Bilateria is a subkingdom of the animal kingdom, consisting of all animals except sponges, cnidarians, and ctenophores. They share common characteristics such as three primary tissue layers, bilateral symmetry, and a coelom. The first known bilaterian, Vernanimacula, lived 600 million years ago in China. Bilaterians have a coelom, which allows for sophisticated movement and organ development. Bilaterians dominate ecosystems and consist of millions of species, while non-bilaterians consist of only about 14,000.
Bilateria is a subregenum, or subkingdom of the animal kingdom in general. It consists of all animals except sponges, cnidarians (jellyfish and relatives), and ctenophores (comb jellies). Bilaterals share many common characteristics, including three primary tissue layers (also called germ layers), bilateral symmetry, a coelom (body cavity), and usually sophisticated organs and body morphology. All complex animals are part of the Bilateria, as two germ layers (possessed by non-bilateria) are not sufficient to sculpt sophisticated morphologies.
The first known bilaterian lived about 600 million years ago in China. With a diameter between 0.1 and 0.2 mm (100 and 200 microns), this tiny spherical animal has been named Vernanimacula, which means “little spring animal”. The animal was given this name because it evolved shortly after the Marinonan glaciation, the most severe ice age in the history of the planet, which lasted about 50 million years. Something closely related to Vernanimacula is probably the common ancestor of every member of Bilateria.
One of the major evolutionary innovations of bilaterals is the coelom or body cavity, which allows organs to develop independently of the body wall and protects them from damage from fluid use. The coelom also allows for a hydrostatic skeleton that the muscles can pull against, allowing for more sophisticated locomotion and movement than in animals without a coelom. Without a coelom, animals are very fragile to external pressure, which can easily damage organs. Except for a few animals that have lost their coelom or have a reduced coelom, all members of Bilateria make use of this evolutionary innovation, including humans and all mammals.
Bilateral fossils appear in the Ediacara fossil beds, which represent the first ecosystems of multicellular life. This includes the shield-shaped Parvancorina and the trilobite-like Spriggina, which may be one of the earliest known predators. Whether Spriggina was a true member of Bilateria is disputed because it exhibited glide symmetry — where its symmetry is shifted to one side by a body segment — rather than true symmetry.
Bilateral organisms first came to dominate the environment in the Cambrian, during which time representatives of most modern phyla evolved. Bilaterians at the time were represented by velvet worms, arthropods such as trilobites, annelid worms, and even early jawless fish. As evolution proceeded, the bilaterians continued to differentiate and dominate ecosystems, as they have done since the dawn of the Cambrian 542 million years ago. Today the Bilaterians consist of millions of species, while the non-Bilaterians consist of only about 14,000.
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