Page 255 - Introduction to Paleobiology and The Fossil Record

242 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 10.2 Molecular classifi cation

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 com-
monly 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.

Demosponges
Calcisponges
Cnidarians
Echinoderms

Hemichordates
Chordates
Arthropods

Priapulids
Bryozoans
Annelids
Brachiopods

Mollusks

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

laide in southern Australia in the late 1940s,
Ediacara biota
this remarkable assemblage has now been
Since the first impressions of soft-bodied documented from 30 localities on fi ve conti-

organisms were identified in the Upper Pro- nents (Fig. 10.7). More than 100 species of

terozoic rocks of Namibia and in the Pound these unique organisms have been described
Quartzite in the Ediacara Hills, north of Ade- on the basis of molds usually preserved in