THE SPECIES PROBLEM WITH FOSSIL ANIMALS 



mind, data on a collection of the Pleistocene bamboo rat 

 Rhizomys were compared with data on three subspecies of the 

 modern Rhizomys sinensis (Fig. 5). The extensive mutual overlap 

 among the four samples in the six measured skeletal features was 

 then used as a basis for considering the fossil a subspecies, R. 

 sinensis troglodytes. 



A similar approach by Kotaka (1953) on recent and fossil 

 populations of the arcid pelecypod Anadara is illustrated in Fig. 

 6. Having concluded that the population in question could best 

 be distinguished on the basis of two shell indices ( H/L and 

 C/L), Kotaka calculated and graphed the 0.95 elliptical contour 

 of each bivariate density distribution. Note that although four 

 fossil and three recent populations are included in this study, 

 each receives the same taxonomic treatment; and there is every 

 reason to suppose that Kotaka's conclusions apply equally to fossil 

 and modern forms. 



An illustration of the taxonomic importance of intrapopulation 

 variation patterns is furnished by Westoll's (1950) study of a 

 large sample of the Permian terebratuloid brachiopod DieJasma 

 elongata (Fig. 7). Frequency polygons of the thickness-length 

 ratio reveal a distinct bimodality in larger size classes, although 

 in all other respects shell characters display unimodal distribution 

 patterns. This bimodality is interpreted as sexual dimorphism. 

 Westoll's brachiopods and Kotaka's clams illustrate the same 

 point: sound taxonomic inferences can be made solely on the 

 basis of the distribution pattern displayed by shell features whose 

 functional significance is incompletely known. 



One troublesome limitation of paleontological data lies in the 

 difficulty (usually the impossibility) ol distinguishing phenotypic 

 from genotypic variation. This problem is particularly evident to 

 the taxonomist analyzing statistically significant morphological 

 differences among a small number of samples whose stratigraphic 

 relationships arc poorly known. If suitable collections arc avail- 

 able for study, however, the taxonomic (but not the genetic) 

 problem may disappear. A case in point is McKerrow's (1953) 

 detailed study ol contemporaneous brachiopod communities sam- 

 pled from many localities along the outcrop of a thin stratigraphic 



