Vendobionta

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Vendobionta
Temporal range: Ediacaran–Middle Cambrian
Ediacaran sea.png
Interpretation of the Ediacaran biota.
Scientific classification e
Kingdom: Animalia
Superphylum: †Vendobionta
Seilacher 1992
Subtaxa
Synonyms
  • Vendozoa, Seilacher 1989

Vendobionts or Vendozoans (Vendobionta) are a group of benthic beings made up of the majority of extinct creatures that were part of the Ediacaran biota. It is a hypothetical group and at the same time, it would be the oldest of the animals that populated the Earth about 580 million years ago, in the Ediacaran or Vendic period. They became extinct when the so-called Cambrian explosion appeared, with the introduction of fauna formed by more recognizable groups and more related to modern animals.[1] It is very likely that it isn't a monophyletic clade and not every genus placed in its subtaxa is an animal.

This biological group is not widely recognized because it is speculative to try to establish phylogenetic relationships between such ancient extinct groups. This is a hypothesis formulated by the German geologist Adolf Seilacher, who even doubts its relationship with the animal kingdom, or its multicellular nature--the group might have originated independently, and could be large unicellular forms. It has also been proposed that they could have been cnidarians, articulated, or a divergent group from all current animals; perhaps a fungal organism, colonial protist, algae, or lichen; although now there is consensus that they could not be photosynthetic. In any case, like the acritarchs, they are considered evolutionary enigmas and were probably an independent and extinct kingdom.[2]

Features[]

Dickinsonia, a proarticulate.

Within the description of this hypothetical group, characteristics have been proposed that could have been common in all the first groups of Ediacaran beings:

  • Structure: Relatively soft body, without hard parts such as armor or skeletons. In appearance, they have been compared to thin inflatable mattresses generally flat and filled with a liquid which has been called plasmodial fluid. Internally they could contain a single compartment (the syncytium)[3] and externally they presented several types of folds or segments (parallel, radial and/or concentric) in addition to, probably, some type of cell wall (such as plants or fungi) that offered resistance to contraction or compaction, thus facilitating fossilization.[4] Despite having soft bodies, their abundant preservation is surprising, to which the absence of burrowing creatures in these sediments surely contributed.[5]
  • Habitat: All were marine and probably also benthic, inhabiting the seabed, from shallow to deep subtidal environments. Even medusoids, which were thought to be swimmers, were actually later concluded to be like polyps[6] or benthic discs anchored to the seabed, or semi-buried endobentonics. This habitat influenced the popular name of the "Ediacara Garden".
Tribrachidium, a trilobozoan.
  • Immobility: Most lacked locomotion. The oldest groups did not have any capacity for movement, as evidenced by the complete lack of ash and sediment disturbance during fossilization.[4] It is believed that there were no musculature or nervous system, given their simplicity, and many were sessile. However, it is considered that the movement would have appeared in Proarticulata and would have been slow and sliding; presumably, mobile representatives such as Spriggina grazed on a lawn of microorganisms (epifaunal grazing). This characteristic of Proarticulata is the main argument to consider that vendobionts, if they were a clade, would be ascribed to the animal kingdom; although the development of its own nervous system would have been independent of the other animals.[7]
  • Food: There is no definitive evidence of the presence of the mouth, anus or a digestive system, although there is speculation about it. It is believed that the feeding could have been by osmosis,[8] just as fungi or bacteria do. The presence of numerous streaks, folds or segments considerably increases the total surface area necessary for the osmotic absorption of nutrients. It was also proposed that they were photosynthetic organisms, however, they lived at different depths, even under 200 meters where light does not reach for photosynthesis. Due to the absence of bite marks in the Ediacaran fossils, it was concluded that the vendobionts were not predators nor were they exposed to them; which could then have made them easy victims of Cambrian predators.
  • Growth: They grow preserving their shape and maintain the same number of segments regardless of size, although the segments are subdivided more and more. It is believed that they did not have an embryonic stage, if so, they would be a different group from the other animals.[7]
  • Reproduction: It would apparently be asexual. No sex organs or gonads have been found.

Age and distribution[]

Charnia, a frondomorph.

The first to appear were frondomorphs (Petalonamae) and simple medusoids 578 Ma (millions of years) ago and are considered the oldest complex life forms. The Ediacaran biota is chronologically divided into three associationsː the Avalon association evokes the oldest site that is in the cliffs of the Avalon Peninsula, Canada (578-560 Ma), showing a postglacial and deep-water habitat; with a completely benthic biota, with a predominance of rangeomorphs, well below the photic zone, at a depth of one kilometer, which rules out that they were algae or lichens.[4] This type of biota achieves great distribution in all marine niches at sea level. worldwide, as an abrupt appearance of macroscopic beings in what was called the Avalon explosion, 37 million years before the Cambrian explosion. These same beings, frondomorphs and medusoids, were the last to disappear in the Cambrian.

The bilateralomorphs (Proarticulata) appeared 555 Ma ago and had more environmental restrictions, being found in tropical seas.[9][10] The White Sea association (560-550 Ma, Russia and Australia), showed a great variety of new fossil footprints and the greatest diversity occurs in rocks that were deposited in well-lit and energetically active shallow waters.[11]

The third and last Ediacaran association is that of Nama (550-541 Ma, Namibia), where a decrease in biodiversity is observed, as a preamble to the massive extinction of the Ediacaran biota. Before the end of the Ediacaran period, proarticulates and trilobozoans would have already become extinct; and the last vendobionts disappear with the arrival of the animals of the Cambrian explosion.[12]

Systematic[]

The classification is very controversial. As a unified group they have been placed primarily in Animalia, but are considered by others to be protists, fungi, terrestrial lichens, or an independent kingdom. Those who see here different unrelated groups, place the proarticulates in Bilateria basally (this opinion is the most upheld among these alternative theories) or as pre-arthropods, the petalonams with cnidarians such as sea pens or as ctenophores, and the trilobozoa and medusoids are considered jellyfish without cnidoblasts and classified in Cnidaria or Coelenterata. Finally, given their antiquity, others consider it speculative to develop a phylogeny or to directly relate these beings to modern animals.

Comparison with protists[]

It has been suggested that vendobionts may come from amoeboid protozoa or protists, and be, for example, similar to xenophiophores,[13] which are unicellular foraminifera that have developed large size, reaching 20 cm. They could therefore have been unicellular beings that developed macroscopic size thanks to the absence of predators, since all the groups have a size that ranges from a few millimeters to exceeding a meter in length, favored by the increase in oxygen; in such a way that the opportunity is opened to consider that they were not animals, fungi or plants, but an independent and extinct kingdom.[2] However, structural studies of these fossils have so far revealed no traces of a shell, testa or carapace organic material (sclerotized), agglutinated or mineralized, the presence of which is implicit in this hypothesis.[14]

It has also been suggested that the Ediacaran biota could be formed by colonial beings, as an intermediate degree between protists and animals.[15] However, the morphological complexity and the absence of layers of stromatolites or other microbial structures,[16] in addition to indications that they were multicellular as seen in discoid forms like Aspidella; would indicate that they were not colonial.[4]

Comparison with coelenterates[]

All vendobionts, except proarticulates, have been classified in Cnidaria by some on the basis of morphological similarities, although differences have also been described.

  • Similarities: Petalonamae shares basic structural characteristics with sea pens cnidarians (Pennatulacea). Similarly, the extant jellyfish impressions and the Proterozoic (Ediacaran) medusoid circular impressions show general similarities in the arrangement and position of radial and concentric structures, as well as a central raised axis.
  • Differences: Recent sea feathers have produced fossil impressions that are more misshapen and irregular than Proterozoic fossils. While in medusoids, concentric rings and radial grooves are more numerous in Proterozoic fossils, as strongly folded or deformed fossils are rare compared to modern jellyfish prints. This could mean that Ediacaran beings, despite not having hard parts, had stiffer or firmer bodies than many modern cnidarians of comparable sizes. Many Ediacaran fossils have no counterpart between existing forms. The structural simplicity of the impressions of existing Ediacaran and cnidarians suggests that their mutual similarities may be due to convergence, however, possible phylogenetic affinities between them cannot be completely ruled out.[17]

With Petalonamae: There are important morphological differences. The current sea feathers are actually a colony of polyps, they grow from a polyp that after losing its tentacles becomes the axis of the colony and from which the other tentaculated polyps grow, they are also quite capable of certain movements . On the other hand, the petalonamos are immobile, they do not seem to be a colony and could rather be closer to the proarticulated ones due to their simple and flat shape, because of their segmentation and because of the lack of a mouth, anus and tentacles, in addition, instead of a axis there is usually a middle suture that can be zigzagging if the segments are alternate. An analysis of the growth and development of Charnia fossils through laser imaging of the holotype reveals that it cannot be related to modern cnidarians such as sea feathers, with which it has been compared for so long, because they have opposite growth polarities.[18]

Cyclomedusa, a medusoid.

With medusoids: There are morphological differences between the jellyfish and their globose and gelatinous shape, compared to the flat, discoid medusoids, with relief that is greater on the dorsal side and firmer due to the indications that would reveal the existence of a rigid wall surrounding the body . It can be suggested that there is no evidence that medusoids share biological characteristics with coelenterates; there is no mouth, no two-layered body wall enclosing a single cavity. This, and the evidence for a rigid outer wall, adds to recent doubts about the concept of an Ediacaran fauna that would have been dominated by soft-bodied coelenterates.[19]

Comparison with articulates[]

Spriggina, a proarticulate of the group Cephalozoa.

Proarticulata have been compared to modern articulated animals such as arthropods or annelids. However, other anatomical features of the proarticulates, mainly the absence of truly complete segmentation, articulated limbs, and any other lateral processes, do not agree with this interpretation. The body of the proarticulates consisted of two rows of identical 'semi-segments' (the isomers), right and left, located along their longitudinal axis, which does not correspond to the articulated ones. This type of symmetry is not typical in animals, instead it has been observed in other vendobionts, protists, multicellular colonies and frequently in plants.[20]

In general, Vendobionta is being defined as a group apart from animals, with no proven common origin. However, those in favor of considering them animals, see in the discovery of trails left by some proarticulates that would have locomotion, a decisive argument of the relationship with the metazoans.[20]

Comparison with fungi[]

It has been postulated that Ediacaran beings such as frondomorphs or medusoids, could be related to fungi (Fungi) or to some other fungal organism (slime molds), due to certain characteristics such as multicellularity, lack of movement, indeterminate growth, osmtrophic feeding and resistance to taphonomic shrinkage (ease of fossilization given by the presence of a cell wall).[4] However, there is no evidence of mycelial development, nor presence of sporangia or fruiting bodies. For other Ediacaran fossils, a fungal model is clearly inappropriate.

Gallery[]

Some examples of Ediacaran fossils, which Adolf Seilacher at various times considered to be members of Vendozoa and Vendobionta.

References[]

  1. ^ María Luisa Martínez Chacón. Invertebrate paleontology. Paleontology, Geological and Mining Institute of Spain
  2. ^ a b Seilacher, A. (1992). "Vendobionta and Psammocorallia: lost constructions of Precambrian evolution" (abstract). Journal of the Geological Society. Journal of the Geological Society, London. 149 (4): 607–613. Bibcode:1992JGSoc.149..607S. doi:10.1144/gsjgs.149.4.0607. S2CID 128681462. Retrieved June 21, 2007.
  3. ^ Seilacher A, Grazhdankin D, Legouta A: Ediacaran biota: The dawn of animal life in the shadow of giant protists. In: Paleontological Research. 7, Nr. 1, 2003, S. 43–54.
  4. ^ a b c d e Kevin J. Peterson, Ben Waggoner & James W. Hagadorn 2003, A Fungal Analog for Newfoundland Ediacaran Fossils? Integrative and Comparative Biology, Volume 43, Issue 1, 1 February 2003, Pages 127–136, https://doi.org/10.1093/icb/43.1.127
  5. ^ Stanley, S. M. (1973). "An ecological theory for the sudden origin of multicellular life in the Late Precambrian". Proceedings of the National Academy of Sciences. 70 (5): 1486–1489. Bibcode:1973PNAS...70.1486S. doi:10.1073/pnas.70.5.1486. PMC 433525. PMID 16592084. Retrieved June 21, 2007.
  6. ^ Dave Smith 1999, Cyclomedusa UCMP Berkeley
  7. ^ a b Mark AS McMenamin 1986, The Garden of Ediacara. Discovering the First Complex Life. Columbia University Press. ISBN 9780231500302, 1998
  8. ^ Laflamme, Marc; Shuhai, Xiao; Kowalewski, Michał (2009). "Osmotrophy in modular Ediacara organisms". Proceedings of the National Academy of Sciences. 106 (34): 14438–14443. doi:10.1073/pnas.0904836106. PMC 2732876. PMID 19706530.
  9. ^ Ben Waggoner 2003, The Ediacaran Biotas in Space and Time.
  10. ^ Waggoner, Ben (February 1, 2003). "The Ediacaran Biotas in Space and Time1". Integrative and Comparative Biology. 43 (1): 104–113. CiteSeerX 10.1.1.491.4910. doi:10.1093/icb/43.1.104. PMID 21680415 – via Silverchair.
  11. ^ Stephen Q. Dornbos et al. 2012, Lessons from the fossil record: the Ediacaran radiation. (pdf) Marine Biodiversity and Ecosystem Functioning. First Edition. Edited by Martin Solan, Rebecca J. Aspden, and David M. Paterson.© Oxford University Press 2012.
  12. ^ The Ediacara biota and first mass extinction of metazoan life. Smithsonian's National Museum of Natural History 2016
  13. ^ Zhuravlev, AY 1993, Were Ediacaran Vendobionta multicellulars? Neues Jahrb. Geol. Palëontol. 190ː 299-314.
  14. ^ Waggoner, B. M. 1998. Three-dimensional anatomy and microstructure of some simple “medusoids” from the Vendian of Siberia. Geol. Soc. Amer. Abstrs. Progs , 3034.
  15. ^ Bergström, J. 1991. Metazoan evolution around the Precambrian-Cambrian transition. In A. M. Simonetta and S. Conway Morris (eds.), The early evolution of Metazoa and the significance of problematic taxa, pp. 25–34. Cambridge University Press, Cambridge.
  16. ^ Hagadorn, J. W., and D. J. Bottjer. 1997. Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition. Geology , 251047-1050.
  17. ^ RICHARD D. NORRI 1989, Cnidarian taphonomy and affinities of the Ediacara biota. Lethaia, Volume 22, Issue 4, October 1989 Pages 381–393
  18. ^ Jonathan B Antcliffe & Martin D Brasier. Charnia at 50: Developmental models for Ediacaran fronds. Palaeontology 51(1):11 - 26 · January 2008
  19. ^ T. P. Crimes, A. Insole & B. P. J. Williams 1995, A rigid-bodied Ediacaran Biota from Upper Cambrian strata in Co. Wexford, Eire Geological Journal, Volume 30, Issue 2, June 1995, Pages 89–109
  20. ^ a b A. Yu. Ivantsov 2013, Trace fossils of precambrian metazoans “Vendobionta” and “Mollusks” Stratigr. Geol. Correl. (2013) 21: 252. https://doi.org/10.1134/S0869593813030039

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