Fossils. Traces of the Past

Stratigraphy and the Discovery of Fossils

Back in the 19th century, a scientist named William Smith made a cool observation. After studying layers of rocks (that’s called stratigraphy) and the old living things found in them (paleontology), he realized that different creatures appeared in the rocks in a specific order over time. This made scientists realize that this could be a super helpful tool for learning about the history of life on Earth and figuring out how old certain rocks are. You see, if rock layers in different places have the same types of fossils in them, it means those layers are the same age, even if the rocks themselves look different. This idea is called the principle of faunal succession.

This discovery added to some ideas about rock layers that Nicolas Steno had come up with way back in the 17th century. He figured out that:

Usually, rock layers are laid down flat: this is the principle of original horizontality.
Rock layers tend to be continuous sideways and are the same age throughout that stretch: this is the principle of lateral continuity.
Any rock layer on top of another one is younger, and the one at the bottom is the oldest: this is the principle of superposition.

All these discoveries really changed the field of Geology and what people knew about our planet. They realized that Earth was probably much, much older than some people thought and that it was a very active planet with changes happening over incredibly long periods of time. Also, studying rocks and how they change led to stratigraphy becoming its own branch of Geology. The goal of stratigraphy is to figure out and organize all the local rock layers in a region and then connect those local layers to understand bigger areas of sediment and how they formed.

Index Fossils

Fossils can be really useful for dating the rocks they’re found in, sometimes even more useful than the rocks themselves! That’s because fossils are the remains of living things that lived during a specific period of time. Evolution is a process where living things change over time in ways that are unique and don’t happen again. So, the fossils of these creatures can act like time markers.

However, dating rocks with fossils isn’t always easy. You don’t always find the fossils you need, and depending on what type of creature the fossil is from, it can be easier or harder to use it as a time marker. That’s where the idea of an index fossil comes in. We can say that an index fossil has specific features that make it a good tool for figuring out the relative age of rocks. These features are:

They were found in many different places because they lived in large areas. This makes it easier to find them and even connect rock layers that are far apart.
They lived for a relatively short period of time. This means they changed quickly through evolution and didn’t exist for a super long time (geologically speaking).
There were lots of them, which makes them easier to find.

Today, we have index fossils for all the main groups of animals we know and for many types of plants. Of course, the ones that fossilize best are those with hard parts like bones or shells. Here’s a list of some groups of living things and examples of index fossils that belong to them:

Sponges: Archaeocyathids (Cambrian Period).
Corals: Rugose corals (Ordovician to Permian Periods).
Mollusks: Cephalopods like ammonoids and belemnoids (Mesozoic Era).
Arthropods: Trilobites (Cambrian to Permian Periods).
Brachiopods: Some like Rhynchonella or Pygope (Jurassic to Cretaceous Periods, Mesozoic Era) and others like Terebratula (Miocene to Pliocene Epochs, Cenozoic Era).
Foraminifera: Fusulinids (Carboniferous to Permian Periods).
Plants: Tree-like horsetails like Calamites (Devonian to Permian Periods) and ferns like Pecopteris or Neuropteris (Carboniferous to Permian Periods).
Echinoderms: Crinoids like Encrinus (Triassic Period) and Echinoids like Heteraster (Cretaceous Period).
Reptiles: Dinosaur remains (Mesozoic Era).
Fish: Teeth of Carcharodon (Neogene Period to today).
Mammals: Dinotherium (Neogene Period), Equus (Quaternary Period).

Fossils in Different Geological Eras

Paleozoic Era

The Paleozoic Era was a time period of about 290 million years that belongs to the Phanerozoic Eon. It started when a supercontinent (Pannotia) broke apart and ended when another one (Pangea) formed. The climate generally got warmer, and geologically, it was an active time with different pieces of land moving around, which significantly and sometimes drastically affected the climate and life on Earth. There were at least three major extinction events during this era, with the third one being the largest known extinction, where about 95% of all living species disappeared. Some important fossil groups from the Paleozoic Era include:

Trilobites: Ancient arthropods with bodies divided into three parts. They started to become diverse in the early Cambrian Period, and scientists have identified up to 22,000 species. They all lived in the sea, and most likely on the seafloor. A well-known type often used as an index fossil is Paradoxides.
Graptolites: Strange, floating, colonial creatures related to chordates. Some types are index fossils for the Ordovician Period.
Plants: Remains of tree-like plants like horsetails and ferns from this time are often found as fossils, especially from the Carboniferous and Permian Periods when there was a big increase in the variety of these types of plants.

Mesozoic Era

In general, we can say that the Mesozoic Era was the age of dinosaurs for animals and the age of gymnosperms (specifically cycads) for plants. The climate was very warm, and sea levels were noticeably low. Geologically, it was a period with fewer big changes since the supercontinent Pangea was already formed. Some important fossil groups from the Mesozoic Era include:

Ammonoids: These were cephalopod mollusks that, unlike most modern ones, had a hard outer shell made of aragonite. They evolved quickly and lived all over the world, making them excellent index fossils.
Orbitolinas: Coin-shaped, bottom-dwelling foraminifera that were very common in rocks from the Cretaceous Period.
Dinosaurs: Large reptiles that ruled the Earth during this era, with an amazing variety. Scientists have identified up to 500 different types. Most of them died out in the massive extinction event at the end of the Cretaceous Period. Their hard parts like bones and teeth, and even their footprints (called ichnites), are used today for relative dating.

Cenozoic Era

The Cenozoic Era is the era that covers the last 66 million years of Earth’s geological history. During this time, the continents moved into their current positions, making it a geologically active period that influenced the climate, which was characterized by a long-term cooling trend. Some important fossil groups from the Cenozoic Era include:

Nummulites: Bottom-dwelling foraminifera that were very abundant in sediments from the late Paleocene to the early Oligocene Epochs.
Mammals: Remains of hard parts from types like Hipparion, Equus, and other large Ice Age mammals from the Pleistocene Epoch.

Sources

“Historia de la Tierra. Un planeta singular” (History of the Earth. A Unique Planet) by Anguita F.
“Ciencias de la Tierra. Vol II” (Earth Sciences. Vol II) by Tarbuck and Lutgens
“Guía de fósiles del mundo” (World of Fossils Guide) by Parker S.