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Bilateria

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Bilaterians
Temporal range: Ediacaran–Present, 567–0 Ma[1]
The original bilaterian was likely a marine worm somewhat like Xenoturbella.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Hatschek, 1888
Subdivisions[4]
Synonyms

Triploblasts Lankester, 1873

Bilateria (/ˌbaɪləˈtɪəriə/ BY-lə-TEER-ee-ə)[5] is a large clade or infrakingdom of animals called bilaterians (/ˌbaɪləˈtɪəriən/ BY-lə-TEER-ee-ən),[6] characterized by bilateral symmetry (i.e. having a left and a right side that are mirror images of each other) during embryonic development. This means their body plans are laid around a longitudinal axis (rostral–caudal axis) with a front (or "head") and a rear (or "tail") end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface.[7] Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which have pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo. Cephalization is a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front end.

Bilaterians constitute one of the five main metazoan lineages, the other four being Porifera (sponges), Cnidaria (jellyfish, hydrozoans, sea anemones and corals), Ctenophora (comb jellies) and Placozoa (tiny "flat animals"). For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm and ectoderm. Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others have primary body cavities deriving from the blastocoel, or secondary cavities that appear de novo, in particular the coelom.

Body plan

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Idealised wormlike body plan. Cylindrical body and a direction of movement give the animal head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.

Some of the earliest bilaterians were wormlike, and their body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom.[a] Animals with this bilaterally symmetric body plan have a head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[9][7]

Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth.[10] The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[7] these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis.[11] Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus, while xenacoelomorphs have a bag gut with one opening. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely plesiomorphic body structures.[9][7]

Evolution

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Inferred nature of the ancestor

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Ikaria wariootia, living 571–539 million years ago, is one of the oldest bilaterians identified.[12]
Main article: Urbilateria

The hypothetical most recent common ancestor of all Bilateria is termed the "urbilaterian".[13][14] The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by Ludwig von Graff, Elie Metchnikoff, Libbie Hyman, or Luitfried von Salvini-Plawen [nl]), while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla (flatworms and gastrotrichs) have lost body cavities secondarily (the Archicoelomata hypothesis and its variations such as the Gastrea by Haeckel or Sedgwick, the Bilaterosgastrea by Gösta Jägersten [sv], or the Trochaea by Nielsen).

One hypothesis is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella.[8] Alternatively, it may have resembled the planula larvae of some cnidarians, which have some bilateral symmetry.[15] However, there is evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes).[16]

Bilaterians, presumably including the urbilaterian, share many more Hox genes controlling the development of their more complex bodies, including of their heads, than do the Cnidaria and the Acoelomorpha.[17]

Fossil record

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The first evidence of Bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago.[18] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[19][20] Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.[21] Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old.[22] However, more recent evidence shows these fossils are actually late Paleozoic instead of Ediacaran.[23]

Phylogeny

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See also: List of bilateral animal orders

Bilateria has traditionally been divided into two main lineages or superphyla.[24] The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among the protostomes, Ecdysozoa[25] and Spiralia.[25][26][27] The arrow worms (Chaetognatha) have proven difficult to classify; recent studies place them in the Gnathifera.[28][29][30]

The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence found support for a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha), and the remaining bilaterians.[24] The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004).[24] The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in the Nephrozoa.[24][31] Subsequently the acoelomorphs were placed in phylum Xenacoelomorpha, together with the xenoturbellids, and the sister relationship between Xenacoelomorpha and Nephrozoa confirmed in phylogenomic analyses.[31]

A modern consensus phylogenetic tree for Bilateria, from a 2014 review by Casey Dunn and colleagues, is shown below.[32]

Bilateria

A different hypothesis is that Ambulacraria are sister to Xenacoelomorpha together forming Xenambulacraria. Xenambulacraria may be sister to Chordata or to Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or, as shown here, to Chordata + Protostomia).[33] The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).[34] A 2019 study by Hervé Philippe and colleagues presents the tree, cautioning that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy".[35]

Bilateria

Taxonomic history

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The Bilateria were named by the Austrian embryologist Berthold Hatschek in 1888. In his classification, the group included the Zygoneura, Ambulacraria, and Chordonii (the Chordata).[36][37] In 1910, the Austrian zoologist Karl Grobben renamed the Zygoneura to Protostomia, and created the Deuterostomia to encompass the Ambulacraria and Chordonii.[36][38]

See also

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Notes

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  1. ^ The earliest Bilateria may have had only a single opening, and no coelom.[8]

References

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  2. ^ Bekkouche, Nicolas; Gąsiorowski, Ludwik (2022-12-31). "Careful amendment of morphological data sets improves phylogenetic frameworks: re-evaluating placement of the fossil Amiskwia sagittiformis". Journal of Systematic Palaeontology. 20 (1): 1–14. doi:10.1080/14772019.2022.2109217. ISSN 1477-2019.
  3. ^ Chen, Zhe; Zhou, Chuanming; Yuan, Xunlai; Xiao, Shuhai (2019). "Death march of a segmented and trilobate bilaterian elucidates early animal evolution". Nature. 573 (7774): 412–415. Bibcode:2019Natur.573..412C. doi:10.1038/s41586-019-1522-7. PMID 31485079.
  4. ^ Giribet, Gonzalo; Edgecombe, Gregory (3 March 2020). The Invertebrate Tree of Life. Princeton University Press.
  5. ^ "bilateria". Merriam-Webster.com Dictionary. Merriam-Webster.
  6. ^ "bilaterian". Merriam-Webster.com Dictionary. Merriam-Webster.
  7. ^ a b c d Brusca, Richard C. (2016). "Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation" (PDF). Invertebrates. Sinauer Associates. pp. 345–372. ISBN 978-1-60535-375-3.
  8. ^ a b Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. Bibcode:2016Natur.530...89C. doi:10.1038/nature16520. PMID 26842059. S2CID 205247296.
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  18. ^ Fedonkin, M. A.; Waggoner, B. M. (November 1997). "The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism". Nature. 388 (6645): 868–871. Bibcode:1997Natur.388..868F. doi:10.1038/42242. S2CID 4395089.
  19. ^ Bengtson, S.; Budd, G. (19 November 2004). "Comment on 'small bilaterian fossils from 40 to 55 million years before the Cambrian'". Science. 306 (5700): 1291a. doi:10.1126/science.1101338. PMID 15550644.
  20. ^ Bengtson, S.; Donoghue, P. C. J.; Cunningham, J. A.; Yin, C. (2012). "A merciful death for the 'earliest bilaterian,' Vernanimalcula". Evolution & Development. 14 (5): 421–427. doi:10.1111/j.1525-142X.2012.00562.x. PMID 22947315. S2CID 205675058.
  21. ^ Hagadorn, J. W.; Xiao, S.; Donoghue, P. C. J.; Bengtson, S.; Gostling, N. J.; Pawlowska, M.; Raff, E. C.; Raff, R. A.; Turner, F. R.; Chongyu, Y.; Zhou, C.; Yuan, X.; McFeely, M. B.; Stampanoni, M.; Nealson, K. H. (13 October 2006). "Cellular and Subcellular Structure of Neoproterozoic Animal Embryos". Science. 314 (5797): 291–294. Bibcode:2006Sci...314..291H. doi:10.1126/science.1133129. PMID 17038620. S2CID 25112751.
  22. ^ Pecoits, E.; Konhauser, K. O.; Aubet, N. R.; Heaman, L. M.; Veroslavsky, G.; Stern, R. A.; Gingras, M. K. (June 29, 2012). "Bilaterian burrows and grazing behavior at >585 million years ago". Science. 336 (6089): 1693–1696. Bibcode:2012Sci...336.1693P. doi:10.1126/science.1216295. PMID 22745427. S2CID 27970523.
  23. ^ Verde, Mariano (15 September 2022). "Revisiting the supposed oldest bilaterian trace fossils from Uruguay: Late Paleozoic, not Ediacaran". Palaeogeography, Palaeoclimatology, Palaeoecology. 602. doi:10.1016/j.palaeo.2022.111158.
  24. ^ a b c d Nielsen, Claus (2008). "Six major steps in animal evolution: are we derived sponge larvae?". Evolution and Development. 10 (2): 241–257. doi:10.1111/j.1525-142X.2008.00231.x. PMID 18315817. S2CID 8531859.
  25. ^ a b Halanych, K.; Bacheller, J.; Aguinaldo, A.; Liva, S.; Hillis, D.; Lake, J. (17 March 1995). "Evidence from 18S ribosomal DNA that the lophophorates are protostome animals". Science. 267 (5204): 1641–1643. Bibcode:1995Sci...267.1641H. doi:10.1126/science.7886451. PMID 7886451. S2CID 12196991.
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  31. ^ a b Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. Bibcode:2016Natur.530...89C. doi:10.1038/nature16520. PMID 26842059. S2CID 205247296.
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  34. ^ Peterson, Kevin J.; Cotton, James A.; Gehling, James G.; Pisani, Davide (27 April 2008). "The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society of London B: Biological Sciences. 363 (1496): 1435–1443. doi:10.1098/rstb.2007.2233. PMC 2614224. PMID 18192191.
  35. ^ Philippe, Hervé; Poustka, Albert J.; Chiodin, Marta; Hoff, Katharina J.; Dessimoz, Christophe; et al. (2019). "Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria". Current Biology. 29 (11): 1818–1826.e6. doi:10.1016/j.cub.2019.04.009. hdl:21.11116/0000-0004-DC4B-1.
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  38. ^ Grobben, Karl; Claus, Carl Friedrich Wilhelm (1910). Lehrbuch der Zoologie (2nd ed.). Marburg: Elvert'sche Verlagsbuchhandlung.
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