Please mind the gaps: exploring the limits of our understanding of animal body plan evolution

Ronald Jenner, Department of Zoology, The Natural History Museum, United Kingdom

As we draw nearer to a consensus on the higher-level phylogeny of Metazoa, we must consider the challenges of the next phase of our great quest to understand the evolution of animal body plans. This phase of research will require clarifying the diverse traits that typify different body plans, such as behavioral, morphological, embryological, genomic, and ecological traits, and integrating these into a coherent evolutionary narrative in the context of the established phylogeny. The greatest challenge in achieving this ultimate goal is to overcome the various kinds of gaps in our knowledge. I will review the nature and location of the main gaps in our knowledge, and examine whether and how we will be able to fill them. One type of gap can be bridged relatively easily, at least in principle, by generating new data on the morphology, embryology, ecology, etc. of poorly studied taxa. Another type of gap reflects our limited understanding of the ground patterns of some higher-level taxa, a shortcoming that can be remedied by further research on their internal phylogenetic relationships. A third type of knowledge gap, however, cannot so easily be overcome. The crown body plans of higher (phylum)-level taxa are generally phenotypically very distinct from each other, and in several cases we have little more than our imagination to bridge them. Notably, some recent phylogenomic analyses have united such phenotypically dissimilar taxa that these sister group hypotheses have been given names expressing great surprise (Kryptrochozoa uniting brachiopods and nemerteans; Miracrustacea uniting hexapods with remipedes and cephalocarids). A proper understanding of the divergence of such disparate body plans would be very difficult, unless facilitated by fortuitous discoveries of fossils or living taxa with more or less intermediate body plans. Unless our luck is significantly up in the near future, I expect that most higher-level animal taxa will remain separated by wide gaps in body plan organization. It will therefore remain a great, perhaps even insurmountable challenge to formulate testable scenarios for animal body plan evolution, and to choose among competing hypotheses.