| Vendobionta | |
|---|---|
 | |
| Artist interpretation ofEdiacaran biota. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Superphylum: | †Vendobionta Seilacher (1992)[1] |
| Subtaxa | |
| Synonyms | |
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Vendobionts orVendozoans (Vendobionta) are a proposed very high-level, extinctclade ofbenthic organisms that made up of the majority of the organisms that were part of theEdiacaran biota. It is a hypothetical group. It would be the oldest of theanimals that populated the Earth about 580 million years ago, in theEdiacaran period (formerlyVendian). They became extinct shortly after the so-calledCambrian explosion, with the introduction of fauna forming groups more recognizably related to modern animals, howeversponges may be descended from this clade. It is likely that the whole Ediacaran biota is not amonophyleticclade and not every genus placed in its subtaxa is an animal.
This biological group is not widely recognized; credibility is limited by the expansive speculation needed to establishphylogenetic relationships between such ancient extinct groups. The hypothesis was formulated by the German geologistAdolf 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 beencnidarians, orarticulates; perhaps afungus,colonialprotist,algae, orlichen; or a group divergent from all current animals. The only current consensus is that they could not have beenphotosynthetic. In any case, like theacritarchs, they are considered evolutionary enigmas, and possibly an independent and extinctkingdom.[1]
Within the description of this hypothetical group, characteristics have been proposed that could have been common in all the first groups of Ediacaran organisms:
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 (thesyncytium)[3] and externally they presented several types of folds or segments (parallel, radial, or concentric) in addition to, probably, some type of cell wall (such as plants or fungi) that offered resistance to contraction or compaction, thus promoting fossilization.[4]
All were marine and probably alsobenthic, 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[5] or benthic discs anchored to the seabed, or semi-buried endobentonics. This habitat influenced the popular name of "The garden of Ediacara".
The Ediacarans' abundant preservation is surprising, despite having soft bodies; this almost surely indicates the absence of burrowing creatures in their home sediments.[6]
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 inProarticulata and would have been slow and sliding; presumably, mobile representatives such asSpriggina 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]
There is no definitive evidence of the presence of the mouth, anus, or a digestive system, although internal structures have been identified. It is believed that the feeding could have been by osmosis,[8] just asfungi orbacteria 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 ofCambrian predators.
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]
Vendozoan reproduction would apparently have beenasexual. No sex organs orgonads have been recognized.
The first to appear were frondomorphs (Petalonamae) and simplemedusoids 578 Ma (millions of years) ago and are considered the oldest complex life forms. The Ediacaran biota is chronologically divided into threeassociations:
The classification is very controversial. As a unified group they have been placed primarily inAnimalia, 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 inBilateria basally (this opinion is the most upheld among these alternative theories) or as pre-arthropods, the petalonams with cnidarians such assea pens or as ctenophores, and the trilobozoa and medusoids are considered jellyfish withoutcnidoblasts and classified inCnidaria orCoelenterata. Finally, given their antiquity, others consider it speculative to develop a phylogeny or to directly relate these organisms tomodern animals.
It has been suggested that vendobionts may come fromamoeboidprotozoa or protists, and be, for example, similar toxenophiophores,[12] which are unicellularforaminifera that have developed large size, reaching 20 cm. They could therefore have been unicellular organisms 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.[1] 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.[13]
It has also been suggested that the Ediacaran biota could be formed by colonial organisms, as an intermediate degree between protists and animals.[14] However, the morphological complexity and the absence of layers of stromatolites or other microbial structures,[15] in addition to indications that they weremulticellular as seen indiscoid forms likeAspidella; would indicate that they were not colonial.[4]
All vendobionts, except proarticulates, have been classified inCnidaria by some on the basis of morphological similarities, although differences have also been described.
Petalonamae shares basic structural characteristics with sea pens cnidarians (Pennatulacea). Similarly, the extant jellyfish impressions and theProterozoic (Ediacaran) medusoid circular impressions show general similarities in the arrangement and position of radial and concentric structures, as well as a central raised axis.
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 organisms, despite not having hard parts, had stiffer or firmer bodies than many modern cnidarians of comparable size. Many Ediacaran fossils have no counterpart among existing forms. The structural simplicity of the impressions of existing Ediacaran and cnidarians suggests that their mutual similarities may be due toconvergence; however, possible phylogenetic affinities between them cannot be completely ruled out.[16]
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.[17]
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 withcoelenterates; 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.[18]
Proarticulata have been compared to modern articulated animals such asarthropods orannelids. 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.[19]
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.[19]
It has been hypothesized that Ediacaran organisms such as frondomorphs or medusoids, could be related to fungi (Fungi) or to some other fungal organism (slime molds), due to certain characteristics such asmulticellularity, lack of movement, indeterminate growth, osmotrophic 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 orfruiting bodies. For other Ediacaran fossils, a fungal model is clearly inappropriate.
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