Intergradation and origins of subspecies of the turtle Chrysemys picta: morphological comparisons

Abstract

We present morphological data on the four subspecies of painted turtles (Chrysemys picta) from 20 states in the U.S.A. and three provinces in Canada, and from localities within them that are removed from reported zones of intergradation. The characters used are those of Hartman (1958), which have been routinely used in subsequent studies: the extent of plastral patterning (here quantified by image analysis), the degree of misalignment of carapacial scutes, and the width of the light anterior margins of the second lateral scutes. Our purposes are to present descriptive baseline data for each subspecies for use in comparisons with the results of past and future studies of intergradation, and to critique the oft-accepted theory of Bleakney (1958) regarding the evolutionary origins of the current subspecies, particularly C. p. marginata. Data are given for C. p. bellii from 5 states ranging from Minnesota to New Mexico, for C. p. dorsalis from 3 states (Arkansas, Louisiana, and Mississippi), for C. p. marginata from 4 localities (Ontario, Michigan, Indiana, and Ohio), and for C. p. picta from 11 localities from Nova Scotia to Georgia. We find that (i) a "pure" picta does not exist; there is marginata influence throughout picta's range, particularly at the extremes; (ii) similarities exist that group dorsalis with picta; (iii) there is a north-south cline of reduced scute alignment in picta; (iv) there is a north-south cline of increasing extent of the plastral pattern in bellii; (v) the question of which subspecies first reached New England after the Wisconsinan glaciation remains unresolved, but it was likely not picta; (vi) there is no compelling reason to accept or reject Bleakney's theory of the distribution of painted turtle subspecies at the height of the Wisconsinan glaciation or his conjecture that marginata arose as in intergrade of bellii and dorsalis; in fact, it could be argued that if marginata is a hybrid, it could be a hybrid of picta and bellii.