Evolution of Bilateria from a Meiofauna Perspective—Miniaturization in the Focus

Meiofaunal life forms are found all over the animal tree of life, and miniaturization seems to have occurred within otherwise macrofaunal clades multiple times. While sponges, comb jellies and cnidarians suggest a macroscopic ancestry for Metazoa, several phyla are exclusively meiofaunal, however, and may evidence a wider microscopic ancestry of some major groups, such as Ecdysozoa and Spiralia/Lophotrochozoa. This is an unsolved debate, which should be tackled from a synthesis of zoomorphological, palaeontological, molecular and phylogenetic approaches to test alternative scenarios. Advances in microscopic techniques have led to a renaissance in anatomical studies that allows for new and detailed examination of both extant and extinct meiofauna, revealing an unseen wealth of information. Likewise, the rapid development in genomic sequencing and analytical toolsmakes detailed reconstructions of meiofauna genomes feasible. The anticipated flood of new morphological and molecular data on meiofauna will broaden integrative and comparative studies and hopefully allow scientists of this generation to answer the long-debated questions of how the animal kingdom evolved and ramified into today’s amazing diversity of life.