SCIENCE FOCUS
Most oil reserves were formed between 65 and 252 million years ago. While this does overlap with the ‘dinosaur times’, oil is a marine sediment made of the remains of algae and plankton.
Skeletons of prehistoric reptiles such as plesiosaurs and ichthyosaurs (neither of which count as dinosaurs) have been found in the same geological layers as oil and they may have contaminated the oil deposit. But to say that oil is made from dinosaurs is like saying that bread is made from insects, just because the odd one occasionally falls into a flour mill.
Oil sediments formed in shallow seas that were teeming with life near the surface but stagnant and dead on the seabed. As dead plankton and microorganisms rained down, they buried the ones below them faster than they could decay. This trapped the organic matter in an oxygen-deprived layer that sank lower and lower as it was compressed from above. After 100 million years of this, the bottom layers were under several kilometres of clay and sand, and the heat and pressure at these depths converted the organic material into oil.
In contrast, a five-tonne plesiosaur falling dead to the seafloor would be very unlikely to remain undisturbed for long enough to be safely buried. Instead, it would be a temporary oasis for fish, crustaceans and worms that would quickly strip away the organic parts. We see this happening today when whale corpses fall to the seabed.
