Acoel development supports a simple planula-like urbilaterian

Molecular approaches to the study of development and evolution have had profound effects on our understanding of the nature of the evolutionary process. Developmental biologists became intoxicated with fanciful notions of reconstructing genetic pathways of morphogenesis while evolutionary biologists were sobered by the fallacy of reconstructing organismal relationships along increasing grades of morphological complexity. Increased taxon sampling and improvements in analytical techniques are providing a new approach and are forcing biologists to move past historical biases to allow more accurate mapping of morphological and developmental characters through evolutionary time. Here, we discuss the possible developmental and morphological features of the 'urbilaterian', the triploblastic animal with anterior-posterior and dorsoventral axes and predecessor of the protostome-deuterostome ancestor. We argue that this animal, with features resembling acoelomorph flatworms, was far simpler morphologically than the protostome-deuterostome ancestor despite possessing a nearly complete eubilaterian genome. We show that the deployment of some genes expected to pattern the protostome-deuterostome ancestor is not deployed in acoels in the predicted manner and thus might have been co-opted after the evolution of the urbilaterian. We also identify the developmental changes related to gastrulation that gave rise to the urbilaterian from a simpler cnidarian-like ancestor.