Abstract

In recent years, scientists have made remarkable progress reconstructing the animal phylogeny. There is broad agreement regarding many deep animal relationships, including the monophyly of animals, Bilateria, Protostomia, Ecdysozoa, and Spiralia. This stability now allows researchers to articulate the diminishing number of remaining questions in terms of well-defined alternative hypotheses. These remaining questions include relationships at the base of the animal tree, the position of Xenacoelomorpha, and the internal relationships of Spiralia. Recent progress in the field of animal phylogeny has important implications for our understanding of the evolution of development, morphology, genomes, and other characters. A remarkable pattern emerges—there is far more homoplasy for all these characters than had previously been anticipated, even among many complex characters such as segmentation and nervous systems. The fossil record dates most deep branches of the animal tree to an evolutionary radiation in the early Cambrian with roots in the Late Neoproterozoic.