Biology:Medusoid

Short description: Ediacaran fossils of uncertain classification

Medusoid Temporal range: 560 Ma PreЄ Є O S D C P T J K Pg N ↓ Ediacaran
Fossil of Mawsonites, up to 35 cm wide.
Scientific classification
Kingdom:	Animalia
Superphylum:	†Vendobionta
Phylum:	†Medusoids Hatschek, 1888

Medusoids or discoids are extinct organisms that are superficially similar to jellyfish that lived during the end of the Precambrian, the Ediacaran, and early Cambrian periods. It is believed that they were among the oldest animals, together with the petalonams, due to the discovery of Medusoid fossils from approximately 580 million years ago. They were part of the Ediacaran fauna.

Initially, it was theorized that they were free-swimming, but it was later concluded that they were either polyp-like benthic discs anchored to the seabed, or semi-buried endobentonic organisms. Their classification has been systematically controversial, often being grouped into Cnidaria, although they could have been primordial Vendozoans.

Features

Cyclomedusa.

Although they have been compared to current jellyfish, they were most likely benthic in habit, perching on the seabed and lacking locomotive ability. They were mostly circular with concentric folds or discs, except when adjacent individuals interfere with the growth of one another, causing the individuals to grow into an irregular form. Occasionally, there may be a somewhat prominent central formation or even tentacles. They usually have many radial segment lines, but the markings do not match the musculature pattern of modern jellyfish. There is also a hypothesis that these organisms could have been microbial colonies. They could measure from a few millimeters to a meter in diameter. Its reproduction was asexual, by division or budding.

Medusoids are often erroneously reported, when it comes to inorganic processes that leave concentric circular marks or when it comes to the anchor bulbs of Petalonamae individuals.

History

The first fossils discovered were of Aspidella in 1868, a disk-shaped organism. They were discovered by Scottish geometer Alexander Murray, who found them under Cambrian strata and postulated that they were the first signs of life. The proposal was rejected by his peers because of its simple shape, as they were considered to be gas escape structures or inorganic concretions. Not until 1959, when Martin Glaessner reexamined Murray’s findings, was the age of the fossil Medusoids recognized and, thus, other new discoveries made.

Classification

The round shape is reminiscent of jellyfish, which is why they were initially classified with them in Cnidaria or Radiata, particularly in the Scyphozoa or Hydrozoa classes; it was even held that of the 7 groups of cnidarians represented in the Ediacara fauna , 4 appear to be ancestral to living taxa.

However, these fossils do not clearly present typical jellyfish characteristics such as well-defined tentacles, muscle markings, gonads, or the presence of cnidoblasts; on the contrary, well-preserved fossils indicate that the oldest true jellyfish are actually 505 million year old organisms from the Middle Cambrian and measured less than one centimeter.

Today they are considered an evolutionary enigma, they have not been assigned to any clades, and could be related to the hypothetical Vendobionta group, given their age and simplicity.

Genera

The following Ediacaran genera may be morphologically medusoid with round, radiated, or central concentric structure. Some specimens have other forms and have, thus, been classified as cnidarians or coelenterates by some authorities.

• †Arkarua
• †Aspidella
• †Brachina
• †Corumbella
• †Cyclomedusa
• †Eoporpita
• †Ediacaria
• †Eoandromeda
• †Haootia (polipoid)
• †Hiemalora (anemonoid)
• †Inaria (?)
• †Kimberella (molluscoid)
• †Mawsonites
• †Medusina
• †Nimbia
• †Pomoria
• †Solza
• †Studenicia

Then, during the Cambrian, the following group of medusoids is proposed:

• Class †Psammocorallia
  • †Asterosoma
  • †Beltanelliformis
  • †Brooksella
  • †Nemiana
  • †Protolyellia
  • †Spatangopsis

References

Wikidata ☰ Q3028869 entry