What are fossils and why are they important?

Fossils are the remains of ancient life, often buried under soil or water.  Fossils can be bones, shells, plants, insects, or the tracks and impressions left by life.  Their age can range from thousands to billions of years old.  Most fossils are fragments of the original animal.  Finding complete specimens is considered rare.

Not all geologic layers contain fossils. To become fossilized, the carcass must be protected from scavengers and oxygen breathing microbes.  This can be accomplished by falling to the bottom of the sea, being silted with mud (marine or freshwater), covered by volcanic ash, falling into an acidic bog, or dieing in a dry cave.

Sedimentary rocks often contain fossils.  The most common, limestone, is composed of marine animals: forminifera, shells, urchins, corals, arthropods, the occasional fish, and marine reptile.  Inland, shales, siltstones, mudstones, and sandstones contain freshwater and land dwelling animals.

More than 90% of all fossils come from marine invertebrates.  Many help date layers and are called index fossils.  These were animals that were prolific for a time but then changed or died out. Some layers of dry land origin do not contain visible fossils. Scientists have recently begun using pollen and spores caught in the old soils as index fossils.  These microscopic fossils are in both land and sea layers, tying them together.  This way a more accurate timeline can be established.

Fossils Are Not All The Same

Actual bone, shell, and calcified structures --These are the hard parts of animals that were buried and preserved.

Petrification (petrifaction) – As an animal or plant slowly dissolves, minerals in the surrounding matrix grow in the small hollows.  Submersion in water rich in minerals is required.  This process can maintain detail at the cellular level.  This is the most common form of dinosaur and tree fossils one sees. 

Mummification – In very dry climates, soft tissue of animals is can be preserved.  This is very rare.

Coal – Plants buried in swamps do not decay quickly.  The material often becomes peat.  In the Carboniferous Period, great swampy forests covered the shorelines.  The climate cooled and the seas covered the land.  Layers of limestone deposited on top.  Compression, heat, and time transformed the plants' natural carbon compounds, preserving material as coal.  Plant and animal fossils can be quite detailed.

Films – A thin layer of carbon left after a plant or animal has been dissolved.  Delicate features, like leaves and feathers, can be perfectly preserved.

Amber – Resin of broadleaf conifers and legume trees naturally polymerizes (a chemical process) into a non-water-soluble material.  If a plants and animals become encase in the sticky goop, they are frozen in time as mummies.

Trace fossils – Fossils that are not part of the animal itself.

a. Natural impressions – A plant or animal part is pressed into soft mud leaving shallow marks.

b. Natural molds and casts – A mold is stone surrounding a fossil that has impression details.  A cast requires a hollow space that is filled with a secondary soil or mineral.  An external mold is where the fossil has dissolved and has been completely replaced.  An internal mold, usually a shell,  has the inside of the fossil filled, then the outside is dissolved or broken away.

c. Tracks – Footprints and tail drags from bugs to Sauropods are preserved in soft mud flats.

d. Burrows – Tunnels made by ancient animals that became filled with secondary sediments.  Generally, the fossil of the animal who made the tunnels are not found.

e. Coprolite – Petrified animal feces.  Many contain fossilized remains of what the animal ate.

What is Parallel (or Convergent) Evolution?

We learn about fossils by studying living animals with similar attributes.  The theory states that animals with similar habits will develop similar body shapes.  Large animals like elephants can give us clues on how the huge suropods moved.  Mosasaurs and whales are both exclusively marine animals with flippers and streamline bodies, but Mosasaurs are reptiles (not dinosaurs), and whales are mammals.

A little history of Paleontology

Two hundred years ago scientists made the first attempt to add fossils into the growing list of the animal kingdom; but how they became rock remained a mystery.  They rationalized the fossils using a new set of standards.  Giants were things of mythology; so large leg bones and teeth became enormous elephants and crocodiles.  Shell shaped rocks could be simplified as mineral growths, or be placed on top of mountains by a great flood.  Some of their theories proposed reasonable explanations, others did not.

Arguing ideas and facts, scientists examined the evidence.  Each theory held its credulity or died.  England led the way to modern paleontology with an abundance of new ideas.  William Smith proposed (early 1800’s) that the layers in soil and rock could be identified and classified by their fossils.  In an attempt to group the huge reptile bones, Richard Owen, coined the term Dinosauria (fearfully great lizards) in 1842.  In 1859 Darwin published The Origin of Species that proposes animals with similar features may have a common ancestor even after they are no longer the same species.

The first dinosaur fossil fragments of North America were described by Joseph Leidy of Philadelphia in 1855.  In 1858, he helped excavate the first articulated dinosaur.  (Articulated means, bones of one animal are found together instead of scattered.)  Found in Haddonfield, New Jersey, he named it Hadrosaurus foulkii.  This animal obviously stood on two legs.  This important aspect revolutionized the concept of dinosaurs.

The first articulated dinosaur of Europe was found and described twenty years later giving paleontologists of North America a head start.

Cladogram

A cladogram links animals related to one another.  Scientists measure, compare, and interpret fossils to determine relationship.  This process requires years of meticulous study.  Each new specimen must be evaluated.  Often, a new branch must be added to the “tree” to include new groups.  It is understood that intermediary forms are not and may never be known. 

Sometimes the data changes the placement of an animal.  For example, Tyrannosaurus, which was once a considered a Carnosaur, is now placed in the Coelurosauria.  For convenience sake, and the lack of specimens, paleontologists had placed all the large Theropods under Carnosauria and the small ones under Coelurosauria.  With the discovery and study of more fossils, the concept of these groups has changed.  Generally, Coelurosauria has specimens closer in relationship to birds.

Why do scientific names change?

It is amazing and very confusing how many plants and animals have multiple scientific names.

1. People study the same thing unaware of the others doing the same.  Published papers use different names.  When it is determined that multiple names describe the same thing the one first published becomes its official name and the others are kept as synonyms.

2. Sometimes the things described are the same but because of age, sex, or environment look totally different.

3. As information becomes available, like DNA, we understand relationships better.  Classification changes.