Ecdysozoa is a proposed super-grouping of eight animal phyla, characterized by ecdysis and absence of locomotor cilia in reproductive cells. It includes arthropods, the largest group, and some of the earliest life forms to evolve hard shells and venture onto land. The group’s membership is controversial, but a genetic study in 2008 strongly supported it.
Ecdysozoa is a proposed super-grouping of eight animal phyla (out of approximately 37) – Arthropoda, Onychophora, Tardigrada, Kinorhyncha, Priapulida, Loricifera, Nematoda and Nematomorpha. It’s a controversial grouping, but a large genetic study in Nature in 2008, “Large Phylogenomic Sampling Improves Resolution of Animal Tree of Life,” strongly supported Ecdysozoa as a naturally occurring category. The group was originally proposed as a result of genetic studies using 18S ribosomal RNA genes in 1997.
The main feature of the Ecdysozoa part is the moulting of a three-layered cuticle in a process called ecdysis (from which the group gets its name). Another shared feature is the absence of locomotor cilia in reproductive cells: spermatozoa are amoeboid. Ecdysozoa embryos do not undergo spiral cleavage, while other protostome embryos do. Ecdysozoa are all protostomes, one of the two major groups of animals, the other being the deuterostomes.
Ecdysozoa is a proposed superphylum within Bilateria, an unclassified subregnum (sub-kingdom) within animals in general. Its largest group is the Arthropods, which includes all insects, crustaceans and arachnids, and is estimated to include a total of 5-8 million species, the majority of all animal life. Ecdysozoa are the most numerous and prolific organisms on the planet, and their combined biomass exceeds that of all approximately seven billion humans.
The Ecdysozoa includes what were some of the first life forms to evolve hard shells, such as the trilobite arthropods, whose emergence defines the beginning of the Cambrian Period, 542 million years ago, and the first life forms to venture onto land . The earliest land animal fossil, Pneumodesmus newmani, a 1-cm millipede, is a member of the Ecdysozoa. It is likely that nematodes, which cover the abyssal plains of the oceans and are the most numerous multicellular life form on Earth, were actually the first to land, but they don’t fossilize well.
Ironically, the cuticle of nematodes has a different chemical composition than all other Ecdysozoa, so their group membership is also the most suspect. A microscopic member of Ecdysozoa, tardigrades, are known to be able to withstand extreme temperatures and radiation that would kill a human being hundreds of times over. I’m the only animal that could survive the rigors of outer space for any significant amount of time.
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