PHENETIC DISCRIMINATION OF BIOMETRIC SIMPLETONS - PALEOBIOLOGICAL IMPLICATIONS OF MORPHOSPECIES IN THE LINGULIDE BRACHIOPOD GLOTTIDIA

The extreme morphological simplicity of lingulide brachiopod shells ma kes them particularly useful for investigating the species-level taxon omic resolution of the fossil record as well as the relationships betw een taxonomy, morphological complexity, and evolutionary rates. Lingul ides have undergone little change in shell morphology and have had low taxonomic diversity since the Paleozoic. Is this pattern an evolution ary phenomenon or an artifact of the shell's simplicity? Multivariate methods were used to establish morphogroups among seven populations of four extant species of Glottidia. Six characters (three shell dimensi ons and three internal septa) were measured for 162 specimens from fie ld and museum collections. All populations follow similar allometric t rajectories: internal septa display positive allometry and shell dimen sions display negative allometry. The allometric pattern may reflect D 'Arcy Thompson's Principle of Similitude. Principal component analysis does not reveal any distinct clusters in Glottidia morphospace but su ggests that some differences independent from ontogeny exist among the populations. Size-free canonical variate analysis indicates the prese nce of five size-invariant groups that are statistically distinct. boo tstrap-corrected error rates indicate that four specimens are enough t o classify a sample correctly at alpha = 0.05 and eight specimens at a lpha = 0.01. The groups are consistent with neontological classificati on with the exception of two populations of G. pyramidata identified b y discriminant analysis as two distinct groups. The size-free morphogr oups reflect geographic separation rather than ontogenetic or substrat e differences among the populations.Despite the morphological simplici ty of the shell, size-free multivariate analysis of Glottidia delineat es groups that offer taxonomic resolution comparable with the neontolo gical classification. The method offers a promising tool for identifyi ng natural morphogroups on the basis of few morphological characters. Moreover, the agreement between neontological taxonomy and the morphog roups suggests that the size-free approach can be applicable for evalu ating the reality of the low diversity and turnover rates observed in the fossil record of lingulide brachiopods (= Family Lingulidae). Assu ming that the neontological species of Glottidia are biologically mean ingful, this study shows that morphological simplicity of lingulides d oes not necessarily result in taxonomic underresolution. Our analysis, as well as several previous case studies, suggests that taxonomic div ersity and turnover rates do not have to be dependent on the morpholog ical complexity of preservable parts. In many cases, when rigorous qua ntitative methods are employed, the differences in the rates of morpho logical evolution may be a real evolutionary phenomenon and not artifa cts of morphological complexity.