Deuterostome
Deuterostomes | |
---|---|
Diversity of deuterostomes | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Clade: | ParaHoxozoa |
Clade: | Bilateria |
Clade: | Nephrozoa |
Superphylum: | Deuterostomia Grobben, 1908 |
Clades | |
|
Deuterostomes (from
In deuterostomy, the developing embryo's first opening (the
Deuterostomia's sister clade is
Systematics
History
Initially, Deuterostomia included the phyla
While protostomes as a monophyletic group has strong support, research has shown that deuterostomes may be paraphyletic, and what was once considered traits of deuterostomes could instead be traits of the last common bilaterian ancestor. This suggests the deuterostome branch is very short or non-existent. The Xenambulacraria's sister group could be both the chordates or the protostomes, or be equally distantly related to them both.[13]
Classification
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)
- Subphylum
- Subphylum
- Clade Ambulacraria
- Phylum Hemichordata
- Class Planctosphaeroidea
- Class Enteropneusta (acorn worms)
- Class Pterobranchia
- Class
- 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)
- Subphylum Asterozoa
- Phylum
- Phylum
There is a possibility that Ambulacraria is the sister clade to Xenacoelomorpha, and could form the Xenambulacraria group.[14][15][16]
Notable characteristics
In both deuterostomes and protostomes, a
Most deuterostomes display
In deuterostomes the
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).[17][18] 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.
Except for the
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.[19]
Formation of mouth and anus
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.
Origins and evolution
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.[20] It seems likely that the 555 million year old Kimberella was a member of the protostomes.[21][22] That implies that the protostome and deuterostome lineages split some time before Kimberella appeared — at least 558 million years ago, and hence well before the start of the Cambrian 538.8 million years ago,[20] i.e. during the later 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.[23]
The last common ancestor of the deuterostomes had lost all innexin diversity.[24]
Fossils of one major deuterostome group, the
On the other hand, fossils of early chordates are very rare, as non-vertebrate chordates have no
Phylogeny
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.[32][13] 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.[33]
Bilateria |
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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.[13][34][35][36][37][38][39][40]
Fossil record
Deuterostomes have a rich fossil record with thousands of fossil species being found throughout the
and as such their true affinity remains uncertain.References
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- ^ Wade, Nicholas (30 January 2017). "This Prehistoric Human Ancestor Was All Mouth". The New York Times. Retrieved 31 January 2017.
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- ^ a b Brusca, R.C.; Brusca, G.J. (1990). Invertebrates. Sinauer Associates. p. 669.
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- ^ Acorn worm genome reveals gill origins of human pharynx | Berkeley News
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- ^ Connexins evolved after early chordates lost innexin diversity
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- ^ Hahn, G; Pflug, H. D (1985). "Polypenartige Organismen aus dem Jung-Präkambrium (Nama-Gruppe) von Namibia". Pascal-Francis. Retrieved 13 March 2024.
- ^ M. A. Fedonkin (1996). "Ausia as an ancestor of archeocyathans, and other sponge-like organisms". In: Enigmatic Organisms in Phylogeny and Evolution. Abstracts. Moscow, Paleontological Institute, Russian Academy of Sciences, p. 90-91.
Further reading
- 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. PMID 18192178.