Urbilaterian

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Phylogenetic position of the urbilaterian
Urbilaterian ↓

The urbilaterian (from German ur- 'original') is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry.

Contents

Appearance

Its appearance is a matter of debate, for no representative has been (or may or may not ever be) identified in the fossil record. Two reconstructed urbilaterian morphologies can be considered: first, the less complex ancestral form forming the common ancestor to Xenacoelomorpha and Nephrozoa; and second, the more complex (coelomate) urbilaterian ancestral to both protostomes and deuterostomes, sometimes referred to as the "urnephrozoan". Since most protostomes and deuterostomes share features — e.g. nephridia (and the derived kidneys), through guts, blood vessels and nerve ganglia— that are useful only in relatively large (macroscopic) organisms, their common ancestor ought also to have been macroscopic. However, such large animals should have left traces in the sediment in which they moved, and evidence of such traces first appear relatively late in the fossil record — long after the urbilaterian would have lived. This leads to suggestions of a small urbilaterian (around 1 mm) which is the supposed state of the ancestor of protostomes, deuterostomes and acoelomorphs.

Dating the urbilaterian

The first evidence of bilateria in the fossil record comes from trace fossils in sediments towards the end of the Ediacaran period (about 570  million years ago), and the first fully accepted fossil of a bilaterian organism is Kimberella , dating to 555  million years ago. [1] There are earlier, controversial fossils: Vernanimalcula has been interpreted as a bilaterian, but may simply represent a fortuitously infilled bubble. [2] Fossil embryos are known from around the time of Vernanimalcula ( 580  million years ago), but none of these have bilaterian affinities. [3] This may reflect a genuine absence of bilateria, however it is likely this is the case as bilateria may not have laid their eggs in sediment, where they would be likely to fossilise. [4]

Molecular techniques can generate expected dates of the divergence between the bilaterian clades, and thus an assessment of when the urbilaterian lived. These dates have huge margins of error, though they are becoming more accurate with time. More recent estimates are compatible with an Ediacaran bilaterian, although it is possible, especially if early bilaterians were small, that the bilateria had a long cryptic history before they left any evidence in the fossil record. [5]

Characteristics of the urbilaterian

Eyes

Light detection (photosensitivity) is present in organisms as simple as seaweeds; the definition of a true eye varies, but in general eyes must have directional sensitivity, and thus have screening pigments so only light from the target direction is detected. Thus defined, they need not consist of more than one photoreceptor cell. [6]

The presence of genetic machinery (the Pax6 and Six genes) common to eye formation in all bilaterians suggests that this machinery - and hence eyes - was present in the urbilaterian. [6] The most likely candidate eye type is the simple pigment-cup eye, which is the most widespread among the bilateria. [6]

Since two types of opsin, the c-type and r-type, are found in all bilaterians, the urbilaterian must have possessed both types - although they may not have been found in a centralised eye, but used to synchronise the body clock to daily or lunar variations in lighting. [7]

Complexity

Proponents of a complex urbilaterian point to the shared features and genetic machinery common to all bilateria. They argue that (1) since these are similar in so many respects, they could have evolved only once; and (2) since they are common to all bilateria, they must have been present in the ancestral bilaterian animal.

However, as biologists' understanding of the major bilaterian lineages increases, it is beginning to appear that some of these features may have evolved independently in each lineage. Further, the bilaterian clade has recently been expanded to include the acoelomorphs — a group of relatively simple flatworms. This lineage lacks key bilaterian features, and if it truly does reside within the bilaterian "family", many of the features listed above are no longer common to all bilateria. [8] Instead, some features — such as segmentation and possession of a heart — are restricted to a sub-set of the bilateria, the deuterostomes and protostomes. Their last common ancestor would still have to be large and complex, but the bilaterian ancestor could be much simpler. [8] However, some scientists stop short of including the acoelomorph clade in the bilateria. This shifts the position of the cladistic node which is being discussed; consequently the urbilaterian in this context is farther out the evolutionary tree and is more derived than the common ancestor of deuterostomes, protostomes and acoelomorphs. [9]

Genetic reconstructions are unfortunately not much help. They work by considering the genes common to all bilateria, but problems arise because very similar genes can be co-opted for different functions. For instance, the gene Pax6 has a function in eye development, but is absent in some animals with eyes; some cnidaria have genes which in bilateria control the development of a layer of cells that the cnidaria do not have. This means that even if a gene can be identified as present in the urbilaterian, we cannot necessary tell what the gene's function was. [8] Before this was realised, genetic reconstructions implied an implausibly complex urbilaterian. [5]

The evolutionary developmental biologist Lewis Held notes that both centipedes and snakes use the oscillating mechanism based on the Notch signaling pathway to produce segments from the growing tip at the rear of the embryo. Further, both groups make use of "the obtuse process of 'resegmentation', whereby the phase of their metameres shifts by half a unit of wavelength, i.e. somites splitting to make vertebrae or parasegments splitting to form segments." [10] Held comments that all this makes it difficult to imagine that their urbilaterian common ancestor was not segmented. [10]

Two hypotheses of the different characters and organ systems of the urbilaterian: the "complex" and "planula-like" urbilaterian. It is important to note that none of these representations shows an animal which existed or exists, and different combinations of these two organisms can be proposed by some authors (for example, an unsegmented urbilaterian with a centralized nervous system). These two representations are only two "extremes" of different hypotheses. Urbilateria horizontal english.tif
Two hypotheses of the different characters and organ systems of the urbilaterian: the "complex" and "planula-like" urbilaterian. It is important to note that none of these representations shows an animal which existed or exists, and different combinations of these two organisms can be proposed by some authors (for example, an unsegmented urbilaterian with a centralized nervous system). These two representations are only two "extremes" of different hypotheses.

Reconstructing the urbilaterian

The absence of a fossil record gives a starting point for the reconstruction — the urbilaterian must have been small enough not to leave any traces as it moved over or lived in the sediment surface. This means it must have been well below a centimetre in length. As all Cambrian animals are marine, one can reasonably assume that the urbilaterian was too. [8]

Furthermore, a reconstruction of the urbilateria must rest on identifying morphological similarities between all bilateria. While some bilateria live attached to a substrate, this appears to be a secondary adaptation, and the urbilaterian was probably mobile. [8] Its nervous system was probably dispersed, but with a small central "brain". Since acoelomorphs lack a heart, coelom or organs, the urbilaterian probably did too — it would presumably have been small enough for diffusion to do the job of transporting compounds through the body. [8] A small, narrow gut was probably present, which would have had only one opening — a combined mouth and anus. [8] Functional considerations suggest that the surface of the bilaterian was probably covered with cilia, which it could have used for locomotion or feeding. [8]

As of 2018 there is still no consensus on whether the characteristics of the deuterostomes and protostomes evolved once or many times. Features such as a heart and a blood-circulation system may therefore not have been present even in the deuterostome-protostome ancestor, which would mean that this too could have been small (hence explaining the lack of fossil record). [5]

Possible models of the Urbilaterian

It is possible that the common ancestor of all bilaterals looked similar to:

Colonial-Pennatulacean hypothesis: (Colonialy fusion of cnidarian-like)

The proposal that bilaterals arose from the fusion between pennatulacean-like cnidarian zooids was granted by Dewel, implies that the body plans of bilaterals originated from a colonial ancestor. [12]

This proposal has little or no support in the existing data, and has been commonly used as a justification against the sedentary/semi-sedentary models of urbilaterians as a whole. [13] [14]

Larval Hypothesis (Pelagic larvae and adult ancestor)

Panarticulata hypothesis: (Segmentated annelid-like ancestor)

Cloudinomorpha hypothesis: (Biphasic Sedentary sessile adult and Pelagic larvae)

Presence of an embryonic structure similar to a protoconch (embryonic dome) in Cloudinidae and Pterobranchia, this structure along with other characteristics could have been present in the common ancestor of the bilaterals. Cloudinomorpha homology.jpg
Presence of an embryonic structure similar to a protoconch (embryonic dome) in Cloudinidae and Pterobranchia, this structure along with other characteristics could have been present in the common ancestor of the bilaterals.

The recent model by Alexander V. Martynov and Tatiana A. Korshunova revives the idea of a sessile sedentary biphasic ancestor. [14]

Consider that the urbilaterian is an organism whose adult life is sessile sedentary with a juvenile or free and pelagic larval phase. This hypothesis is a derivative of Nielsen's larval hypothesis, but now also considering the homology of the adult forms of choanozoans (except Ctenophora [15] ). It also considers various phylogenetic, paleontological and molecular data, relates the adult and ancestral form of anthozoans (from which jellyfish, [16] placozoans, nephrozoans, [17] and perhaps proarticulate [18] are derived), in turn derived from an ancestral organization shared between choanoflagellates, sponges and parahoxozoans.

The current strong bias towards a mobile urbilaterian is considered to cause problems with palaeontological and morphological data in relation to groups within and outside Bilateria.

So members of Proarticulata are an evolutionary dead end rather [14] than the ancestors of nephrozoans. It is possible that the Cloudinids ( Cloudina , [19] [20] Conotubus [21] and Multiconotubus [22] ) are basal (and therefore bilateral) nephrozoans, because they have considerable similarity with the tubariums of sedentary pterobranchs, as well as with the shells of semi-mobile hyoliths and mobile mollusks, this taking into account the ontogeny of the cloudinids. [14] [20]

Potential homology between nephrozoans through a sedentary-pelagic ancestor, among which the embryonic structure similar to a protoconch (violet), the digestive tube (red), the stolon and tail (blue), the head shield and potential derivatives (yellow), and the oral lobes (green). Nephrozoa.jpg
Potential homology between nephrozoans through a sedentary-pelagic ancestor, among which the embryonic structure similar to a protoconch (violet), the digestive tube (red), the stolon and tail (blue), the head shield and potential derivatives (yellow), and the oral lobes (green).

This implies that the Cloudinomorpha is not a polyphyletic group as would have been proposed [23] but rather is a paraphyletic grade from which several taxa derive that may or may not conserve the ancestral clonality of basal metazoans, but instead of cloudinids having an annelid-type gut, it would instead be a U-shaped digestive tube, in fact the relationship between Cloudina and annelids is denied.

The hypothesis of annelid-like ancestor is rejected, due to the independent evolution of segmentation and complete metamerism of several groups of bilaterians (annelids, panarthropods, chordates and proarticulates); On the other hand, the urbilaterian would be an animal with a U-shaped gut, with deuterostomic characteristics that hemichordates and lophophorates among other groups conserve, a stolon that holds the organism inside a tube secreted from the embryonic form as a dome or protoconch, a semi-metamerism derived from the formation of mesoderm from the gastrovascular cavity of an anthozoan-like animal. [17]

This form of urbilaterian: [14]

The common ancestor of modern bilaterals would then be more similar to modern pterobranchs, although they would not be completely identical to them.

The location of Ctenophora (Myriazoa hypothesis) [15] should not change the hypothesis since it has been left aside taking only into account the molecular and morphological development of Choanoflagellatea, Porifera and Cnidaria.

See also

Related Research Articles

The cloudinids, an early metazoan family containing the genera Acuticocloudina, Cloudina and Conotubus, lived in the late Ediacaran period about 550 million years ago. and became extinct at the base of the Cambrian. They formed millimetre-scale conical fossils consisting of calcareous cones nested within one another; the appearance of the organism itself remains unknown. The name Cloudina honors the 20th-century geologist and paleontologist Preston Cloud.

<span class="mw-page-title-main">Bilateria</span> Animals with embryonic bilateral symmetry

Bilateria is a large clade or infrakingdom of animals called bilaterians, characterized by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front and a rear end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo. Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end.

<span class="mw-page-title-main">Coelom</span> The main body cavity in many animals

The coelom is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it remains undifferentiated. In the past, and for practical purposes, coelom characteristics have been used to classify bilaterian animal phyla into informal groups.

<span class="mw-page-title-main">Ecdysozoa</span> Superphylum of protostomes including arthropods, nematodes and others

Ecdysozoa is a group of protostome animals, including Arthropoda, Nematoda, and several smaller phyla. The grouping of these animal phyla into a single clade was first proposed by Eernisse et al. (1992) based on a phylogenetic analysis of 141 morphological characters of ultrastructural and embryological phenotypes. This clade, that is, a group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes.

<span class="mw-page-title-main">Eumetazoa</span> Basal animal clade as a sister group of the Porifera

Eumetazoa, also known as diploblasts, Epitheliozoa or Histozoa, are a proposed basal animal clade as a sister group of Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a eumetazoan in the ParaHoxozoa. The competing hypothesis is the Myriazoa clade.

<i>Kimberella</i> Primitive Mollusc-like organism

Kimberella is an extinct genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the gastropods, although its affinity with this group is contentious.

<span class="mw-page-title-main">Acoelomorpha</span> Phylum of marine, flatworm-like animals

Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, such as algae and corals. With the exception of two acoel freshwater species, all known acoelomorphs are marine.

<span class="mw-page-title-main">Symmetry in biology</span> Geometric symmetry in living beings

Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symmetry down its centre, or a pine cone displays a clear symmetrical spiral pattern. Internal features can also show symmetry, for example the tubes in the human body which are cylindrical and have several planes of symmetry.

<span class="mw-page-title-main">Animal</span> Kingdom of living things

Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals form a clade, meaning that they arose from a single common ancestor.

As understanding of the events of the Cambrian becomes clearer, data have accumulated to make some postulated causes for the Cambrian explosion look improbable. Some examples are the evolution of herbivory, vast changes in plate tectonic rates or orbital motion, or different evolutionary mechanisms in force.

The Cambrian explosion is an interval of time approximately 538.8 million years ago in the Cambrian period of the early Paleozoic when a sudden radiation of complex life occurred, and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 to 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.

<span class="mw-page-title-main">Deuterostome</span> Superphylum of bilateral animals

Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into four phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is thought to be a member of Deuterostomia.

<span class="mw-page-title-main">Protostome</span> Clade of animals whose mouth develops before the anus

Protostomia is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's members, although the reverse is typically true of its sister clade, Deuterostomia. Well-known examples of protostomes are arthropods, molluscs, annelids, flatworms and nematodes. They are also called schizocoelomates since schizocoely typically occurs in them.

<span class="mw-page-title-main">Embryological origins of the mouth and anus</span> Important characteristic for separating animals into protostomes and deuterostomes

The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.

The Urmetazoan is the hypothetical last common ancestor of all animals, or metazoans. It is universally accepted to be a multicellular heterotroph — with the novelties of a germline and oogamy, an extracellular matrix (ECM) and basement membrane, cell-cell and cell-ECM adhesions and signaling pathways, collagen IV and fibrillar collagen, different cell types, spatial regulation and a complex developmental plan, and relegated unicellular stages.

<span class="mw-page-title-main">Spiralia</span> Clade of protostomes with spiral cleavage during early development

The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa. The term Spiralia is applied to those phyla that exhibit canonical spiral cleavage, a pattern of early development found in most members of the Lophotrochozoa.

In evolutionary developmental biology, inversion refers to the hypothesis that during the course of animal evolution, the structures along the dorsoventral (DV) axis have taken on an orientation opposite that of the ancestral form.

<span class="mw-page-title-main">Avalon explosion</span> Proposed evolutionary event in the history of metazoa, producing the Ediacaran biota

The Avalon explosion, named from the Precambrian faunal trace fossils discovered on the Avalon Peninsula in Newfoundland, eastern Canada, is a proposed evolutionary radiation of prehistoric animals about 575 million years ago in the Ediacaran period, with the Avalon explosion being one of three eras grouped in this time period. This evolutionary event is believed to have occurred some 33 million years earlier than the Cambrian explosion, which had been long thought to be when complex life started on Earth.

<span class="mw-page-title-main">Xenacoelomorpha</span> A deep-branching bilaterian clade of animals with a simple body plan

Xenacoelomorpha is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies, which was then confirmed by phylogenomic analyses of molecular data.

<span class="mw-page-title-main">Xenambulacraria</span> Animal clade containing xenoturbellids, acoelomorphs, echinoderms and hemichordates

Xenambulacraria is a proposed clade of animals with bilateral symmetry as an embryo, consisting of the Xenacoelomorpha and the Ambulacraria.

References

  1. ↑ Further details are available at Ediacara biota
  2. ↑ Further details are available at Vernanimalcula .
  3. ↑ Further details are available at Fossil embryos.
  4. ↑ Gostling, Neil J.; Thomas, Ceri-Wyn; Greenwood, Jenny M.; Dong, Xiping; Bengtson, Stefan; Raff, Elizabeth C.; Raff, Rudolf A.; Degnan, Bernard M.; Stampanoni, Marco; Donoghue, Philip C. J. (June 2008). "Deciphering the fossil record of early bilaterian embryonic development in light of experimental taphonomy". Evolution & Development. 10 (3): 339–349. doi:10.1111/j.1525-142X.2008.00242.x. PMID   18460095. S2CID   13233666.
  5. 1 2 3 Budd, G.E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews. 75 (2): 253–295. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID   10881389. S2CID   39772232.
  6. 1 2 3 Arendt, D.; Wittbrodt, J. (Oct 2001). "Reconstructing the eyes of Urbilateria". Philosophical Transactions of the Royal Society of London B. 356 (1414): 1545–1563. doi:10.1098/rstb.2001.0971. ISSN   0962-8436. PMC   1088535 . PMID   11604122.
  7. ↑ Nilsson, E.; Arendt, D. (Dec 2008). "Eye Evolution: the Blurry Beginning". Current Biology. 18 (23): R1096–R1098. Bibcode:2008CBio...18R1096N. doi: 10.1016/j.cub.2008.10.025 . PMID   19081043. S2CID   11554469.
  8. 1 2 3 4 5 6 7 8 9 Hejnol, A.; Martindale, M.Q. (2008). "Acoel development supports a simple planula-like urbilaterian". Philosophical Transactions of the Royal Society of London B. 363 (1496): 1493–1501. doi:10.1098/rstb.2007.2239. PMC   2614228 . PMID   18192185.
  9. ↑ Telford, M.J.; Budd, G.E. (2003). "The place of phylogeny and cladistics in Evo-Devo research". International Journal of Developmental Biology. 47 (7/8). University of the Basque Country Press; 1999: 479–490. PMID   14756323. Archived from the original on 2011-07-16. Retrieved 2011-07-16.
  10. 1 2 Held, Lewis I. (2014). How the Snake Lost its Legs. Curious Tales from the Frontier of Evo-Devo. Cambridge University Press. p. 11. ISBN   978-1-107-62139-8.
  11. ↑ Michaël, Manuel (2009). "Évolution animale : les péripéties de la phylogénie". Encyclopædia Universalis (in French).
  12. ↑ Dewel, R. A. (January 2000). "Colonial origin for Eumetazoa: major morphological transitions and the origin of bilaterian complexity". Journal of Morphology. 243 (1): 35–74. doi:10.1002/(SICI)1097-4687(200001)243:1<35::AID-JMOR3>3.0.CO;2-#. ISSN   0362-2525. PMID   10629096.
  13. ↑ Nielsen, Claus (2013-08-16). "Life cycle evolution: was the eumetazoan ancestor a holopelagic, planktotrophic gastraea?". BMC Evolutionary Biology. 13 (1): 171. Bibcode:2013BMCEE..13..171N. doi: 10.1186/1471-2148-13-171 . ISSN   1471-2148. PMC   3751718 . PMID   23957497.
  14. 1 2 3 4 5 Martynov, Alexander V.; Korshunova, Tatiana A. (2022-08-11). "Renewed perspectives on the sedentary-pelagic last common bilaterian ancestor". Contributions to Zoology. 91 (4–5): 285–352. doi:10.1163/18759866-bja10034. ISSN   1875-9866.
  15. 1 2 Schultz, Darrin T.; Haddock, Steven H. D.; Bredeson, Jessen V.; Green, Richard E.; Simakov, Oleg; Rokhsar, Daniel S. (2023-06-17). "Ancient gene linkages support ctenophores as sister to other animals". Nature. 618 (7963): 110–117. Bibcode:2023Natur.618..110S. doi:10.1038/s41586-023-05936-6. ISSN   1476-4687. PMC   10232365 . PMID   37198475.
  16. ↑ Zhao, Yang; Parry, Luke A.; Vinther, Jakob; Dunn, Frances S.; Li, Yu-jing; Wei, Fan; Hou, Xian-guang; Cong, Pei-yun (2021-12-24), An early Cambrian polyp reveals an anemone-like ancestor for medusozoan cnidarians, doi:10.1101/2021.12.24.474121 , retrieved 2024-08-31
  17. 1 2 Steinmetz, Patrick R. H. (2019-09-01). "A non-bilaterian perspective on the development and evolution of animal digestive systems". Cell and Tissue Research. 377 (3): 321–339. doi:10.1007/s00441-019-03075-x. ISSN   1432-0878. PMC   6733828 . PMID   31388768.
  18. ↑ Runnegar, Bruce (2021-07-12). "Following the logic behind biological interpretations of the Ediacaran biotas". Geological Magazine. 159 (7): 1093–1117. doi: 10.1017/S0016756821000443 . ISSN   0016-7568.
  19. ↑ Hua, H.; Pratt, B. R.; Zhang, L.-Y. (2003-10-01). "Borings in Cloudina Shells: Complex Predator-Prey Dynamics in the Terminal Neoproterozoic". PALAIOS. 18 (4–5): 454–459. Bibcode:2003Palai..18..454H. doi:10.1669/0883-1351(2003)018<0454:BICSCP>2.0.CO;2. ISSN   0883-1351.
  20. 1 2 Cortijo, Iván; Cai, Yaoping; Hua, Hong; Schiffbauer, James D.; Xiao, Shuhai (2015-08-01). "Life history and autecology of an Ediacaran index fossil: Development and dispersal of Cloudina". Gondwana Research. 28 (1): 419–424. Bibcode:2015GondR..28..419C. doi:10.1016/j.gr.2014.05.001. ISSN   1342-937X.
  21. ↑ Cai, Yaoping; Schiffbauer, James D.; Hua, Hong; Xiao, Shuhai (2011-11-01). "Morphology and paleoecology of the late Ediacaran tubular fossil Conotubus hemiannulatus from the Gaojiashan Lagerstätte of southern Shaanxi Province, South China". Precambrian Research. 191 (1): 46–57. Bibcode:2011PreR..191...46C. doi:10.1016/j.precamres.2011.09.002. ISSN   0301-9268.
  22. ↑ Cai, Yaoping; Cortijo, Iván; Schiffbauer, James D.; Hua, Hong (2017-09-01). "Taxonomy of the late Ediacaran index fossil Cloudina and a new similar taxon from South China". Precambrian Research. 298: 146–156. Bibcode:2017PreR..298..146C. doi:10.1016/j.precamres.2017.05.016. ISSN   0301-9268.
  23. ↑ Schiffbauer, James D.; Selly, Tara; Jacquet, Sarah M.; Merz, Rachel A.; Nelson, Lyle L.; Strange, Michael A.; Cai, Yaoping; Smith, Emily F. (2020-01-10). "Discovery of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran Period". Nature Communications. 11 (1): 205. Bibcode:2020NatCo..11..205S. doi:10.1038/s41467-019-13882-z. ISSN   2041-1723. PMC   6954273 . PMID   31924764.
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