NIELS STENSEN: It was not until the middle of the 18th century that the concept of "fossil" entered the conceptual framework of science. The work on fossils of the Danish natural historian Niels Stensen (1638-1687), also known as Steno, near the end of the 17th century, and the posthumous publication in 1705 of physicist/biologist Robert Hooke's (1635-1703) convincing demonstration of the organic nature of fossils, were in part the deciding blows in the long evolution toward the modern meaning of fossils.

At the left is an illustration from Stensen's 1665 paper,The Head of a Shark Dissected, which shows the head of a modern shark with its many parallel rows of triangular teeth, and at the near left, two triangular tooth-like "tongue-stones", which were common constituents of certain rock layers in northern Italy. He carefully documented the similarities between the teeth of the shark and the tongue-stones, and showed that the teeth of modern sharks resembled lithified "tongue-stones" so closely that the only rational view was that tongue-stones were the teeth of large ancient sharks that lived when the sediments in which the tongue-stones are found were accumulating at the bottom of the sea - that tongue-stones were fossils in the modern sense of the word. In making this argument, Stensen provided a reasonable explanation of how the teeth of ancient sharks could be incorporated into rocks, and thus resolved the issue of how organic objects come to occur in rocks of the earth's crust. This was a big step forward.