Vendobionts or Vendozoans (Vendobionta) are a proposed, very high-level group of extinct organisms that lived on the ocean floor during the Ediacaran period, about 580 million years ago. This group is not certain, as scientists have not confirmed its exact classification. It would have been among the earliest animals on Earth, but it became extinct soon after the Cambrian explosion, when new animal groups appeared. Sponges might have evolved from this group. However, it is likely that the Ediacaran biota was not a single family of related organisms, and not all of its genera were animals.

This group is not widely accepted by scientists, as it is difficult to determine how these ancient organisms are related to other life forms. The idea was first suggested by geologist Adolf Seilacher, who questioned whether Vendobionts were animals or multicellular organisms. He proposed they might have developed independently, possibly as large single-celled organisms. Other theories suggest they could have been related to cnidarians, sponges, fungi, algae, or a completely different group of life. Scientists agree they were not photosynthetic. Like acritarchs, Vendobionts remain a mystery in evolutionary history and may represent a unique, now-extinct group of life.

Features

The Ediacaran organisms are described as having soft bodies without hard parts like shells or bones. They are often compared to thin, flat, and filled with a liquid called plasmodial fluid, similar to inflatable mattresses. Inside, they may have had a single compartment called a syncytium, while their outer surfaces showed folds or segments arranged in parallel, radial, or concentric patterns. They might have had a type of wall similar to plants or fungi, which helped them resist being crushed and made fossilization possible.

These organisms lived in the ocean, on the seafloor, in areas ranging from shallow to deep subtidal environments. Even those resembling jellyfish were later found to be similar to polyps or flat, attached structures, or partially buried in the seafloor. This lifestyle led to the nickname "The garden of Ediacara" for their habitat.

Despite having soft bodies, many Ediacaran fossils are well-preserved, which suggests that burrowing animals were not present in their environment. Most of these organisms did not move. The oldest groups showed no signs of movement, such as digging or shifting sediment during fossilization. It is thought they lacked muscles or a nervous system, and many were stationary. However, some later groups, like Proarticulata, may have moved slowly by sliding. These groups might have fed on microorganisms on the seafloor, which supports the idea that they could be related to animals, even though their nervous system developed separately.

There is no clear evidence of mouths, anuses, or digestive systems in Ediacaran fossils, though internal structures have been found. It is believed they absorbed nutrients through their surfaces, like fungi or bacteria. Their many folds and segments increased their surface area, which would help with nutrient absorption. Some scientists suggested they might have used photosynthesis, but this is unlikely because they lived in deep areas where sunlight could not reach. The lack of bite marks on fossils suggests they were not predators or preyed upon, making them vulnerable to Cambrian predators.

As they grew, Ediacaran organisms kept the same shape and number of segments, though the segments became more divided. There is no evidence they had an embryonic stage, which would place them in a different group from other animals.

Reproduction in Ediacaran organisms likely involved asexual methods, as no reproductive organs like gonads have been found.

Age and distribution

The first organisms to appear were frondomorphs (Petalonamae) and simple medusoids 578 million years ago. These are considered the oldest complex life forms. The Ediacaran biota is divided into three groups.

Systematics

The classification of these organisms is very controversial. Some scientists place them in the kingdom Animalia, while others believe they belong to different groups, such as protists, fungi, lichens, or an independent kingdom. Some researchers think these groups are unrelated and place proarticulates in a group called Bilateria at the base of the evolutionary tree. Others suggest they are similar to early arthropods or cnidarians like sea pens or ctenophores. Trilobozoa and medusoids are sometimes classified as jellyfish without cnidoblasts in the group Cnidaria or Coelenterata. Because these organisms are very old, some scientists believe it is too speculative to create a clear family tree or directly link them to modern animals.

Some scientists suggest vendobionts may have come from amoeboid protozoa or protists, similar to xenophyophores, which are large unicellular foraminifera that can grow up to 20 cm. These organisms may have been unicellular but reached large sizes because there were no predators to eat them. Many vendobionts range in size from a few millimeters to over a meter, possibly due to increased oxygen levels. This has led some to believe they may form a separate and now extinct kingdom, not related to animals, fungi, or plants. However, studies of their fossils have not found any evidence of shells, testa, or carapace made of organic or mineral material, which would support this idea.

Some scientists think the Ediacaran biota may have been colonial organisms, a step between protists and animals. However, their complex shapes and the lack of microbial structures like stromatolites, along with evidence of multicellularity in forms like Aspidella, suggest they were not colonial.

Most vendobionts, except proarticulates, have been placed in the group Cnidaria by some scientists because of their similar shapes, though differences have also been noted.

Petalonamae share basic features with cnidarians like sea pens. Fossils of ancient jellyfish and medusoids show similar patterns of radial and concentric structures and a central raised axis.

Recent fossils of modern sea feathers are more distorted and irregular compared to older Proterozoic fossils. Proterozoic fossils show more concentric rings and radial grooves, and fewer deformed fossils are found compared to modern jellyfish. This might mean Ediacaran organisms had stiffer bodies than many modern cnidarians of similar size. Many Ediacaran fossils do not match any living organisms. The simple shapes of these fossils suggest similarities with cnidarians may be due to convergent evolution, though a possible evolutionary link cannot be ruled out.

There are clear differences between modern sea feathers and petalonamae. Sea feathers are colonies of polyps that grow from a central axis, can move slightly, and have tentacles. Petalonamae are immobile, not colonies, and have a flat shape with segments and no mouth, anus, or tentacles. Instead of a central axis, they often have a zigzagging suture line. Studies of Charnia fossils show they grow in the opposite direction to modern sea feathers, suggesting they are not related.

Jellyfish have a round, gelatinous shape, while medusoids are flat and have a more pronounced dorsal side, possibly due to a rigid outer wall. There is no evidence that medusoids share features with cnidarians, such as a mouth or a two-layered body wall enclosing a cavity. This, along with signs of a rigid outer wall, raises doubts about the idea that Ediacaran organisms were mostly soft-bodied cnidarians.

Proarticulata have been compared to modern animals like arthropods or annelids. However, they lack true segmentation, articulated limbs, or other lateral features that define these groups. Their bodies consist of two rows of identical "semi-segments" on either side of a central axis, a symmetry not typical of animals. This type of structure is seen in other vendobionts, protists, and plants.

In general, Vendobionta is considered a group separate from animals, with no clear common origin. Some scientists argue that evidence of movement, such as trails left by proarticulates, supports a link to animals.

Some scientists suggest Ediacaran organisms like frondomorphs or medusoids may be related to fungi or slime molds due to traits like multicellularity, lack of movement, and resistance to fossilization. However, there is no evidence of fungal features like mycelium, sporangia, or fruiting bodies. A fungal model does not fit many Ediacaran fossils.