Vendobionts or Vendozoans (Vendobionta) are a proposed very high-level, extinct clade of benthic organisms that is made up of the majority of the organisms that were part of the Ediacaran biota. It is a hypothetical group. It would be the oldest of the animals that populated the Earth about 580 million years ago, in the Ediacaran period (formerly Vendian). They became extinct shortly after the so-called Cambrian explosion, with the introduction of fauna forming groups more recognizably related to modern animals. It is likely that the whole Ediacaran biota is not a monophyletic clade 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 establish phylogenetic relationships between such ancient extinct groups. The hypothesis was 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, or articulates; perhaps a fungus, colonial protist, alga, or lichen; or a group divergent from all current animals. The only current consensus is that they could not have been photosynthetic. In any case, like the acritarchs, they are considered evolutionary enigmas, and possibly an independent and extinct kingdom.
Features
Within the description of this hypothetical group, characteristics have been proposed that could have been common in all the first groups of Ediacaran organisms:
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) 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.
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 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.
Mobility 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. 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.
Diet 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, 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.
Reproduction Vendozoan reproduction would apparently have been asexual. No gonads (sex organs) have been recognized.
Age and distribution
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 associations:
The Avalon association is the oldest fossil bed. It is found in the cliffs of the Avalon Peninsula, Newfoundland (578–560 Ma), and shows a postglacial, deep-water habitat. The Avalonian organisms are completely benthic, 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. The Avalon biota achieve great distribution in all marine niches, worldwide: There was an abrupt appearance of macroscopic organisms in what was called the Avalon explosion, 37 million years before the Cambrian explosion. These same organisms – frondomorphs and medusoids – were the last to disappear during the extinction of all Vendobionts in the Cambrian. The bilateralomorphs (Proarticulata) appeared 555 Ma ago and had more environmental restrictions, being found in tropical seas. 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. The 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.
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