BIO 113 — Dinosaurs
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Lab 2
Types of Fossils

See also Rocks Page
See also Lagerstätte Page

Molds and Casts

Molds are formed when an organism leaves an impression in the sediments
crinoid brachiopod mold brachiopod mold
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 (with some mineralization to produce the surface color)

Casts are formed when sediments fill a mold and thus take on the shape of the original organism
crinoid brachiopods Trilobite
Cast of a section of crinoid (sea lily) stem Cast of a brachiopod shell Cast of a trilobite
Ammonite Horn coral bryozoan
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 nautiloid internal mold of a Turritella brachiopod internal mold
Internal mold of a nautiloid mollusk Internal mold of a Turritella snail Internal mold of a brachiopod


Permineralization: minerals form in the internal spaces of porus material such as bone or wood
stegosaurus bones Hadrosaur vertebra dinosaur bone
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 trees Petrified wood Araucaria cone
Petrified logs at Petrified Forest Nat. Pk., AZ Polished slice of petrified wood Araucaria cone, cut in half; unlike casts, permineralization can preserve internal structures

Replacement Mineralization: mineral content of shells and other tissues is replaced by different minerals
mineralized crinoids mineralized brachiopods pyritized ammonite
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shells; note different color and texture from the surrounding matrix The shell of this Pleuroceras ammonite has been replaced by the mineral pyrite
Craspedites ammonite cut ammonite graptolite
In this Craspedites ammonite, the original external shell has dissolved away and the internal divisions have been replaced by another mineral, leaving these wavy suture lines Ammonites, such as this Cleoniceras, have sealed, internal chambers in which minerals form by crystallizing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 The organic material of this graptolite (a colony of tiny marine invertebrates) was replaced by minerals that left a white film on the rock

Mixed Fossilization Methods
ReedopsTrilobite concretion Alethopteris leaflet
The 3-dimensional shape of this trilobite was preserved through the mold and cast process, but its thin cuticle (exoskeleton) underwent replacement mineralization to produce the dark color. Internal structures are not preserved. Concretions result from minerals precipitating around decomposing remains; this one has been split open to reveal an unidentified fossil. The fossils may be preserved as molds and casts or by mineralization This leaflet from a seedfern is preserved within a concretion. It has undergone replacement mineralization, with the soft organic material replaced by a white mineral


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
seedfern graptolites amphibian
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

Unaltered Remains

More recent fossils such as shells and teeth may be relatively little altered and still contain much of their original mineral content. Generally, they are still considered fossils if they are more than 10,000 years old.
snail fossil mammoth hair dire wolf femur
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 This piece of dire wolf femur from the La Brea tar pits. The tar perfectly preserves the bones so that most of its original mineral content is still intact, despite being between 10 - 40 thousand years old

The sections above categorize (body) fossils by the ways they were formed. Below, fossils are categorized by what they represent: dead remains, actions and behaviors, or chemical traces.

Body Fossils

Body fossils are any kind of fossil that preserves evidence of the anatomy of past life. All of the fossils described above are body fossils. For vertebrates such as dinosaurs, the most common type of body fossil are permineralized bones, but can also include molds and casts, such as skin impressions.
fish fossil Thescelosaurus Vertebra Titanosaur eggshell
Articulated skeletons, such as this Diplomystus fish, are preserved with all (or at least most) of the bones in the position they had in life. It requires rapid burial before scavengers or the elements can scatter the parts. Disarticulated skeletons (where the bones have separated from each other and moved) and isolated bones provide less information, but are more common than articulated skeletons. Vertebra of the dinosaur Thescelosaurus. Eggs and egg shells, such as this fragment from a titanosaur, are also considered body fossils

Trace Fossils

Trace fossils (also called ichnofossils) are indirect evidence of the presence of animals that preserves evidence of their behavior. Trace fossils include tracks, burrows, and coprolites (fossil feces)
grallator print Cheilichnus track burrow
Footprint of a theropod (predatory) dinosaur. 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
coprolite coprolite dinosaur coprolite
The coprolite (fossilized feces) of an unknown mammal The coprolite of a fish Coprolite from an unknown dinosaur (Late Jurassic Period; Utah)
gastroliths Leaf w/ insect bites dugong rib with shark bites
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 Poplar leaf, Populus wilmattae (body fossil), with insect damage (arrows; trace fossil) Dugong rib (body fossil) with shark tooth marks (trace fossil)

Chemical Fossils

Chemical fossils preserve the chemical traces of past life without preserving any structure
coal tiger iron amber
Coal is an example of a chemical fossil derived from peat, the partially decomposed bodies of plants The alternating red and dark layers in banded iron formation was produced by periods of presence or absence of microbes such as cyanobacteria that altered chemical reactions in ancient marine sediments Amber is a chemical fossil derived from the resin or sap of certain trees. In addition, amber can also contain body fossils of small plants and animals that became trapped in the amber, such as these insects (insets)

Miscellaneous Types

conodont dendrites Chrysanthemum stone
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 and determining past environments. 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. This flower-like pattern is a mineral deposit called chrysanthemum stone and thus it is a Pseudofossils rather than a true plant fossil.
This page last updated 15 July 2017 by Udo M. Savalli ()
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