Basal metazoans are the earliest animals in the evolutionary tree, including cnidarians, porifera, ctenophores, placozoans, and extinct species. Trichoplax adhaerens is the most basal living metazoan, with a small genome and unclear relation to other animals. Cnidarians were found to split from other metazoans earlier than sponges, challenging assumptions about evolution. Mesozoans are a simplified form of metazoan, and the Ediacaran fauna is a group of extinct, tufted organisms from the past.
The term “basal metazoans” refers to the animals at the base of the metazoan evolutionary tree (multicellular animal). The term is not very well defined and may refer to cnidarians (jellyfish and relatives), porifera (sponges), ctenophores (comb jellies), placozoans (the only animal phyla with only one species, Trichoplax adhaerens), and extinct species that can being more primitive than the common ancestor of all living metzoans (also called stem metazoans, mostly Ediacaran fauna).
The most basal of all living metazoans may be Trichoplax adhaerens, a very small (0.5mm) balloon-like animal named for its propensity to attach itself to the sides of a glass aquarium. Trichoplax has the smallest genome of any known animal, with only 50 megabases of DNA and 6 chromosomes. Sequencing of the Trichoplax genome is currently underway. It is suspected that it may be related to cnidarians and ctenophores. However, some recent molecular studies have suggested that Trichoplax may have separated from the rest of the animals after sponges and cnidarians.
Although sponges had long been suspected to be the most basal of all major metazoan groups, a 2007 genetic study determined that, indeed, cnidarians split from other metazoans earlier than sponges. This is quite a surprise, as cnidarians are clearly more complex than sponges. The finding shows that morphology (appearance) cannot be used as a yardstick for how early an organism separated from others, as some animals become less complex over time, not more. This contradicts many popular assumptions about evolution.
Another interesting group of basal metazoans are the mesozoans, once thought to be intermediaries between protozoa (single-celled organisms) and mesozoans (multicellular organisms), now considered a simplified form of metazoan or a true basal metazoan. The two main Mesozoic groups are the Rhombozoa and the Orthonectida, simple organisms of which little is known. They are tiny marine invertebrate parasites and some have only a few dozen cells. To learn more about these animals, their genomes will need to be sequenced.
One group of extinct metazoans, perhaps the most basal of all metazoans, maybe not, is the Ediacaran fauna, a group of tufted, sack-like organisms that lived very long in the past, during the Ediacaran Period, between about 600 and 542 millions of years ago. These animals have been classified by some workers into their own group, Vendobiota, on the basis of their similar “padded” appearance, although there is great controversy on this. The Ediacaran fauna has been described as a “failed early experiment on animals”. Since this group has long been extinct, we will never get our hands on its genetic material and will limit ourselves to making assumptions based solely on crude morphology.
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