The Question of Metazoan Monophyly and the Fossil Record

Abstract

As metazoans, almost inevitably we have a chauvinistic interest in our origins. We must also acknowledge, however, that the acquisition of specialized tissues - notably muscles to enable macroscopic motility, and nerves to transmit information - have transformed the world. Nevertheless, matters in biology are seldom clear-cut and amongst the smaller metazoans there are distinct overlaps with some of the more complex protistans, perhaps most strikingly with the ciliates. This overlap is perhaps most familiar from the miniaturized representatives of the sandy meiofauna (e.g. Giere 1993), the curious and probably degenerate diphyletic mesozoans (e.g. Katayama et al. 1995; Hanelt et al. 1996; Pawlowski et al. 1996; cf Cavalier-Smith 1993, who regards mesozoans as multicellular protists), and the recently discovered Symbion (Funch and Kristensen 1995; Funch 1996). Moreover, the recent assignment of the endoparasitic myxozoans, long thought to be protistans (e.g. Cavalier-Smith 1993), to the Metazoa, albeit without agreement as to whether they belong within the Bilateria (Smothers et al. 1994; Schlegel et al. 1996; see also Hanelt et al. 1996 and Pawlowski et al. 1996) or Cnidaria (Siddall et al. 1995) is a timely reminder that the concept of this Kingdom is more protean than is popularly imagined.