Fossils

WHAT IS A FOSSIL?

Fossils (from the Latin fossus, literally meaning "having been dug up") are the preserved remains or traces of animals, plants, and other organisms from the remote past.  A preserved specimen is called a "fossil" if it is older than some minimum age, most often the arbitrary date of 10,000 years ago.  Hence, fossils range in age from the youngest at the start of the Holocene Epoch to the oldest from the Archaean Eon, up to 3.4 billion years old.

Just like current living organisms, fossils vary in size from microscopic, such as single bacterial cells only one micrometer in diameter, to gigantic, such as dinosaurs and trees many meters long and weighing many tons.  A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the exoskeletons of invertebrates.  Preservation of soft tissues is rare in the fossil record.  Fossils may also consist of the marks left behind by the organism while it was alive, such as the footprint or feces of a reptile.  These types of fossil are called trace fossils, as opposed to body fossils.

Ammonites
Mollusks
Trilobites


Ammonites – Important Fossils Prized For Millennia

Ammonites are an extinct group of marine cephalopods, a class of the phylum Mollusca.  Although ammonites existed with and in appearance looked much like the still-living shelled nautilus, they were more nearly related to the shell-less coleoid cephalopods, including octopuses, squid and cuttlefish.  In fact, paleontologists sometimes casually refer to ammonites as “squid in a shell.”  The coleoids are considered to be among the most intelligent of invertebrate species, which may have applied to the ammonites, as well.

Ammonites are first known from rocks of the Devonian Age, about 400 million years ago (mya), and became extinct at the end of the Cretaceous Period, 65 mya.  Because of their abundance and species variety, the tendency of their hard shells to make durable, identifiable fossils, and the long span of geologic time during which they existed, ammonites are excellent index fossils.  They provide valuable information about the age of the sedimentary rocks in which they are found and the oceanic environment from which the sediments were deposited.

Ammonite shells tended to grow in coiled or planispiral shapes, similar to snails.  The name ammonite was inspired by the coiled shape of their spiral shells, which somewhat resemble coiled ram’s horns.  Because the Egyptian god Ammon was typically depicted wearing ram’s horns, the Roman scientist Pliny the Elder (who died in the A.D. 79 eruption of Mt. Vesuvius, near Pompeii) called these coiled fossils ammonis cornua, Latin for the “horns of Ammon,” from which their present English name is derived.  Near the end of the Cretaceous Period, some ammonite varieties began to grow partially non-spiraled and/or helical shells, and these unusual ammonites are referred to as heteromorphs.  Many ammonite varieties tend to be prized fossils, and heteromorphs are among the most popular.


Mollusks - Coming Soon


Trilobites - Ancient Movers and Shakers

The trilobites were very ancient marine arthropods that first appeared in the Cambrian Period (about 525 million years ago, or mya) and lived until the end of the Permian Period, about 250 mya, nearly the entire Paleozoic Era.  The name “trilobite” refers to the fact that their hard-shelled bodies consisted of three distinctly separate parts, or lobes: the cephalon (head), thorax (central body with legs) and pygidium (tail).  Because of their body armor, trilobites are among the best-preserved of Paleozoic animals, with more than 17,000 species having been identified.  Their closest living relative appears to be the horseshoe crab, which still occupies a very similar marine environment.

The trilobites were likely among the most widespread and mobile of the early shelled invertebrates.  These creatures lived on the sea bottom, and were most likely scavengers, predators and/or filter-feeders, this perhaps varying with the species and size of the trilobite.  Trilobites had bulbous, protruding compound eyes, which are often well-preserved in skeletons of larger trilobites.  Their thorax had between 2 to 61 segments, although the number was typically less than twenty.  Attached to these segments on the underside were pairs of legs, and fossil trilobite tracks are occasionally identified in fine shales and mudstones.  As these arthropods grew, they molted their shells and re-grew larger ones, which explains why so many more trilobite body segments are found as fossils than whole trilobites.  When attacked by a larger predator, trilobites rolled up their bodies to present a round armored ball, just as do modern pill bugs, or “roly-polys.”  The trilobite Phacops is commonly found fossilized in this position.