ALTERNATE VIEWS OF DEUTEROSTOME CLADISTIC RELATIONSHIPS
| The morphological tradition of grouping phyla in the deuterostomes assumes that sessile and simple is primitive and that moving and complex is derived. This, to a large extent is a consequence of our bias as members of the vertebrates. Figure A shows the traditional view of the deuterostome relationships (e.g. Nielsen 2001 and Brusca and Brusca 2003) in which the echinoderms are the most primitive and the chordate groups monophyletic and the most derived. Putnam et al. (2008; Figure B) examined the amphioxus genome relative to whole genomes of other deuterostomes and found that the cephalochordates were not sisters to the vertebrates, but basal in the whole chordate clade. Furthermore, the echinoderm + Hemichordate clade was more complex. The analysis of Delsuc et al. (2006; Figure C) was even more surprising in that it concluded that the cephalochordates were associated with the echinoderms. If so, the cephalochordate body plan is primitive, suggesting that the basic vertebrate plan also is primitive. Furthermore, the sessile natures of the echinoderms and tunicates are secondary simplifications. The molecular results call to question the concept of the chordate. In fact, the results of Delsuc et al (2006) indicate that the “chordates” are paraphyletic. |
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| FIGURE A. Standard textbook cladogram of the relative positions of the deuterostomes. Note the sister relationship between cephalochordates and vertebrates. | FIGURE B. In this view, the cephalochordates are basal in the deuterostomes. Note the sister relationships between the vertebrates and the tunicates as well as between the echinoderms and the hemichordates. | FIGURE C. This cladogram, which is equivocal regarding the hemichordates, suggests that the cephalochordates are sisters to the echinoderms. |
| By Jack R. Holt. Last revised: 01/21/2009 |
