About Fossils
Oak
Fossils are windows into our geologic past;
we look through them to help understand the evolution of life on Earth.
But it is only under rare circumstances that any organic remains petrify,
so what we actually see are little snapshots of the history of life—not
all that unlike what is displayed in this exhibit. Many of the images displayed
here are “petrifactions”—fossils in which the mineral chert
(a cryptocrystalline form of quartz) has faithfully preserved internal structure
at the cellular level. With this degree of detail, paleontologists can compare
modern organisms with their fossil counterparts, revealing much about the
tempo and pattern of our ancient past in microcosm.
One lesson we learn from the photographs presented
in this exhibition is the progressive evolution of the plant kingdom. The
stromatolite from two billion years ago contains evidence of a microbial
world that covered the Earth's surface long before plants evolved. By Carboniferous
time, vast coal swamps covered much of North America and Europe. The plants
that populated these swamps included a mixture of ferns, giant lycopods,
and conifers whose ancestors persist today. But other groups, like the pteridosperms
(tree ferns) are long extinct. These early seed-bearing plants, or angiosperms,
belonged to the groups that eventually gave rise to the flowering plants
that dominate the contemporary landscape.
In the petrified oak (above right) we see
wood with large water-conducting cells called “vessels”—one
of the evolutionary innovations that characterize the angiosperms. Overall,
plant fossils demonstrate that water is the driving selective force in the
evolution of major plant groups. Plants developed woody tissue for water
transport from soil to leaf, and over time, seeds replaced spores as the
principal means of reproduction and dispersal. The angiosperms have achieved
complete independence from free films of water in their reproduction.
Stromatolite
These fossils also demonstrate an opposite lesson: that organisms can resist evolution. Many here have remained unchanged over millions of years. The woody tissue of the conifers evolved just before the great coal swamps first appeared, yet it is virtually indistinguishable from that of modern Araucaria growing today in the Southern Hemisphere. The Jurassic cones of this plant are close to the modern forms—rather amazing, given that they are over 100 million years old. Gingko is another living fossil ). Growing just outside in the courtyard between Devlin and Lyons Halls, this distinctive tree would have shaded the dinosaurs. Gingko, and other such plants are sensitive to carbon dioxide concentration in the atmosphere today and paleobotanists are using the fossil record of Gingko leaves to study climate changes over the millions of years. Species of cyanobacteria that formed stromatolites similar to the one seen in the exhibition (left) have been extant for over one billion years. These organisms demonstrate remarkable evolutionary stasis and their particular histories tell us much about early environmental evolution on Earth.