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.

a, Diagram showing position of embryo and yolk sac within the mother. b, Artist’s reconstruction of Materpiscis gen. nov. giving birth.
Long JA, Trinajstic K, Young GC, Senden T (2008) Live birth in the Devonian period. Nature 453:650-652.