Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria.

Abstract

The bilaterally symmetric animals (Bilateria) are considered to comprise two monophyletic groups, Protostomia and Deuterostomia. Protostomia contains the Ecdysozoa and the Lophotrochozoa; Deuterostomia contains the Chordata and the Xenambulacraria (Hemichordata, Echinodermata and Xenacoelomorpha). Their names refer to a supposed distinct origin of the mouth (stoma) in the two clades, but these groups have been differentiated by other embryological characters including embryonic cleavage patterns and different ways of forming their mesoderm and coeloms. Deuterostome monophyly is not consistently supported by recent studies. Here we compare support for Protostomia and Deuterostomia using five recently published, phylogenomic datasets. Protostomia is always strongly supported, especially by longer and higher quality genes. Support for Deuterostomia is always equivocal and barely higher than support for paraphyletic alternatives. Conditions that can cause tree reconstruction errors - inadequate models, short internal branch, faster evolving genes, and unequal branch lengths - correlate with statistical support for monophyletic deuterostomes. Simulation experiments show that support for Deuterostomia could be explained by systematic error. A survey of molecular characters supposedly diagnostic of deuterostomes shows many are not valid synapomorphies. The branch between bilaterian and deuterostome common ancestors, if real, is very short. This finding fits with growing evidence suggesting the common ancestor of all Bilateria had many deuterostome characteristics. This finding has important implications for our understanding of early animal evolution and for the interpretation of some enigmatic Cambrian fossils such as vetulicolians and banffiids.