«One can only believe entirely, perhaps,
in what one cannot see.» Orlando VIRGINIA WOOLF, 1928 |
«Hardly any bivalve family rivals the immense number of species that Pectinidae Rafinesque-Schmaltz, 1815 (scallops) evolved during nearly 250 million years of phylogeny. Apart from sheer diversity, also commercial importance and the aesthetics of the shell have attracted considerable interest in this family. Yet, the phylogenetic origin of the Pectinidae has been controversial.
The current consensus is that two key characters define the Pectinidae (Waller 2006). The first is the alivincular-alate ligament condition (Hautmann 2004), which is characterized by the internal position of the resilium with respect to the lateral parts of the ligament and by a resilium with an uncalcified median core. The internal position of the resilium results from the ventrally directed growth of this structure, whereas the lateral parts of the ligament remain more or less at the dorsal margin, with no ventral shift as shell growth proceeds. This is an easily recognizable difference to the primitive alivincular-areate ligament type, in which the lateral ligament and submarginal resilium shift jointly in a ventral direction during ontogeny (Hautmann 2004). The second key character of pectinids is a comb-like row of denticles along the ventral edge of the byssal notch termed ctenolium. The ctenolium strengthens byssal attachment by spreading byssal threads where they pass over the disk (Waller 1984). Pectinidae share the alivincular-alate ligament system with the geologically older Entoliidae Korobkov, 1960 (herein understood to include ‘‘Pernopectinidae’’ and ‘‘Entolioididae’’), whereas a ctenolium is unique to Pectinidae. Waller (2006) suggested that the similarity in the ligament system sufficiently establishes monophyly of Entoliidae and the pectinid clade. By contrast, the traditional view first expressed by Newell (1938) is that Entoliidae were too specialized to give rise to Pectinidae, implying that the shared presence of an alivincular-alate ligament system evolved convergently. The major problem in unravelling the origin of pectinids is the lack of sufficient morphological data on early representatives of this family. It is generally agreed that the oldest taxon with a ctenolium and thus the earliest known pectinid is Pleuronectites laevigatus Schlotheim, 1820 from the Anisian-Ladinian (Middle Triassic) of the Germanic basin (Waller 1984; Newell and Boyd 1995). Waller (2006, p. 331) mentioned that ‘‘Praechlamys’’ reticulata (Schlotheim, 1823), which first appeared in the Lower Wellenkalk (Bithynian stage of the Anisian) and thus only slightly later than P. laevigatus (first appearing in the Upper Buntsandstein [Aegean stage]; Schmidt 1938, p. 95; for chronostratigraphy see Deutsche Stratigraphische Kommission 2002), also had a ctenolium. However, this observation has not been illustrated, and the ligament system of ‘‘Praechlamys’’ reticulata is unknown.» The first scallop
MICHAEL HAUTMANN, 2010 |