Vendobionts or Vendozoans (Vendobionta) are a proposed, very high-level group of extinct benthic organisms that include most of the life forms from the Ediacaran biota. This group is hypothetical, meaning scientists are not certain about its exact classification. It would represent some of the earliest animals on Earth, living about 580 million years ago during the Ediacaran period (formerly called the Vendian). These organisms became extinct soon after the Cambrian explosion, when new groups of animals evolved that are more closely related to modern species. However, sponges may have descended from this group. Scientists believe the Ediacaran biota is not a single, unified group, and not all genera within it are animals.
This biological group is not widely accepted by scientists. Its credibility is limited because establishing relationships between such ancient and poorly understood organisms requires much speculation. The idea was first proposed by German geologist Adolf Seilacher, who questioned whether this group was related to animals or even multicellular. He suggested they might have developed independently and could have been large single-celled organisms. Other theories suggest they might have been cnidarians, articulates, fungi, colonial protists, algae, lichens, or a group unrelated to all known animals. The only agreement among scientists is that they were not photosynthetic. Like acritarchs, Vendobionts are considered evolutionary mysteries and may represent a separate, now-extinct kingdom.
Features
The Ediacaran organisms are described as having certain shared traits that might have been common among the earliest groups of these life forms. These organisms likely had soft bodies without hard parts like armor or bones. Their appearance has been compared to thin, flat, inflatable mattresses filled with a liquid called plasmodial fluid. Inside, they may have had a single compartment called a syncytium, and on the outside, they had folds or segments arranged in parallel, radial, or concentric patterns. They might also have had a type of cell wall, similar to plants or fungi, which helped resist compression and made fossilization possible.
All Ediacaran organisms lived in marine environments, likely on the seafloor in shallow to deep subtidal areas. Even some organisms that seemed like floating medusas were later found to be similar to polyps or flat, disc-like structures attached to the seabed, or partially buried in the sediment. This seafloor habitat led to the nickname "The garden of Ediacara."
Despite their soft bodies, many Ediacaran fossils are well-preserved. This suggests that there were no burrowing animals in their environment, which could have disturbed the sediment and prevented fossilization. Most of these organisms did not move. The oldest groups had no ability to move, as shown by the lack of signs of movement or sediment disturbance in fossils. Scientists think they lacked muscles or a nervous system due to their simple structure, and many were sessile, meaning they stayed in one place. However, some later groups, like Proarticulata, may have had slow, sliding movement. These groups might have grazed on microorganisms on the seafloor. This ability to move is a key reason scientists consider some Ediacaran organisms, like vendobionts, to be related to animals, even though their nervous systems evolved separately.
There is no clear evidence that Ediacaran organisms had mouths, anuses, or digestive systems. However, internal structures have been found. Scientists believe they may have absorbed nutrients through osmosis, like fungi or bacteria. Their many folds and segments increased their surface area, which would help with osmotic nutrient absorption. Some scientists once thought they might have been photosynthetic, but they lived in deep areas where sunlight could not reach, making photosynthesis unlikely. The lack of bite marks on Ediacaran fossils suggests they were not predators and were not hunted by others, which may have made them vulnerable to Cambrian predators.
Ediacaran organisms grew by increasing the number of subdivisions within their segments, but they kept the same overall shape and number of segments regardless of size. Scientists think they did not have an embryonic stage, which would make them different from other animals.
Reproduction in Ediacaran organisms appears to have been asexual. No sex organs, such as gonads, have been found in fossils.
Age and distribution
The first complex life forms to appear were frond-like organisms (Petalonamae) and simple jellyfish-like creatures 578 million years ago. These are considered the oldest known complex life forms. The Ediacaran biota is divided into three groups based on time.
Systematics
The classification of these organisms is very controversial. Some scientists place them mainly in the Animalia kingdom, but others think they belong to protists, fungi, lichens, or a separate kingdom. Scientists who believe these groups are unrelated place proarticulates in Bilateria (a group of animals) or as early ancestors of arthropods. Petalonamae are sometimes grouped with cnidarians, like sea pens or ctenophores. Trilobozoa and medusoids are sometimes classified as jellyfish without cnidoblasts in Cnidaria or Coelenterata. Because these organisms are very old, some scientists think it is too speculative to create a family tree or directly link them to modern animals.
Some scientists suggest vendobionts may come from amoeboid protozoa or protists, like xenophyophores, which are large unicellular foraminifera that can grow up to 20 cm. These organisms might have been unicellular but developed large size due to the lack of predators. Their sizes range from a few millimeters to over a meter, possibly because of increased oxygen levels. This could mean they were not animals, fungi, or plants, but an extinct kingdom. However, studies of their fossils show no evidence of shells, hardened structures, or mineralized parts, which contradicts this idea.
Some scientists think the Ediacaran biota might be colonial organisms, a step between protists and animals. However, their complex shapes and the lack of stromatolites or microbial layers, along with evidence of multicellularity in forms like Aspidella, suggest they were not colonial.
Most vendobionts, except proarticulates, are sometimes classified in Cnidaria due to similar shapes, though differences exist. Petalonamae share basic features with cnidarians like sea pens (Pennatulacea). Ediacaran medusoids and modern jellyfish have similar patterns of radial and concentric structures and a central raised axis.
Recent sea feather fossils are more misshapen than Ediacaran fossils. Proterozoic medusoids have more concentric rings and radial grooves, and deformed fossils are rare compared to modern jellyfish. This might mean Ediacaran organisms had stiffer bodies than many modern cnidarians. Many Ediacaran fossils have no modern equivalents. Similarities between Ediacaran and cnidarians may be due to convergence, but a shared ancestry cannot be ruled out.
There are key differences between modern sea feathers and petalonamae. Sea feathers are colonies of polyps that grow from a central axis, can move, and have tentacles. Petalonamae are immobile, not colonies, and lack mouths, anuses, or tentacles. They have a middle suture instead of a central axis. Laser imaging of Charnia fossils shows they grow in the opposite direction of modern cnidarians, suggesting they are not related.
Medusoids differ from jellyfish in shape, being flatter and more rigid, possibly due to a hard outer wall. They lack mouths, two-layered body walls, or a single cavity. This challenges the idea that Ediacaran organisms were soft-bodied cnidarians.
Proarticulates have been compared to modern animals like arthropods or annelids. However, they lack complete segmentation, articulated limbs, or lateral processes. Their bodies have two rows of identical "semi-segments" along their axis, a feature not seen in animals but found in other vendobionts, protists, and plants.
In general, Vendobionta is considered a separate group from animals, with no clear common origin. Some scientists argue that trails left by proarticulates suggest movement, linking them to animals.
Ediacaran organisms like frondomorphs or medusoids have been compared to fungi or slime molds due to traits like multicellularity, lack of movement, and resistance to fossilization. However, there is no evidence of fungal structures like mycelium or spores. A fungal model does not fit many Ediacaran fossils.