Paleontologists have long been intrigued by the process of fossilization. After all, it’s sort of a miracle for any animal to be preserved for tens of millions of years. Only one bone in a billion ends up in the fossil record. This means a dinosaur has a better chance of winning the Mega Millions jackpot than being fossilized. Let’s travel back in time and discover how these prehistoric beasts become fossils.
What is a fossil?
When people think about fossils, they usually think about dinosaur bones or teeth. But fossils are more than just a bunch of bones. A fossil refers to anything that proves an ancient organism was once alive. So this includes their eggs, nests, trails, feeding marks, poop, and other traces.
But here’s the catch: the word ancient here does not refer to any old organism. The preserved remains or traces should be more than 10,000 years old. Because dinosaurs lived around 230 million years ago, any evidence of their existence is considered fossils.
We can also say that the world’s most recent fossils are the giant sloths, mastodons, mammoths, and saber-toothed tigers that went extinct between the Late Pleistocene and the early Holocene. All man-made objects are artifacts and not fossils.
Two Main Types of Dinosaur Fossils
Any preserved body part of a dinosaur is a body fossil. This includes bones, teeth, horns, claws, bone plates (osteoderms), and skin impressions.
Trace fossils are a record of dinosaurs’ lifestyles and behavior. They’re helpful to scientists because a dinosaur’s anatomy cannot tell everything about their day-to-day activities, including what went in or out of their bodies.
Bromalites are any former food items associated with the digestive tract of a fossil animal. These trace fossils tell us what dinosaurs ate and how they digested their food. Examples of bromalites include:
- Gastrolite (fossil stomach stone)
- Enterolite (fossil stomach contents)
- Cololite (fossil intestinal contents)
- Regurgitalites (fossil puke)
- Coprolite (fossil poop)
Dinosaur tracks or footprints are other important trace fossils. Even if the dinosaurs that made the tracks cannot be identified, they still reveal vital information about the animal’s life habits and environment.
How do dinosaurs become fossils?
It all starts with death. What is left from an animal after it dies may be fossilized. Scientists learned that for a dinosaur to be preserved, it requires a particular type of environment, but it also has to do with luck.
Most dinosaur remains disappear without being preserved. Other animals eat or carry them away. If it’s lucky, the dinosaur’s body is quickly buried by sediment.
This process can happen in a number of ways. For instance, a river might sweep away a carcass and deposit it in a lake where it sinks to the bottom. Or, a dinosaur could die in a sandstorm and be buried by blown-in sand.
Over time, the sediment around the body hardens into rock. This process can take millions of years. In the meantime, the carcass decomposes, leaving behind a cavity in the shape of the dinosaur.
Eventually, minerals from the groundwater fill up this cavity. As they do, they slowly harden into a fossil. And voila! You have a dinosaur fossil! The rocks containing the fossil are then returned to the surface through the earth’s movements. The buried fossil is exposed through erosion and weathering.
Types of Fossil Preservation
There are many ways a fossil can be preserved, depending on the environment the organism was in when it died. Here are the common ways that dinosaur fossils are formed.
Many biological tissues, such as bone and wood, are full of tiny pores and hollows where chemicals could seep through and crystalize inside the fossil, preserving its original structure. This kind of preservation is common among dinosaur bones and petrified wood.
Another common type of fossilization is recrystallization. In this process, the minerals that make up an original bone or shell change into a more stable mineral made of the same chemical components. The appearance of the fossil remains the same, though finer details are often lost as new crystals form. This usually occurs among shelled organisms.
Some fossils do not contain their original biological structure anymore. Instead, their chemical and organic components are dissolved and replaced by a new chemical structure. One example is some prehistoric marine reptiles in Australia called plesiosaurs, in which the bones have been entirely replaced by opal.
Plants and soft organisms are usually preserved through carbonization. Their remains look like flattened pancakes beneath the weight of the rock, but the original film (usually reduced to a black “coalified” film) of their soft tissues is still visible.
Casts and Molds
This is when an organism leaves an imprint in soft sediment, which hardens and preserves the shape of the organism. An example of this is a dinosaur track or footprint.
Paleontologists can determine the age of a fossil and geologic events using relative dating and numerical dating.
Scientists compare fossils to other fossils found in the same area in relation to rock layers above and below them. This method is based on the Law of Superposition, also known as Steno’s Law. The law states that in an undisturbed stack of layered rocks, the layers at the top of the pile are younger than those at the bottom.
Relative dating allows us to place objects in chronological order without having any knowledge about their actual age.
For example, if Fossil A is found between two layers of rock that are known to be 10 million years old, then scientists can say that the sample is also about 10 million years old. If Fossils B and C are found deeper down, then we know that they are older than Fossil A.
Another way to date fossils is through numerical dating (older books call it absolute dating). It’s a fairly young method that measures the amount of radioactive decay that has occurred in an element found within the material. This technique gives a numerical age such as thousands of years or millions of years, which makes it more precise than relative dating.
Radioactive dating relies on the fact that parent isotopes of an element in a rock decay into daughter isotopes at a constant rate over time. Uranium decays into lead; potassium decays into argon. The time it takes for half of the parent isotopes to decay is called a half-life.
For example, we find uranium in a fossil and know that its half-life is 700 million years. What we do is measure the amount of uranium and lead in the fossil in relation to its half-life. Let’s say we found that the value of uranium is 50%, then we know that one half-life has passed. If it’s 25% then two half-lives have passed.
Since each half-life is equivalent to 700 million years, then the age of the fossil sample with 25% uranium is 1.4 million years.
Dinosaur Fossil Sites
Many fossils were unearthed in the most unlikely areas. But don’t dig a hole in your driveway or backyard. Dinosaur fossils are extremely rare. Not all rocks will yield dinosaur bones.
Fossils are primarily found in sedimentary rocks and sometimes in low-grade metamorphic rocks. Sedimentary rocks are stratified and layered rocks that are formed under conditions ideal for preservation.
Some parts of the world have more dino fossils than others. North America, for instance, is a treasure trove for paleontologists. At least three lagerstätten are found in the United States alone.
A lagerstätte (plural lagerstätten) is an area containing an exceptional collection of fossils. Fossil sites are considered lagerstätten if there are abundant fossils in the area from one particular period (concentration–lagerstätten) or if the quality of preserved fossils is incredibly high (conservation-lagerstätten). Over 50 sites around the world are described as lagerstätten.
Hell Creek Formation
|End of Cretaceous/Start of Cenozoic||Montana, North Dakota, South Dakota, and Wyoming||Lagerstätten|
Noteworthy because it appears to capture the last few hours after the impact of the asteroid that initiated the K-T extinction event.
|Ellisdale Fossil Site|
Has produced the largest and most diverse group of terrestrial animals of the Late Cretaceous.
|Ghost Ranch Site||Triassic||New Mexico||Lagerstätten|
Uncovered thousands of well-preserved skeletons of Coelophysis dinosaurs.
|Egg Mountain Site|
Two Medicine Formation
|Cretaceous||Montana||Dinosaurs, eggs, and young|
|Woodbine, Texas Arlington Archosaur Site|
|Cretaceous||Texas||Dinosaurs and crocodilians|
|Jurassic National Monument|
|Jurassic||Utah||Dinosaurs and other vertebrates|
|Aguja Formation||Late Cretaceous||Texas||Dinosaur coprolites|
|Portland Formation||Early Jurassic||Connecticut, Massachusetts||Dinosaur coprolites and footprints|
|Cedar Mountain Formation||Cretaceous||Utah||Eggs or nests, dinosaurs, amphibians, fish, mammals, other reptiles|
|Dakota Formation||Late Cretaceous||Nebraska||Eggs or nests, dinosaurs, and dinosaur tracks|
|Fruitland Formation||Late Cretaceous||Colorado, New Mexico||Dinosaur tracks, eggs or nests|
|Kalparowits Formation||Cretaceous||Utah||Eggs or nests, dinosaurs, turtles, crocodilians|
|Mooreville Chalk Formation||Late Cretaceous||Alabama, Mississippi||Dinosaur eggs, primitive fish|
|Glen Rose Formation||Cretaceous||Texas||Dinosaur tracks|
Scientists haven’t been successful in extracting DNA from any fossil cells yet. DNA degrades rapidly after death. So even if they find DNA soon, it would probably be contaminated or chemically altered.
The first complete dinosaur skeleton ever found is that of the Scelidosaurus, which was discovered more than 160 years ago and was only properly identified in 2020. The first 100% complete skeleton of a Tyrannosaurus rex was also discovered in 2006.
Fossilized bones of dinosaurs do not decompose because they are replaced by minerals that don’t break down easily. When an animal dies, its body starts to decay and fall apart. But if it’s buried quickly by sand or mud or other material, then it won’t be able to rot properly and will end up as a fossil instead!
Featured Image Credit: Gary Todd, via Wikimedia Commons