Do fossil fuels really come from dinosaurs?

No. Fossil fuels are genuinely fossils, but they are not the remains of Tyrannosaurus or Triceratops. They are the transformed remains of billions of tonnes of algae, plankton, spore-bearing ferns and woody plants. Coal comes mainly from the giant tree ferns and lycopods of Carboniferous swamps; oil and gas come mainly from phytoplankton and zooplankton in ancient seas and lakes.

Coal's origin lies in the Carboniferous, roughly 300 to 360 million years ago. Atmospheric oxygen then stood at about 35%, far above today's 21%, and that oxygen-rich air let giant tree ferns and lycopods blot out the sky — and let insects grow to alarming sizes, with the dragonfly Meganeura reaching a 75 cm wingspan. But the decisive factor was not the abundance of trees. It was an evolutionary gap lasting tens of millions of years: trees had just evolved lignin, the tough polymer that lets wood stand upright, but the fungi and bacteria capable of breaking lignin down did not yet exist. Fallen giants sank into stagnant acidic swamps, neither rotting nor disappearing, and accumulated layer upon layer into thick carbon-rich peat.

That peat was later buried beneath miles of sediment. The weight of the overlying rock squeezed out water and volatiles such as oxygen and nitrogen, concentrating the carbon-carbon bonds in a process called coalification. The deeper the burial and the higher the temperature, the harder the coal, progressing through the ranks: peat, lignite, sub-bituminous and bituminous coal, and finally anthracite. Anthracite is the diamond of coals, roughly 95% pure carbon — the final memory of a forest that refused to rot three hundred million years ago.

Oil and natural gas came from a different kitchen: the oceans and ancient lakes. Picture a warm shallow sea 150 million years ago, crowded with phytoplankton and zooplankton. When they died they settled onto an anoxic seabed, were not decomposed, and turned into a waxy solid called kerogen, sealed inside shale and limestone. As the source rock was buried deeper it entered the oil window, a narrow band of roughly 65°C to 150°C, where catagenesis cracked kerogen into the shorter liquid hydrocarbon chains we call crude oil. Above 150°C — the gas window — those molecules crack further into the simplest hydrocarbon of all, methane. That is why oil and gas so often occur together: they are the same geological kitchen cooking at different temperatures.

One detail is worth pausing on. Coal exists because of a long era in which nobody came to clean up. Had lignin-eating decomposers evolved a few tens of millions of years earlier, Carboniferous organic matter would simply have cycled, the carbon would have returned to the atmosphere, and no coal would have accumulated. The vast energy inheritance on which industrial civilisation was built traces back to an accidental evolutionary gap. The image of liquefied dinosaurs, reinforced by corporate logos and children's television, is neat, vivid and almost entirely wrong.

The dinosaurs were merely neighbours of the plants and microorganisms that actually became fuel.

This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第二章 2.1