Choanomonada `--Metazoa `==Porifera (paraphyletic?) `==Radiata (paraphyletic?) `--Bilateria `==Acoelomorpha (paraphyletic?) |--Protostomia | |--Ecdysozoa | `--Lophotrochozoa `--Deuterostomia
The deuterostomes make up on of the earliest divisions with the Bilateria. Compared to the protostomes, the deuterostomes are far less diverse, including the echinoderms and hemichordates but having as their main claim to fame the fact that Chordata belong to this sub-division. The characters uniting these taxa are mostly developmental (Miller & Harley, 1996). The name Deuterostomia ("second mouth") refers to the development of the embryo blastopore into the anus, with the mouth developing as a secondary opening, in contrast to protostomes ("first mouth") with the process reversed. The initial embryonic cleavage in deuterostomes is radial, with cleavage planes either parallel or perpendicular to the vertical axis of the embryo, and indeterminate (each early embryonic cell retains the capacity to develop into a complete embryo if isolated from other cells), and coelom development is from outpockets of the gut (enterocoelous). Deuterostomes are often remarkably asymmetric. Molecular data have supported monophyly of the Deuterostomia.
Bilateria `--Deuterostomia |--Vetulicolia |--Yunnanozoa |--+--Xenoturbella | `--Ambulacraria | |?--Dinomischida | |--Hemichordata | `--+--Vetulocystidae | `--+--Stylophora | `--+--Soluta | `--+--Cincta | `--+--Ctenocystoidea | `--+--Helicoplacoidea | `--+--Edrioasteroidea | `--Echinodermata `--Chordata |--Urochordata `--+--Cephalochordata `--Craniata `--Vertebrata
Deuterostomia is here defined as all animals more closely related to Homo (Chordata) and Echinus (Echinodermata) than to Scarabaeus (Arthropoda) or Helix (Mollusca). It should be noted that "deuterostomy" (the anus developmentally proceeding the mouth) is not unique to Deuterostomia, but is also found in Bryozoa, Brachiozoa and Chaetognatha. On this page, "Deuterostomia" and "deuterostome" always refer to the clade, while "deuterostomy" and "deuterostomous" refer to the developmental pathway.
The evolutionary relationships of deuterostomes have gone through a fair amount of contention in recent years. Traditionally. mainly after the work of Hyman, deuterostomes have been placed quite high up in the animal tree, forming a clade with the other deuterostomous taxa nestled within the coelomate clade. Within the Deuterostomia, chordates and hemichordates were united by possession of ciliated pharyngeal gill pores/slits and a stiffened notochord-like structure (the notochord in chordates, the stomochord in hemichordates). The similarities in larval form between hemichordates and echinoderms were regarded as ancestral for deuterostomes, and subsequently lost in chordates.
Humans being the chauvanistic animals we are, there's a lot of appeal in this tree, which appears to sit us at the top of ever-increasing complexity. Unfortunately, recent phylogenies, mostly molecular, rather put the kybosh on this view (Maddison, 2002). Deuterostomia is much lower down, probably the second branch of Bilateria to split off after the Acoelomorpha. Compared to its sister group, the omnipresent and spectacularly diverse Protostomia (containing the arthropods, molluscs and nematodes as its particular success stories), the deuterostomes are merely minor players. Interestingly, while protostomy has been generally regarded as ancestral to deuterostomy, modern phylogenies suggest the opposite, as shown by this phylogeny based on Giribet et al. (2000) where D = deuterostomous and P = protostomous:
0--Deuterostomia (D) `--+--Chaetognatha (D) `--+--Bryozoa (D) `--+--Ecdysozoa (P) `--+--Platyzoa (P) `--+--Brachiozoa (D) `--other Trochozoa (P)
While deuterostomy probably represents a secondary reversal in Brachiozoa (other phylogenies place Brachiozoa nested within other Trochozoa, reinforcing this probability), protosotomous taxa as a whole are unmistakably nested within deuterostomous taxa.
Within the Deuterostomia, molecular data have argued convincingly for a rearrangement of the main relationships (Cameron et al. 2000). The primary division of living members appears to be between the chordates on one hand and the echinoderms + hemichordates (forming a clade called Ambulacraria) on the other. This suggests that the characters shared between chordates and hemichordates may be ancestral for deuterostomes as a whole, and subsequently lost in echinoderms - a not-too-surprising state of affairs considering the greatly altered morphology of echinoderms. Heinzeller & Welsch (1999) regarded the hydrocoel of echinoderms as homologous with the notochord of chordates, as suggested by similarities in development and gene expression. The tornaria-type larva of Ambulacraria can no longer be assumed to be ancestral for Deuterostomia - it may represent an autapomorphy of Ambulacraria.
Recently, a molecular study has suggested that the obscure worm Xenoturbella might also fall within the Deuterostomia, specifically as the sister taxon to Ambulacraria. We therefore give it a look here. Also covered are the Vetulicolia, a group of Cambrian fossils that may represent the stem-group of crown deuterostomes, as well as a number of fossil groups regarded as stem echinoderms.
Not covered for now are a few other fossil groups that have been less certainly referred to Deuterostomia at one time or another, such as Odontogriphidae, Cambroclavida and Dinomischus. Also absent are the Chaetognatha (arrow-worms), which have been considered the basalmost branch of the Deuterostomia in the past, but are currently regarded by many as basal protostomes (Helfenbein et al., 2004).
MAK020407, ATW050814 Palaeos com, phylogeny CKT050815, this page MAK061012