Chasing the urmetazoon: Striking a blow for quality data?

The ever-lingering question: “What did the urmetazoan look like?” has not lost its charm, appeal or elusiveness for one and a half centuries. A solid amount of organismal data give what some feel is a clear answer (e.g. Placozoa are at the base of the metazoan tree of life (ToL)), but a diversity of modern molecular data gives almost as many answers as there are exemplars, and even the largest molecular data sets could not solve the question and sometimes even suggest obvious zoological nonsense. Since the problems involved in this phylogenetic conundrum encompass a wide array of analytical freedom and uncertainty it seems questionable whether a further increase in molecular data (quantity) can solve this classical deep phylogeny problem. This review thus strikes a blow for evaluating quality data (including morphological, molecule morphologies, gene arrangement, and gene loss versus gene gain data) in an appropriate manner.

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► A variety of “urmetazoan hypotheses” and phylogenetic trees is in discussion.
► Confusing pictures arise from molecular sequence data, so-called “quantity data”.
► Morphological and structural molecular data (“quality data”) seem to favor the placula hypothesis.
► The relative weighting and evaluation of both data types is an unresolved question.
► “Quality data” may become crucial to resolve relationships at the base of Metazoa.