The oldest known fossil of a 500 million-year-old jellyfish was found in Utah. Wow...looks just like the modern jellyfish on the right. It's interesting that jellyfish have pretty much always looked like jellyfish, birds have pretty much always looked like birds, etc., etc.. Yet, we're also supposed to *imagine* that birds evolved from dinosaurs. Crazy stuff.
From the article:
The rich detail of the fossils allowed the team to compare the cnidarian (the phylum to which jellyfish, coral and sea anemones belong) fossils to modern jellyfish. The comparison confirmed that the fossils were, in fact, jellyfish and pushed the earliest known occurrence of definitive jellyfish back from 300 million to 505 million years ago.
The fossils also offer insights into the rapid species diversification that occurred during the Cambrian radiation, which began around 540 million years ago and when most animal groups start to show up in the fossil record, Lieberman said.
The complexity of these early jellyfish seems to suggest that either the complexity of modern jellyfish developed rapidly about 500 million years ago, or that jellyfish are even older and developed long before that time.
Over at Young Cosmos you can find recently discovered fossilized imprints of 330-million-year-old salamander-like amphibians. Kinda cool...