 All the sea mud still contains shells, and the shells are petrified together with the mud’’-         Leicester Code, folio 10vOther Renaissance naturalists studied invertebrate ichnofossils during the Renaissance, but none of them reached such accurate conclusions.Leonardo's considerations of invertebrate ichnofossils are extraordinarily modern not only when compared to those of his contemporaries, but also to interpretations in later times.In fact, during the 1800s invertebrate ichnofossils were explained as fucoids, or seaweed, and their true nature was widely understood only by the early 1900s.