LSC 294 — Dinosaurs

Udo Savalli Home

Lab 2
Types of Fossils

See also Rocks Page

Molds and Casts

Molds are formed when an organism leaves an impression in the sediments
Mold of crinoid (sea lily) stems. Sea lilies are primitive relatives of sea stars that have a long stalk	Mold of a brachiopod shell. Brachiopods are superficially similar to clams and scallops, but unrelated and with a differnt internal anatomy	Mold of a brachiopod shell
Casts are formed when sediments fill a mold and thus take on the shape of the original organism
Cast of a section of crinoid (sea lily) stem	Cast of a brachiopod shell	Cast of a trilobite
Cast of an ammonite shell (extinct relatives of octopus and squid)	Cast of a horn coral (an extinct group of corals)	Cast of a bryozoan (a colony of tiny coral-like animals)
Internal molds form when sediments fill the interior spaces of a shelled animal such as a snail
Internal mold of a marine snail	Internal molds of marine snails	Internal mold of a brachiopod

Mineralization

Permineralization: minerals form in the internal spaces of porus material such as bone or wood
Stegosaurus bones in situ at Dinosaur Nat. Mon., UT	Cut section of a Hadrosaur vertebra revealing internal structure	Dinosaur bone piece, cut and polished
Petrified logs at Petrified Forest Nat. Pk., AZ	Polished piece of petrified wood	The color of permineralized fossils can vary greatly, depending on the local chemistry at the time of formation. This pale colored Oreodont (Miniochoerus gracilis) skull comes from the White River Formation of Nebraska
Replacement Mineralization: mineral content of shells replaced by different minerals
These mineralized crinoid stem sections were exposed by dissolving the matrix in acid	Mineralized brachiopod shells; note different color and texture from the surrounding matrix	The shell of this Pleuroceras ammonite has been replaced by the mineral pyrite
In this Craspedites ammonite, the original external shell has dissolved away and the internal divisions have been replaced by another mineral, leaving these wavy suture lines	Ammonites have sealed, internal chambers in which minerals form by crystallizing out of solution (and thus formed differently than internal molds from sediment), often forming geode-like crystals	Concretions result from minerals precipitating around decomposing remains; this one has be split open to reveal an unidentified fossil

Carbonization

Compression, heat and pressure drive away lighter, more volatile atoms, leaving behind a film of carbon. Commonly observed in plant fossils as well as some animal fossils
Carbon films of seed fern fronds	Graptolites (small worm-like animals that form colonies that look like hack-saw blades; each 'tooth' is an individual animal)	A thin carbon film has produced the body outline of this aquatic amphibian

Trace Fossils

Trace fossils (also called ichnofossils) are indirect evidence of the presence of animals, such as tracks, burrows, and coprolites (fossil feces)
Footprints of a theropod (predatory) dinosaur at Dinosaur State Park, CT. The print is named Grallator but it is unknown exactly which dinosaur made it	These tracks, called Cheilichnus, were made by a small "mammal-like reptile" in Arizona 260 million years ago	This burrow was likely made by a marine worm
Gastroliths ("stomach stones") are rocks that have been swallowed by animals to aid digestion by helping to grind up food (or for buoyancy in aquatic animals); they are recognized by their smooth contours and location in the gut region of dinosaur fossils	The coprolite (fossilized feces) of an unknown mammal	The coprolite of a fish

Miscellaneous Fossil Types

This beetle (inset) was trapped in amber, the fossilized resin of a tree	These conodonts are examples of microfossils, which are just very tiny fossils produced by any of the above methods (such as mineralization). Although easy to overlook, they are important for aging sediments	Coal is an example of a chemical fossil in which there is a chemical remnant of the original organism (in this case, plants) without necessarily preserving the form
More recent fossils such as shells and teeth may be relatively little altered and still contain much of their original mineral content, such as this whelk	Ice can preserve organic material for thousands of years, such as this wooly mammoth hair from Siberia	Pseudofossils are rock patterns or shapes that look like fossils but are produced by chemical or geological processes and do not represent organisms. These superficially plant-like patterns are called dendrites and are caused by the mineral Manganese Oxide seeping into the limestone.

Lagerstätte

Lagerstätte are fossil-bearing sites with extraordinary diversity and completeness or quality of preservation. They provide a detailed look into specific prehistoric environments
A fresh-water fish, Knightia eocena	A dragonfly fossil (at Fossil Butte Nat. Monument)	A primitive bird, Pseudocrypturus cercanaxius (at Fossil Butte Nat. Monument)
These three fossils all come from the Green River Formation in SW Wyoming and adjacent states. They represent extensive lakes in a subtropical climate from the Eocene Epoch, about 50 million years ago
An unidentified shrimp	A brittlestar, Ophiopetra sp.	Archaeopteryx, the oldest known bird (actual fossil photographed at the Wyoming Dinosaur Center)
These three fossils all come from the Solnhofen Limestone in Germany. This site was a shallow salt-water lagoon during the late Jurassic Period, 150 million years ago.
A fern, Pecopteris sp.	Annularia sp., a relative of modern horsetails	A shrimp, possibly Acanthotelson sp.
These three fossils all come from the Mazon Creek Fossil area of Illinois. Fossils occur primarily in ironstone concretions that formed in a tropical swamp 300 million years ago.