Level: Introductory

A fossil is a record, preserved in rock, of a creature that lived in the past. The record may be the result of an organism’s activities, like a burrow or trackway, or of the organism itself like its shape, or all or part of the organism. Everybody realizes that fossils are not forming today. So, what was different in the past? These organisms were overwhelmed and buried in sediments (gravel, sand, loam or clay) that had been stripped by rushing water from land surfaces and were carried along in these energetic water currents. When the moving water slowed down, the sediments dropped out of suspension, burying creatures on surfaces lying below or which were also being carried along in the rushing torrents. In order to be preserved as a fossil, the trace of a living organism needs rapid permanent burial by sediments that soon turn to rock. So, the essence of fossilization is rapid permanent burial.

Trace Fossil

These are evidences of the past activities of organisms. Such traces could include footprints or trackways, burrows or animal droppings (coprolite). Scientists can figure out quite a lot about the lifestyle of an organism by examining such trace fossils. They can figure out for example how big the creature was, where it lived (or at least where the footprints were made) and whether the animal was moving quickly or slowly. Hints about these issues are possible if there are several footprints making a trackway and how deep in the sediment the prints are and how far apart the footprints are.  And when it comes to animal droppings, scientists grind up the rock to find out what kinds of diet fragments are revealed in the excrement. [This is digested food that an animal eliminates from the body.]

There is one feature about trace fossils that is not immediately obvious. All these traces would normally disappear very quickly as a result of weathering. Anything exposed on a soil surface would soon disappear as the result of wind or rain or moving water. The footprints, burrows, animal droppings etc would have to be buried quickly by other sediments, filling in the cavities or surrounding potentially rotting material. Those infill sediments would soon need to be further buried and turned to rock before they themselves could be negatively affected by weathering. So, a trace fossil needs fast permanent burial.

Mineral Replacement

A common way for fossils to form is when a hard part of an organism (that does not decay quickly) gets buried and turned into rock. This happens especially to sea shells, bones and wood like tree trunks. Minerals from the sediment are carried by water into the hard remains of the creature. Eventually all the original components of the bone, or shell or wood are gone and only rock remains.  Typically, the minerals in the object look quite different from the surrounding rock and they may actually preserve some of the internal structure of the object. Thus, we have petrified (turned to rock) dinosaur bones, petrified wood and petrified sea shells. This process may take a few years, but not thousands of years. Scientists can carry out experiments to see how long it takes to turn an object into rock. Some people even make money selling petrified modern objects like teddy bears! See footnote.

Molds and Casts

Some soft creatures were buried very suddenly so that their shape was preserved by the surrounding sediment. The sediments harden into rock as the interior creature decays away. Sometimes the shape is all that is preserved. For example, something like a jellyfish may have been trapped by sediments. The jellyfish decays, leaving an impression in the rock of its outside contours. Eventually more minerals may fill in the cavity so that we have the original shape of the creature (mold) and the preserved interior cavity (cast). Nothing is left of the original animal. Any creature as soft as a jellyfish, obviously has to be extremely quickly buried if anything is to be preserved.

Compression

Sometimes the hardened skin of an organism like an insect or shrimp may not decay immediately but it is rapidly buried and flattened. Suppose we find a fossil of such an organism. Obviously, we do not have insect carcasses or shrimp carcasses piling up anywhere today. They  are quickly eaten or decay. Again, what is needed to form such a fossil is rapid permanent burial.

Skeletons with bones still connected

One of the most interesting kinds of fossils are articulated skeletons. That means that the bones of a creature were still attached to each other as they were in life when rapid burial occurred. In life, bones are connected to each other by tendons. These parts decay very quickly after a creature dies and the bones then fall apart and are scattered. Soon the bones disappear too. That means an articulated skeleton was permanently buried very quickly before decay could take place. The smaller the bone, and the smaller the creature, the more amazing is the situation in which we find an articulated skeleton.

 In the Royal Tyrrell Museum, for example, in the Theropod Pavilion, a skeleton of Gorgosaurus libratus (adinosaur) is displayed. Almost the entire skeleton is present, right down to the last bone, only one half centimeter long, at the tip of the tail. That means that this carcass had to be very rapidly and permanently buried before even very early stages of decay could set in! Permanently buried means that it was buried deeply enough so that nothing could later remove those sediments and expose the body. Gorgosaurus was much like Albertosaurus. With a creature that large, a lot of sediment must have been involved to permanently bury it!Articulated skeletons therefore mean that the creature was very rapidly and permanently buried!

Bone Beds

Lastly sometimes we find collections of individual bones all smashed and collected together, buried in sediments deposited from rushing water currents. These bone beds may reveal a collection of different kinds of animal, or they may represent a whole herd of creatures including specimens of all ages. In cases where a whole herd of one species is involved, it is evident that a sudden catastrophe overtook these animals. In Dinosaur Provincial Park, for example, the bones of thousands of Centrosaurus dinosaurs were deposited in a bone bed that extends for about 8 km. Some bone beds like the one at Pipestone Creek near Grande Prairie (Pachyrhinosaurus) are so concentrated that there are about 150 bones per square meter. These animals were huge and heavy, about 7 m long (like a bus) and 3 m (10 ft) tall. This is taller than the ceiling in a normal room. Think about the powerful water currents that would be required to tip these huge animals over, drown them and smash the bones together.

Fossil Summary

To preserve an object in rock, we need sudden permanent burial in sediments that are usually carried along by powerful water currents. Volcanoes and landslides can suddenly and permanently bury some creatures, but these disasters are not anywhere near so widespread in their effects as rushing water currents. Catastrophic flooding on a scale far greater than we ever see today, was involved.

Footnote: There is an article on fast fossilization (24 hours) on the Answers in Genesis website (https://answersingenesis.org/fossils/how-are-fossils-formed/fossils-in-just-24-hours/).  This is in reference to a study carried out by supporters of evolution who nevertheless were able to produce convincing looking fossils in 24 hours. This article is at https://newatlas.com/lab-made-fossils/55619/.Book reference: Gary and Mary Parker. The Fossil Book. Master Books. 77 pp.