On display are fossils found within just a few miles of the Sierra College campus in Rocklin. The skull on top is from a Pleistocene bison that existed here in the Sacramento Valley 20,000 years ago. Also on display are the hoof, leg, pelvis and jaw bones of an extinct horse. A scapula, tooth and tibia are evidence for the extinct giant ground sloth.
Older fossils include the shoulder of a marine turtle, a shark tooth and sea urchin spines. The underwater history of the area is also shown from the nautilus shells, ammonites, baculites, clams and snails. Other bison horns as well as mammoth tusk and a molar attest to the diversity of fauna in the area. The flora is represented by fossil leaves and a pine cone from the Ione Formation of 50 million years ago. Pine needles and live oak leaves come from the Mehrten Fm of 10 mya.
This log of petrified wood, which has become opalized, was found within a few miles of campus. It is of Eocene age, 37-57 million years ago, and possibly was a semi-tropical hardwood tree trunk which had washed down a river. There were many large logs buried in these ancient river gravels (which are now under housing developments).
Petrified wood is the result of a process called petrifaction (or petrification), meaning “to change into stone.” The process involves mineral emplacement, in which dissolved minerals are carried by groundwater into the porous parts of buried wood (or shells or bones) where they crystallize out and settle, filling the pores. An object that has been impregnated with minerals in this way is denser, heavier, and more resistant to destruction than it was originally. The term petrifaction is also used to designate the process in which minerals completely replace the original material, which has been slowly dissolving away. A distinction is sometimes made between these two processes, the first being referred to as permineralization, the latter being called mineralization.
During the Triassic Period, from about 225 million to 190 million years ago, the two dominant flora were cycadophytes and conifers. The cycadophytes were relatively small cycad-like trees; some of the conifers grew to about 220 ft (60 m) in height and 10 ft (3 m) in diameter. The conifers often grew in large forests, in marshes, and in other very humid habitats. Many of these wetlands had previously been shallow seas, and at the end of the Triassic the shallow seas returned and submerged the forests. The drifting logs or upright stumps of the forest remains were eventually buried in volcanic ash or other sediments, and the process of permineralization began.
The waters penetrating the sediments were rich in mineral salts, which reacted with the plant matter inside the cell walls of the trees. The most common of these minerals were silica, occurring as quartz, of which chalcedony and jasper were part; calcium carbonate, occurring as calcite; pyrite and dolomite; and iron and magnesium oxides, which give petrified wood colors of red, yellow, and orange. As the mineral salts penetrated into the cells and started to crystallize, they usually left the cell walls almost completely intact. Thus petrified wood, although rock-like in density and weight, consists of both organic (1% to 15%) and mineral matter. Complete crystallization of the cell cavities was a very rapid and extremely efficient process that left fossils so well preserved, with such excellent clarity and transparency, that they can be studied through a microscope in the same manner as recent biological material.
Eventually, the shallow seas once more withdrew, leaving great depths of sediment behind. Erosion by wind, rain, and other agents began and eventually exposed the petrified trees. Petrified wood can be seen in the Petrified Forest National Park in Arizona. Among other interesting exposures of petrified wood is the one located in Yellowstone National Park, Wyoming. This is not a single forest but a vertical succession of 27 individual forests preserved in more than 2,000 ft (600 m) of volcanic debris. These specific forests are from the Eocene Epoch, about 50 million years ago. Not only have the tree trunks been preserved, but the impressions of leaves, twigs, needles, and cones have also been fossilized in the volcanic ash layers.
A fossilized back bone segment of an elephant, possibly a Gomphothere, was donated to the museum by Christopher Bronny. This thoracic vertebra probably eroded out of the 10 million year old Mehrten formation. It was found in Sacramento County and is now an addition to the museum’s Ancient Elephant display case.