I say "spontaneous" because I didn't have to do anything except gather up Nico's latest drawings and ask him what they were pictures of:
First he did a picture of a bat and a picture of a pteranodon, highlighting the difference in wing structure:
Then he drew a family of right whales: mommy, daddy, and baby:
He pointed out that he included the daddy whale's zizi (which we learned about here and here) and explained that the baby whale is a girl whale.
Then he drew a cross-section of a dolphin, showing its skeleton and a fish it has just eaten:
He also drew a liopleurdon with a cross-section showing the skeleton:
...and did the same for the mosasaur:
I'd never heard of either one of those until my kids got hooked on those BBC specials and the corresponding website. It seems crazy, but I swear they have dinosaurs today that we didn't have when I was a kid...
And here's a picture of a squid with a cross-section showing its skeleton:
If you look closely, you can see the squid's spine and ribs, as well as the bones in its tentacles...
Okay, it leaves something to be desired in terms of accuracy, but it's a pretty good guess for a five-year-old!!! He's turning six next week -- Happy Birthday Science-boy!!! :D