Deuterostome

Initially, Deuterostomia included the phyla Brachiopoda,^[6] Bryozoa,^[7] Chaetognatha,^[8] and Phoronida^[6] based on morphological and embryological characteristics. However, Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses, leading to the removal of the lophophorates which was later combined with other protostome animals to form the superphylum Lophotrochozoa.^[9] The arrow worms may also be deuterostomes,^[8] but molecular studies have placed them in the protostomes more often.^[10][11] Genetic studies have also revealed that deuterostomes have more than 30 genes not found in any other animal groups, but which yet are present in some marine algae and prokaryotes. This could mean they are very ancient genes that were lost in other organisms, or that a common ancestor acquired them through horizontal gene transfer.^[12]

This is the generally agreed upon phylogeny of the deuterostomes:

• Superphylum Deuterostomia
  • Phylum Chordata
    • Subphylum Cephalochordata (lancelets)
    • Clade Olfactores
      • Subphylum Tunicata (tunicates)
      • Subphylum Vertebrata
        • Superclass Agnatha (jawless fish)
        • Infraphylum Gnathostomata (jawed fish)
          • Class Chondrichthyes (cartilaginous fish)
          • Superclass Osteichthyes (bony fish - includes tetrapods)
  • Clade Ambulacraria
    • Phylum Hemichordata
      • Class Planctosphaeroidea
      • Class Enteropneusta (acorn worms)
      • Class Pterobranchia
    • Phylum Echinodermata
      • Subphylum Asterozoa
        • Class Asteroidea (starfish)
        • Class Ophiuroidea (brittle stars)
      • Subphylum Blastozoa †
      • Subphylum Crinozoa (sea lillies and extinct relatives)
      • Subphylum Echinozoa
        • Echinoidea (sea urchins)
        • Holothuriodea (sea cucumbers)

There is a possibility that Ambulacraria is the sister clade to Xenacoelomorpha, and could form the Xenambulacraria group.^[13][14][15]

 

In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, and also occurs in certain protostomes, such as the lophophorates.

Most deuterostomes display indeterminate cleavage, in which the developmental fate of the cells in the developing embryo is not determined by the identity of the parent cell. Thus, if the first four cells are separated, each can develop into a complete small larva; and if a cell is removed from the blastula, the other cells will compensate. This is the source of identical twins.

In deuterostomes the mesoderm forms as evaginations of the developed gut that pinch off to form the coelom. This process is called enterocoely.

Another feature present in both the Hemichordata and Chordata is pharyngotremy — the presence of spiracles or gill slits into the pharynx, which is also found in some primitive fossil echinoderms (mitrates).^[16][17]

A hollow nerve cord is found in all chordates, including tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord. In the early embryonic stage, it looks like the hollow nerve cord of chordates.

Both the hemichordates and the chordates have a thickening of the aorta, homologous to the chordate heart, which contracts to pump blood. This suggests a presence in the deuterostome ancestor of the three groups, with the echinoderms having secondarily lost it.^{[citation needed]}

The highly modified nervous system of echinoderms obscures much about their ancestry, but several facts suggest that all present deuterostomes evolved from a common ancestor that had pharyngeal gill slits, a hollow nerve cord, circular and longitudinal muscles and a segmented body.^[18]

The defining characteristic of the deuterostome is the fact that the blastopore (the opening at the bottom of the forming gastrula) becomes the anus, whereas in protostomes the blastopore becomes the mouth. The deuterostome mouth develops at the opposite end of the embryo, from the blastopore, and a digestive tract develops in the middle, connecting the two.

In many animals, these early development stages later evolved in ways that no longer reflect these original patterns. For instance, humans have already formed a gut tube at the time of formation of the mouth and anus. Then the mouth forms first^{[citation needed]}, during the fourth week of development, and the anus forms four weeks later, temporarily forming a cloaca.

 

Bilateria, one of the five major lineages of animals, is split into two groups; the protostomes and deuterostomes. Deuterostomes consist of chordates (which include the vertebrates) and ambulacrarians.^[19] It seems likely that the 555 million year old Kimberella was a member of the protostomes.^[20][21] That implies that the protostome and deuterostome lineages split long before Kimberella appeared, and hence well before the start of the Cambrian 538.8 million years ago,^[19] i.e. during the earlier part of the Ediacaran Period (circa 635-539 Mya, around the end of global Marinoan glaciation in the late Neoproterozoic). It has been proposed that the ancestral deuterostome, before the chordate/ambulacrarian split, could have been a chordate-like animal with a terminal anus and pharyngeal openings but no gill slits, with active suspension feeding strategy.^[22]

The last common ancestor of the deuterostomes had lost all innexin diversity.^[23]

Deuterostomes have a rich fossil record with thousands of fossil species being found throughout the Phanerozoic. There are also a few earlier fossils that may represent deuterostomes, but these remain debated. The earliest of these disputed fossils are the tunicate-like organisms Burykhia and Ausia from the Ediacaran period. While these may in fact be tunicates, others have interpreted them as cnidarians^[24] or sponges,^[25] and as such their true affinity remains uncertain. Another Ediacaran fossil, Arkarua, may represent the earliest echinoderm, while Yanjiahella from the early Cambrian (Fortunian) period is another notable stem group echinoderm.^[26][27]

Fossils of one major deuterostome group, the echinoderms (whose modern members include sea stars, sea urchins and crinoids), are quite common from the start of Stage 3 of the Cambrian, 521 million years ago^[28] starting with forms such as Helicoplacus. Two other Cambrian Stage 3 (521-514 mya) species, Haikouichthys and Myllokunmingia from the Chengjiang biota, are the earliest bodyfossils of fish,^[29][30] whereas Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is now regarded as a primitive chordate.^[31] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate,^[32] whereas Spartobranchus is an acorn-worm from the Burgess Shale, providing proof that all main lineages were already well established 508 mya.

On the other hand, fossils of early chordates are very rare, as non-vertebrate chordates have no bone tissue or teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from the Permian-aged Paleobranchiostoma, trace fossils of the Ordovician colonial tunicate Catellocaula, and various Jurassic-aged and Tertiary-aged spicules tentatively attributed to ascidians.^{[citation needed]}. Fossils of Echinodermata remain very common after the Cambrian. Fossils of Hemichordata are less common, except for graptolites until the Lower-Carbonoferous.

Below is a phylogenetic tree showing consensus relationships among deuterostome taxa. Phylogenomic evidence suggests the enteropneust family, Torquaratoridae, fall within the Ptychoderidae. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources.^[33][34] The approximate dates for each radiation into a new clade are given in millions of years ago (Mya). Not all dates are consistent, as of date ranges only the center is given.^[35]

Support for the clade Deuterostomia is not unequivocal. In particular, the Ambulacraria are sometimes shown to be related to the Xenacoelomorpha. If true, this raises two possibilities: either the Ambulacraria are taken out of the deuterostome-protostome dichotomy (in which case the grouping Deuterostomia dissolves, with Chordata and Protostomia grouped together as Centroneuralia), or the Xenacoelomorpha are re-positioned next to Ambulacraria within the Deuterostomia as in the above diagram.^{[34][36][37][38][39][40][41][42]}

• Swalla, B. J.; Smith, A. B. (2008). "Deciphering deuterostome phylogeny: Molecular, morphological and palaeontological perspectives". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 363 (1496): 1557–1568. doi:10.1098/rstb.2007.2246. PMC 2615822. PMID 18192178.

 

Wikispecies has information related to Deuterostomia.

 

Wikimedia Commons has media related to Deuterostomia.

• Introduction to the Deuterostomia UCMP
• Deuterostomia at Encyclopædia Britannica