後生動物 (metazoa) 是屬於動物類，雖然在埃迪卡拉紀時期的水母 (刺細胞動物 Cnidaria)， 被列在最早期的化石記錄裡，但是海棉 (海棉動物 Porifera) 有可能比水母還要更早進化。 (依據化石的研究記錄) 多數被礦物化的動物門類之骨骼，最早出現在寒武紀時期, 其年代已超過5億年之久。也有另一種聽似有理的說詞是，一些沒有留下任何化石證據的生物門，也有可能在寒武紀時的生物爆發 (Cambrian explosion) 中進化了。凱文・彼得森認為，最早期的後生動物大概出現在5億7300-6億5600萬年前。
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相關連結
1. The Museums of Cape Town have a marvelous introduction to the kingdom of animals (Metazoa) with photographs and descriptions of the various phyla.
2. A page from U Cal Berkeley about the Metazoa fossil record suggests it may be possible that nearly all the phyla of metazoa evolved in the Cambrian aside from the Cnidaria, which evolved earlier, in the Ediacaran (Vendian).
3. Were the Cnidaria present in the Ediacaran Era, or did they evolve later, in the Cambrian. This article from the New York Times discusses fossil and DNA evidence that seem to suggest conflicting dates for Cnidaria (600 MYA or 540 MYA).
4. The Museum Victoria in Australia has a page about the oldest evidence of life, including the earliest animal life.
5. Another possible ancestor of early metazoa would be the bilaterians who lived 580-600 million years ago.
This work by Eric & Chun-Chih Hadley-Ives is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.