Today's Sunday Stamps theme is invertebrates. Go here for links to more stamps.
Some time ago, Smash sent me this great envelope with a vintage stamp titled Synthetic Fuels, from a set of 4 designed by Charley Harper.
I have not received the others, however one of them is titled Fossil Fuels, and features the extinct trilobite.
Trilobites were marine arthropods, and had well-developed exoskeletons, which are readily fossilized. Over 17,000 species have been identified, and they were diverse in their biological niches.