Annelida and Arthropoda are Not Sister Taxa: A Phylogenetic Analysis of Spiralian Metazoan Morphology

Abstract

Annelids and arthropods have long been considered to be each other's closest relatives, as evidenced by similarities in their segmented body plans. In the first cladistic analysis of metazoan morphology accompanied by an explicit data matrix, Schram (Meglitsch and Schram, 1991, Invertebrate zoology, 3rd edition, Oxford Univ. Press, New York) suggested tentative support for this conventional “Articulata” hypothesis. Our reanalysis of the Schram data matrix yielded weak support for an alternative “Eutrochozoa” grouping of annelids, molluscs, and certain other spiralian phyla, exclusive of arthropods. Likewise, recent 18S ribosomal RNA sequence comparisons have favored the Eutrochozoa hypothesis. This study presents a new analysis of 141 independently assembled characters, purported to represent the current state of knowledge of metazoan morphology and embryology. This maximum parsimony analysis resulted in robust support of Eutrochozoa. For this data compilation and method of analysis, the Articulata hypothesis could only be supported by adding multiple ad hoc proposals of evolutionary events. Instead, the more parsimonious Eutrochozoa hypothesis is favored as the best-supported current reconstruction of higher level animal genealogy.