Since Darwin’s time, the origins and relationships of the bilaterian animals have remained unsolved problems in historical biology (Conway Morris 2000). One of the central difficulties is characterizing the common ancestor of the protostomes and deuterostomes. We argue that an unresolved conceptual puzzle has plagued the many attempts to describe this Urbilaterian, or, in Erwin and Davidson’s (2002) terminology, the protostome-deuterostome ancestor (PDA). Any organism sophisticated enough to be a realistic candidate for the PDA, with such characters as an anterior-posterior axis, gut, and sensory organs, must itself have been constructed by a developmental process, or by what we term an ontogenetic network (Ross and Nelson 2002). But the more biologically plausible the PDA becomes, as a functioning organism within a population of other such organisms, the more it will tend to “pull” (in its characters) towards one or another of the known bilaterian groups. As this happens, and the organism loses its descriptive generality, it will cease to be a good candidate Urbilaterian.
Ross, Marcus R. and Nelson, Paul A., "Problems with Characterizing the Protostome-Deuterostome Ancestor" (2004). Faculty Publications and Presentations. 80.