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Bilateria is a subregenum, or subkingdom of the Animal Kingdom in general. It consists of all animals except for sponges, cnidarians (jellyfish and relatives), and ctenophore (comb jellies). Bilaterians 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 Bilateria, as two germ layers (possessed by non-bilaterians) is not enough to sculpt sophisticated morphologies.
The first known bilaterian lived about 600 million years ago in China. Just between 0.1 and 0.2 mm (100 and 200 microns) in diameter, this tiny spherical animal was 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 key evolutionary innovations of bilaterians is the coelom or body cavity, which allows the organs to develop independently of the body wall and shields them from damage using fluid. The coelom also allows for a hydrostatic skeleton for muscles to pull against, enabling more sophisticated locomotion and movement than animals without a coelom. Without a coelom, animals are very fragile to external pressure, which can easily damage organs. Except for a few animals which have lost their coelom or have a reduced coelom, all members of Bilateria make use of this evolutionary innovation, including humans and all mammals.
Bilaterian fossils appear in the Ediacaran fossil beds, which represent the first ecosystems of multicellular life. This includes the shield-shaped Parvancorina and the trilobite-like Spriggina, which could be one of the first known predators. It is controversial as to whether Spriggina was a true member of Bilateria because it displayed glide symmetry -- where its symmetry is diplaced on one side by one body segment -- rather than true symmetry.
Bilaterian organisms first came to dominate the environment in the Cambrian, during which 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, bilaterians continued to differentiate and dominate the ecosystems, as they have done since the dawn of the Cambrian, 542 million years ago. Today, bilaterians consist of millions of species, while non-bilaterians only consist of approximately 14,000.
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