James R. Garey* and Andreas Schmidt-Rhaesa

From the Symposium on Evolutionary Relationships of Metazoan Phyla:
Advances, Problems and Approaches

SYNOPSIS
   Molecular studies have revealed many new hypotheses of metazoan evolution in recent years.  Previously, using morphological methods, it was difficult to relate  "minor" animal groups representing microscopic metazoans to larger, more well known groups such as arthropods, molluscs annelids.  From molecular studies, it appears that acanthocephalans evolved from rotifers, that priapulids share common ancestry with all other molting animals (Ecdysozoa), and that  flatworms, gnathostomulids and rotifers form a sister group to the remaining non-molting protostomes (Lophotrochozoa), both taxa together named Spiralia.  The lophophorate phyla (phoronids, brachiopods and bryozoans) appear as protostomes, allied with annelids and molluscs, not with deuterostomes.  These findings present a very different view of metazoan evolution, and clearly show that small and simple animals do not necessarily represent ancestral or primitive taxa.

Figure 1.  Proposed phylogeny of protostomes based on morphological and molecular analyses. Platyhelminthes + Gnathostomulida + Rotifera + Acanthocephala may be the sister group to Lophotrochozoa.  Only a few key characters are given. 1: Blastopore becomes the anus. 2: Ventral lateral nerve chord.  3: Molting by ecdysis. 4: Spiral cleavage. 5: Filiform sperm without accessory centriole.  6. Biciliary terminal cell in the protonephridia.  7: Jaws composed of rods imbedded in a cuticular matrix.  8: Internal layer in the syncytial epidermis.