14 | Great Geologists
Nicolas Steno
One day in October 1666, two fishermen
from the town of Livorno landed a huge
shark they had caught off the coast of
Tuscany. News of this reached Grand
Duke Ferdinand II in Florence who ordered
the head to be brought to the city to be
dissected by a Danish anatomist attached
to his court. The anatomist was Nicolaus
Steno (also known by his native name
Niels Stensen). The results of his study
were to provide the sparks that ignited the
sciences of paleontology and geology as
we know them now.
Steno was born in 1638, the son of a
Copenhagen goldsmith. At the age of 19,
he entered the University of Copenhagen
to study medicine. He then travelled to
Holland and Paris where he honed his
skills in anatomy and precise dissection.
During the mid-17th century, the
understanding of how muscles, the
heart and brain function was based on
deductive reasoning rather than direct
observation. By dissecting muscles in
detail, he determined they were bundles
of contractile fibres, not balloons that
were inflated by an ‘animating spirit’.
The heart was not made of any special
substance nor was it an internal cauldron
of boiling blood but a muscle. As for
the brain, René Descartes, the French
mathematician and philosopher, had
concluded the pineal gland sitting at its
centre was the location of the soul and
manipulated the body like a puppet. In
a public lecture in 1665 in Paris, Steno
demonstrated that this could not be the
case, as the gland could not move and
gyrate as Descartes had supposed.
within rocks within the Earth, but Steno
determined that the shark’s teeth and the
glossopetrae were very much the same
— in other words glossopetrae were
fossilized shark’s teeth. This was reported
in a paper published in 1667 (Canis
carchariae dissectum caput – ‘A Shark’s
Head Dissected’) and spurred his interest
in geology. With the support of Grand
Duke Ferdinand, he proceeded to travel
around Tuscany for the next few months
to study its fossils and geology.
Portrait of Nicolas Steno (c. 1666–1667).
Unsigned but attributed to Florence court
painter Justus Sustermans.
These discoveries attracted the attention
of Grand Duke Ferdinand de’ Medici
in Florence. Ferdinand had a strong
interest in science, supporting Galileo and
sponsoring an academy of his disciples.
Steno was thus invited to his court as in-
house physician, but also to educate and
entertain.
So it came about that the shark caught
by the Tuscan fisherman came to Steno
in Florence. The dissection was routine,
but Steno was struck by the similarity
of the shark’s teeth to certain medicinal
stones called glossopetrae or tongue
stones. These were thought to grow
The result was his masterpiece published
in 1669 — De solido intra solidium
naturaliter contento dissertationis
prodomus (‘Preliminary discourse to a
dissertation on solids naturally contained
within a solid’). This set out some basic
principles upon which the science of
geology could be built. He recognized
that rocks containing fossils had originally
been soft sediments hardened into
rock after burial. Therefore, fossils were
organic in origin, rather than being odd
facsimiles of living creatures and their
constituent body parts that happened
to be embedded in rock. This matched
the observations of the English scientist
Robert Hooke who had utilized the newly
invented microscope to note that the
microstructure of fossil wood was similar
to charcoal (Micrographia, 1665).
The position of these fossils on land,
often far from the sea and high above it,
was less easy to explain. Steno favoured
their deposition in sediments relating
to the biblical flood, left behind when
waters receded. In Steno’s time, Earth
history and human history were viewed