It’s not often that something as delicate as details of the reproductive tract get preserved, but here’s a phenomenal fossil of a Devonian placoderm containing the fragile bones of an embryo inside, along with the tracery of an umbilical cord and yolk sac.
This is cool: it says that true viviparity, something more than just retention of an egg internally, but also the formation of specialized maternal/embryonic structures, is at least 380 million years old. Hooray for motherhood!
Here’s a reconstruction of what the animal would have looked like in life, as it is giving birth to its young.
Long JA, Trinajstic K, Young GC, Senden T (2008) Live birth in the Devonian period. Nature 453:650-652.