Da Vinci realised people were wrong about the origin of Italy’s fossils (Image: Ted Spiegel/Corbis)

It was to be Leonardo da Vinci’s most impressive work yet. In 1483, the Duke of Milan, Ludovico Sforza, commissioned the up-and-coming artist to create a huge bronze statue of a horse, standing over 7 metres tall. Da Vinci spent the next 10 years perfecting a full-size clay model. Sadly, it was never cast in bronze. Tonnes of the metal were needed, and Sforza ended up using the earmarked supplies to make weapons for use against invading French troops. When the French army took Milan in 1499, its archers used da Vinci’s clay horse for target practice.

Those years in Milan were nevertheless important for da Vinci, and not only for the many masterpieces he painted in that time. The polymath was also working on a very different project inspired by an intriguing feature of the surrounding countryside: embedded in the rocks there appeared to be a multitude of small stone sea creatures. “The hills around Parma and Piacenza show abundant molluscs and bored corals still attached to the rocks,” da Vinci wrote a few years later. “When I was working on the great horse in Milan, certain peasants brought me a huge bagful of them.”

Could da Vinci picture the creature behind this network of burrows?

Da Vinci recorded his observations of these and other fossils in a secret notebook now known as the Codex Leicester. His findings have long been known to be ahead of their time, but a new analysis suggests that the work was even more advanced than previously thought, …