Box Molecular classification

Can molecular data help? Kevin Peterson and his colleagues (2004, 2005) have presented a minimum evolution analysis (see p. 129) based on amino acid data derived from housekeeping genes (Fig. 10.6). The cladogram separates the Deuterostomia (echinoderms + hemichordates) from the Proto-stomia, which includes the Spiralia (mollusks + annelids + nemerteans + platyhelminthes) and the Ecdysozoa (arthropods + priapulids). Both are united within the Triploblastica that, together with the cnidarians, forms the Bilateria; the Eumetazoa comprise the Bilateria + Cnidaria and the meta-zoan clade is completed with the addition of the calcisponges and demosponges. Thus the last common ancestor of the Metazoa was probably rather like a modern sponge. The tree, however, lacks data from a number of problematic groups such as the Bryozoa and Brachiopoda, both commonly united on the basis of their lophophores. Moreover to date it has proved impossible to resolve polychotomies such as that including the mollusks, annelids and brachiopods (see also Aguinaldo & Lake 1998).

These molecular results are being increasingly accepted by zoologists as analysis of different gene datasets produce the same results. The hunt is now on for morphological characters of some of the major clades discovered by molecular means. A good example is the shedding of the exoskeleton (ecdysis) by the Ecdysozoa, a strong morphological synapomorphy that had once been thought to have evolved convergently in arthropods, nematodes and the others.

Figure 10.6 Phylogenetic relationships among the main invertebrate groups. (Phylogeny courtesy of Kevin Peterson.)

Ediacara biota_

Since the first impressions of soft-bodied organisms were identified in the Upper Pro-terozoic rocks of Namibia and in the Pound Quartzite in the Ediacara Hills, north of Ade laide in southern Australia in the late 1940s, this remarkable assemblage has now been documented from 30 localities on five continents (Fig. 10.7). More than 100 species of these unique organisms have been described on the basis of molds usually preserved in

Ma 500-

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