Old Earth Ministries Online Earth History Curriculum
Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)
This curriculum is presented free of charge for use by homeschooling families.
NOTE: If you found this page through a search engine, please visit the intro page first.
Chapter 1 - Earth History Overview
Lesson 6: Fossils
Latin fossus, literally "having been dug up") are the preserved remains or
traces of animals, plants, and other organisms from the remote past. The
totality of fossils, both discovered and undiscovered, and their placement
in fossiliferous (fossil-containing) rock formations and sedimentary layers
(strata) is known as the fossil record. The study of fossils across
geological time, how they were formed, and the
evolutionary relationships between
are some of the most important functions of the science of
Chapter 1 - Overview
observations that certain fossils were associated with certain rock
led early geologists to recognize a geological timescale in the 19th
century. The development of
techniques in the early 20th century allowed geologists to determine the
numerical or "absolute" age of the various strata and thereby the included
Smith (1769-1839), an English canal engineer, observed that rocks of
different ages (based on the
law of superposition)
preserved different assemblages of fossils, and that these assemblages
succeeded one another in a regular and determinable order. He observed that
rocks from distant locations could be correlated based on the fossils they
contained. He termed this the principle of faunal succession.
Early naturalists well understood the similarities and differences of living species, leading Linnaeus to develop a hierarchical classification system still in use today. It was Darwin and his contemporaries who first linked the hierarchical structure of the great tree of life in living organisms with the then very sparse fossil record. Darwin eloquently described a process of descent with modification, or evolution, whereby organisms either adapt to natural and changing environmental pressures, or they perish.
When Charles Darwin wrote On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, the oldest animal fossils were those from the Cambrian Period, now known to be about 540 million years old. The absence of older fossils worried Darwin about the implications for the validity of his theories, but he expressed hope that such fossils would be found, noting that: "only a small portion of the world is known with accuracy." Darwin also pondered the sudden appearance of many groups (i.e. phyla) in the oldest known Cambrian fossiliferous strata.
Darwin's time, the fossil record has been pushed back to between 2.3 and 3.5
billion years before the present. Most of these Precambrian fossils are
microscopic bacteria or
macroscopic fossils are now known from the late
Ediacaran biota (also called Vendian biota)
dating from 575 million years ago collectively constitutes a richly diverse
assembly of early multicellular
Rarity of fossils
Fossilization is an exceptionally rare occurrence, because most components
of formerly-living things tend to decompose relatively quickly following
death. In order for an organism to be fossilized, the remains normally need
to be covered by sediment as soon as possible. However there are exceptions
to this, such as if an organism becomes frozen,
desiccated, or comes to rest in an
environment. There are several different types of fossils and fossilization
Types of Preservation
occurs after burial, as the empty spaces within an organism (spaces
In some cases the original remains of the organism have been completely dissolved or otherwise destroyed. When all that is left is an organism-shaped hole in the rock, it is called an external mold. If this hole is later filled with other minerals, it is a cast. An internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail.
Replacement and recrystallization
Replacement occurs when the shell, bone or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.
Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state. Often what remains is a carbonaceous film.
Bioimmuration is a type of preservation in which a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton. Usually it is a sessile skeletal organism, such as a bryozoan or an oyster, which grows along a substrate, covering other sessile encrusters. Sometimes the bioimmured organism is soft-bodied and is then preserved in negative relief as a kind of external mold. There are also cases where an organism settles on top of a living skeletal organism which grows upwards, preserving the settler in its skeleton. Bioimmuration is known in the fossil record from the Ordovician to the Recent.
Microfossil is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cut-off point between "micro" and "macro" fossils is 1 mm, although this is only an approximate guide. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters foraminifera and coccolithophores) or component parts (such as small teeth or spores) of larger animals or plants. Microfossils are of critical importance as a reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.
Fossil resin (colloquially called amber) is a natural polymer found in many types of strata throughout the world, even the Arctic. The oldest fossil resin dates to the Triassic, though most dates to the Tertiary. The excretion of the resin by certain plants is thought to be an adaptation for protection from insects and to seal wounds caused by damage elements. Fossil resin often contains other fossils called inclusions that were captured by the sticky resin. These include bacteria, fungi, other plants, and animals. Animal inclusions are usually small invertebrates, predominantly arthropods such as insects and spiders, and only extremely rarely a vertebrate such as a small lizard. Preservation of inclusions can be exquisite, including small fragments of DNA.
Living fossil is an informal term used for any living species which is apparently identical or closely resembles a species previously known only from fossils—that is, it is as if the ancient fossil had "come to life."
This can be (a) a species or taxon known only from fossils until living representatives were discovered, such as the lobe-finned coelacanth, primitive monoplacophoran mollusk, and the Chinese maidenhair tree, or (b) a single living species with no close relatives, such as the New Caledonian Kagu, or the Sunbittern, or (c) a small group of closely-related species with no other close relatives, such as the oxygen-producing, primordial stromatolite, inarticulate lampshell Lingula, many-chambered pearly Nautilus, rootless whisk fern, armored horseshoe crab, and dinosaur-like tuatara that are the sole survivors of a once large and widespread group in the fossil record.
Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil. If the species concerned were short-lived (in geological terms, lasting a few hundred thousand years), then it is certain that the sediments in question were deposited within that narrow time period. The shorter the lifespan of a species, the more precisely different sediments can be correlated, and so rapidly evolving types of fossils are particularly valuable. The best index fossils are common, easy-to-identify at species level, and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is minor.