Chordate evolution and the three-phylum system

Abstract

Traditional metazoan phylogeny classifies the Vertebrata as a subphylum of the phylum Chordata, together with two other subphyla, the Urochordata (Tunicata) and the Cephalochordata. The Chordata, together with the phyla Echinodermata and Hemichordata, comprise a major group, the Deuterostomia. Chordates invariably possess a notochord and a dorsal neural tube. Although the origin and evolution of chordates has been studied for more than a century, few authors have intimately discussed taxonomic ranking of the three chordate groups themselves. Accumulating evidence shows that echinoderms and hemichordates form a clade (the Ambulacraria), and that within the Chordata, cephalochordates diverged first, with tunicates and vertebrates forming a sister group. Chordates share tadpole-type larvae containing a notochord and hollow nerve cord, whereas ambulacrarians have dipleurula-type larvae containing a hydrocoel. We propose that an evolutionary occurrence of tadpole-type larvae is fundamental to understanding mechanisms of chordate origin. Protostomes have now been reclassified into two major taxa, the Ecdysozoa and Lophotrochozoa, whose developmental pathways are characterized by ecdysis and trochophore larvae, respectively. Consistent with this classification, the profound dipleurula versus tadpole larval differences merit a category higher than the phylum. Thus, it is recommended that the Ecdysozoa, Lophotrochozoa, Ambulacraria and Chordata be classified at the superphylum level, with the Chordata further subdivided into three phyla, on the basis of their distinctive characteristics.

Hemichordate models

“…They are less closely related to chordates than to echinoderms, with which they comprise the clade Ambulacraria, a name coined by Metschnikoff [20] (Figure 2). However, the question of whether colonial tube-dwelling pterobranchs are sister to vermiform enteropneusts or whether they should be nested within them has been much debated [21][22][23][24].…”

A New Find of the Whale-fall Lancelet <i>Asymmetron inferum</i> (Cephalochordata) near a Hydrothermal Vent at Oomuro-dashi Submarine Volcano in the Izu Islands, Pacific Coast of Japan, as a Possible Case to Prove the Stepping-stone Hypothesis

“…This dispersal stepping-stone hypothesis has been suggested by finds of whale-fall species of the phyla Annelida and Mollusca also in the reducing vent and/or seep habitats, and vice versa (Smith and Baco 2003;Lorion et al 2013;Sumida et al 2016). Here will be given the first such example for the phylum (or subphylum) Cephalochordata (for taxonomic rank see Satoh et al 2014).…”

Patterning of brain precursors in ascidian embryos

“…Ascidians belong to the urochordates, or tunicates, a phylum of invertebrate chordates closely related to vertebrates (Delsuc et al, 2006;Satoh et al, 2014). Ascidian embryos develop with very few numbers of cells and a fixed cell lineage, features enabling the stepby-step analysis of developmental cell fate choices with a single-cell level of precision (Hudson, 2016).…”

“…The chordate super-phylum is characterised by a well patterned dorsal tubular central nervous system (CNS) (Satoh et al, 2014). Ascidians belong to the urochordates, or tunicates, a phylum of invertebrate chordates closely related to vertebrates (Delsuc et al, 2006;Satoh et al, 2014).…”