He taught in Fano until 1798, when the French invasion of the Papal States compelled him to return to Bologna, where he was appointed keeper of the botanical garden.

The French naturalist Georges Cuvier met him in Bologna in 1810 and was so impressed by his knowledge that he invited him to Paris where he spent fourteen months.

Though he had already prepared great part of the materials necessary for its completion, and although the many articles contributed by him to various journals of natural history amply demonstrate the extent and accuracy of his knowledge, the work has unfortunately been left incomplete.

He had the honour of introducing to his countrymen the discoveries of Buckland, Lyell, De la Beche, and the other members of the English school.

His ability in this branch of science had been recognized even at an early period by Cuvier, who freely confessed his obligations to Ranzani for some important information of which he availed himself in his great work, Le Règne Animal.