Deciphering deuterostome phylogeny: molecular, morphological, and palaeontological perspectives

CHAPTER 9

Deciphering deuterostome phylogeny: molecular, morphological, and palaeontological perspectives
Andrew B. Smith and Billie J. Swalla

9.1

Introduction

Deuterostomes form one of the three major divisions of the Bilateria and are sister group to the Lophotrochozoa plus Ecdysozoa (Eernisse and Peterson, 2004; Philippe et al., 2005a; Telford et al., 2005; Dunn et al., 2008). Traditionally the group was recognized on the basis of a shared embryonic development pattern: gastrulation occurs at the vegetal pole and the blastopore becomes the anus, while the mouth forms secondarily (Chea et al., 2005). Analysis of molecular data has consistently found the deuterostome grouping, with generally high levels of support (Turbeville et al., 1994; Wada and Satoh, 1994; Halanych et al., 1995; Cameron et al., 2000). Five major clades make up the Deuterostomia: craniates, cephalochordates, echinoderms, hemichordates, and tunicates (see Figure 9.1). Because vertebrates, including ourselves, belong to the craniates, there has long been a fascination about their invertebrate origins. Theories of chordate evolution have abounded for over 100 years, but it is only in the last 10 to 15 years that deuterostome relationships have come into sharp focus, driven largely by new data from molecular genetics and the fossil record, and new analyses of traditional morphological and ontogenetic data. From this plethora of information, some complementary, others supporting contradictory conclusions, a more coherent picture of the phylogeny and early evolution of deuterostomes is starting to emerge. Here we review four key areas where there has been the most heated debate in the last 5 years: phylogenetic relationships of the major deuterostome groups; the earliest fossil record and
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divergence times of deuterostome groups; the evolution of body axes; and the characteristics of the ancestral deuterostome body plan.

9.2 Deuterostome phylogenetic relationships
Until 10