In order to test the relationships of the genera within the Dakoticancridae, a cladistic analysis was designed and run. The study employed PAUP 3.1.1 to analyze the data matrix (Swofford, 1990). The program was run on a Macintosh computer.
Eight characters were selected as the basis for developing the data matrix (Fig. 3). These characters were those of the dorsal carapace and were selected because they reflected significant differences between the taxa and they were well enough preserved in each of the taxa to be measured. Each of the characters reflected an aspect of the morphology of the crabs that was judged to be sufficiently independent of the others that they would not be redundant. The hypothetical ancestor, against which the taxa were feature was assigned character states judged to be most primitive.
Because the number of taxa and the resulting data matrix was small, an exhaustive search was performed. The search resulted in a total of 15 trees of which 5 were equally parsimonious. For each, the consistency index was 1.000, the retention index was 1.000, and the homoplasy index was 0.000.
Examination of these five revealed that four of the trees shared the common character of placing Tetracarcinus at the base of the tree. As a result, and because Tetracarcinus is the first of the dakoticancrids to appear in the fossil record, the sole tree in which that genus appeared to be more derived was excluded from consideration. Using the criterion of order of appearance in the fossil record resulted in selection of one tree (Fig. 4) that realistically represents the arrangement of genera within the family.
Fig. 3. Data matrix and list of character states used in cladistic analysis of genera of Dakoticancridae.