1845 laqueatum Conrad, 1830 leptopleura Conrad, 1845 virginianum Conrad, 1839

Dall (1900): Cardium (Cerastoderma) , section Cerastoderma : acutilaqueatum laqueatum leptopleura virginianum californiense Deshayes, 1839 nuttallii Conrad, 1837 blandum Gould, 1850 pseudofossile Reeve, 1844 ciliatum Fabricius, 1780 decoratum Grewingk, 1850 waltonianum sp. nov. pansatrum sp. nov. druidicum sp. nov. taeniopleura sp. nov.

Dall (1900): Cardium (Cerastoderma) , section Dinocardium : phlyctaena sp. nov. chipolanum sp. nov. taphrium sp. nov. robustum Solander, 1786

Mansfield (1932): Cardium (Cerastoderma) virginianum laqueatum blountense subsp.nov. acutilaqueatum ‘sp. cf. taphrium ’

Gardner (1944): Cerastoderma laqueatum acutilaqueatum virginianum

Oleksyshyn (1959): Cerastoderma chancellorensis sp. nov. Compares it to laqueatum

Olsson (1967): Planicardium gen. nov. Cardium virginianum

Campbell et al. (1975): Planicardium acutilaqueatum virginianum

Keen (1980) classified Planicardium as a subgenus of Clinocardium because the “beaks [are] prosogyrate, [with] the intial part or tip lying above anterior cardinal teeth.”

Stanley (1986): Planicardium acutilaqueatum virginianum taeniopleura

Campbell (1993): Clinocardium (Planicardium) :

“Large shells externally resembling Dinocardium , but with a hinge resembling Cerastoderma ”: acutilaqueatum taeniopleura virginianum

Campbell et al. (1995): Clinocardium (Planicardium) acutilaqueatum

Pliocene of the Atlantic coastal plain. C. virginianum had been considered by many authors ( Table 9) to be one of several closely related Miocene to Pliocene species of Cerastoderma ( Conrad, 1863; Dall, 1900; Mansfield, 1932:; Gardner, 1944). Dall also included in Cerastoderma a number of Miocene to Recent species now considered to belong to Dinocardium and Clinocardiinae . Stewart (1930) also included Dinocardium and species now considered clinocardiines in his concept of Cerastoderma .

Many of the species that that had been allied to C. virginianum were placed in the new genus Chesacardium by Ward (1992), including C. acutilaqueatum Conrad, 1839 , which Stanley (1986) and Campbell et al. (1975) had placed in Planicardium . Ward (1992) compared Chesacardium with Dinocardium but did not mention Planicardium .

As discussed in Methods, Keen (1980), followed by Campbell (1993) and Campbell, Campbell & Carter (1995), classified Planicardium as a subgenus of Clinocardium on the basis of the position of the umbo relative to the cardinal teeth; however, this character is equivocal and I have found several characters that support a relationship of Planicardium with either or both Chesacardium and Dinocardium . Planicardium and Chesacardium are united by two characters (see Results). The cladistic coding of these two taxa differs in three of the 29 characters ( Table 1). Planicardium ’s adductor muscles are impressed into the shell, whereas they are not on Chesacardium ; some of the species assigned to Chesacardium by Ward (1992) actually do have adductor muscles slightly impressed into the shell (Schneider, unpubl.). Planicardium and Chesacardium have differently shaped right posterior cardinal teeth. For both of these characters, Chesacardium is coded the same as Dinocardium . Finally, as discussed in Methods, Chesacardium ’s shell shape is intermediate between that of Dinocardium and Planicardium ( Figs 3 , 6C , 9B ). Indeed, Chesacardium ’s overall morphology appears to be ‘intermediate’ between Dinocardium and Planicardium . It is suggested that a lineage closely related to Dinocardium evolved the more elongate shell shape of first Chesacardium , and then the even more elongate shell shape of Planicardium via peramorphosis, for the adult morphology of Chesacardium has developed beyond that of Dinocardium , and then the adult morphology of Planicardium has developed beyond that of Chesacardium . Chesacardium appears to be a basal grade of Planicardium ; Campbell (1993) even considered Chesacardium a subjective junior synonym of Planicardium . A specieslevel phylogeny of Cardiinae is necessary to determine whether Chesacardium is either a paraphyletic grade of Planicardium or its monophyletic sister group, and whether Planicardium + Chesacardium is the sister taxon to Dinocardium . Furthermore, such a specieslevel phylogeny is necessary to determine whether the Planicardium-Chesacardium lineage is indeed derived peramorphically from some species of Dinocardium or a close relative.

Figures

Figure 3. Shell ontogeny. A, Chesacardium laqueatum (FMNH UC7082), B, Chesacardium laqueatum (FMNH PE3523). C, D, Planicardium virginianum (UNC 11856). All scale bars = 10 mm.

Figure 6. External views of right valves. A, Cardium costatum (ANSP 54110), cardiiform shell shape. B, Nemocardium bechei (ANSP 252661), quadrate-short shell shape. C, Planicardium virginianum (USNM 2831), planiform shell shape. All scale bars = 10 mm.

Figure 9. External views of right valves. A, Europicardium multicostatum (NHM PI TB5), europiform shell shape. B, Dinocardium robustum (ANSP 186595), dinoform shell shape. C, Vepricardium multispinosum (ANSP 54220), circular shell shape. D, Bucardium ringens (ANSP 54234), circular shell shape. All scale bars = 10 mm.