Cladistic analysis of the living cassiduloids (Echinoidea), and the effects of character ordering and successive approximations weighting

Abstract

The crown group of the Neognathostomata, one of the three major clades of irregular echinoids, comprises the paraphyletic order Cassiduloida, the family Neolampadidae, and the order Clypeasteroida. Recent studies have recognized the latter two groups as sister taxa, while suggesting different relationships with other cassiduloids. Here parsimony analyses of 57 qualitative morphologic characters (having 127 states) are employed to determine relationships among all 30 living cassiduloid species and three representative clypeasteroids. The 30 trees (of 144 steps) obtained using unordered multistate characters support the branching sequence: apatopygids (Apatopygus and Porterpygus), echinolampadids (Echinolampas and Conolampas), cassidulids (Oligopodia, Eurkodia, Rhyncholampas and Cassidulus), Studeria, neolampadids and clypeasteroids. Contrary to claims in the literature, ordering multistate characters does not improve resolution, although the majority‐rule consensus of the resulting 84 trees supports the same relationships among genera. Trees found using only characters likely to be preserved in fossils differ only in the arrangement of the neolampadid taxa. Successive approximations weightings are found to be sensitive to arbitrary choices of the scaling factor and truncation. Weightings can lead to increases or decreases in the number of most parsimonious trees and the congruence among trees.

Exhaustion of Morphologic Character States Among Fossil Taxa

“…Neognathostomate echinoids display one of the most linear states : steps relationships (Table 2) and retain fairly high compatibility (Fig. 6), yet Suter's (1994a) analyses of 70 echinoid taxa found more than 70,000 equally parsimonious trees from several ''islands'' (Maddison 1991). Suter also found that most nodes were supported only by characters representing reversals or parallelisms.…”

Probing the cassiduloid origins of clypeasteroid echinoids using stratigraphically restricted parsimony analysis

“…How ever, adding all available fossil data seems to make the problem worse. Suter (1994a) includ ed all cassiduloid genera from the Middle Ju rassic onwards but found many thousands of equally parsimonious solutions. Suter's data matrix not only had a very low character-totaxon ratio (71 taxa and 40 characters) but was one of those analyzed by Wagner (2000a) and found to show high levels of character ex haustion.…”

“…He made separate cladistic analy ses of Arenig, Llanvirn, and Caradoc crinoids in an attempt to reduce problems of homopla sy. However, although this approach resulted in a highly resolved hypothesis in comparison to that of Suter (1994b), it has not been entirely successful. Specifically, it has failed to gener ate reliable support for any of the higher groupings.…”