Every fossil tells a story

When you hear the word fossil, what image comes to mind? The brown bones of a reconstructed Tyrannosaurus
rex standing tall in a museum? A fern’s impression on the face of a sandstone slab? A mammal’s footprint, left in mud, preserved for eternity in rock? You would be correct. All of these are fossils. Simply put, a fossil is any physical evidence of prehistoric life. 

One feature shared by all fossils, no matter type or origin, is age. They are all old, very old indeed, with none created earlier than 10,000 years ago. Only a tiny fraction of living things ever get the chance to become fossils. When life ends, the process of breaking down begins. Insects or other animals; weather; and, of course, the appetites of bacteria, all play a part in making sure nothing is left of most creatures, even us, when it is time to die. A miniscule percentage of all things that have ever lived become fossils. So, if you find one, that’s pretty remarkable!

That being said, we are lucky for the process of fossilization. Studying the remains we have discovered has taught us everything we know about life in ancient times. There are three basic types of fossils, as illustrated below.  

TRACE

These fossils distinguish themselves from other types in that they are neither the preserved remains of the organism, like an altered fossil, nor do they capture its two-dimensional appearance, like an impression fossil. Some examples of trace fossils are tracks, burrows and marks left by feeding. Even plants can leave trace fossils, like the spaces indicating where roots had once been. 

Because they are soft-bodied, the only evidence left by worms and similar creatures are trace fossils. Similar fossils, like footprints of species already known from preserved bones, can provide information about how those creatures stood, walked and ran. The study of trace fossils is ichnology. The Eusarcus scorpionis species seen here on the top of this p from its underside, clearly illustrates why Trilobites are referred to as “sea scorpions”. This trace fossil, Arthrophycus alleghaniensis, records the behavior of an animal, likely a trilobite, as it burrowed through the sediment. The mudrock that surrounded the burrows has weathered away, leaving the siltstone filled burrows in three-dimensional relief.

ALTERED

Perhaps the most familiar form of fossil is, for example, that of a dinosaur’s leg bone. This fossil is not actually a dino’s leg bone, but a record of that bone in which the bone itself has been replace by a different substance. 

The pores of the organic tissues become filled with minerals or the organic matter is replaced with minerals, in that way fossils are formed in the original shape of the tissue or organism, like our leg bone example. The part of the organizsm fossilized like this has literally turned to stone! 

The “golden” fossil pictured here formed when the mineral pyrite, also known as Fool’s Gold, replaced the entire body of this Triarthruseatoni trilobite - even the delicate legs and antennae are preserved!

IMPRESSION

When an animal dies and its body decays, it can leave an imprint. In the rare instance that this outline fills in with minerals from groundwater and surrounding sediment, it can harden to form an impression fossil.

This type of fossil can take many forms. A flat leaf or a fossil fern does not contain any organic matter, but, like a photograph, its impression fossil can give us information about its appearance and structure while it was alive. The shell of this bivalve (clam), above, dissolved away leaving an impression of the inside and outside of the shell.  Bonus points if you can find the trilo-butt!

This blog post is derived from a descriptive panel mounted in the Grand Lobby of Armory Studios NY. It was produced by the staff of Armory Studios NY with guidance from New York State Paleontologist Dr Lisa Amati of the Paleontology Department of The New York State Museum, Albany, NY.