The last radiodonts disappeared in the Devonian, 400 million years ago. Yet some of them (found in sedimentary rocks in China and British Columbia) were fossilized so perfectly that their anatomy can be studied in exquisite detail. Hence this gorgeous diagram. Fabulously weird critters, the radiodonts. They came about as part of what is called the Cambrian Explosion of life forms, beginning 538.8 million years ago (I love the “.8”, they can’t narrow it down to a specific day of the week?) and lasted for I think 13 to as many as 25 million years in a time during which evolution went nuts in a world never really evolved in so dramatically before. Thirteen to twenty five million years actually sounds like an enormous slab of time, but we’re talking deep time here, where thirteen to twenty five million years is just a smallish slab of Earth’s geologic history. Just an hour’s worth of geologic time. Our ancestors began then, too, pathetic little things with a proto notocord, which evolved into spinal columns which is what makes vertebrates vertebrate. Thus you and revolting looking jawless lampreys are actually related, I mean if you retrace your family tree back nearly half a billion years, which I don’t, personally. I stop at Bigfoot.