Timeline Of Prehistoric Life In The Midwest
The Geologic Timescale
When we look back at the entire history of the Earth, it is often useful to be able to divide the entire 4.56 billion year story of the planet into smaller increments. The geologic timescale divides Earth's history into a series of chronologically-ordered divisions which are themselves defined by major events in Earth's history. It is used by geologists and paleontologists to organize and date different rock layers and the fossils they hold. You may have heard of some of the geochronologic units found on the geologic timescale before; the Cretaceous and Jurassic periods are some well known examples. The boundaries between geochronologic units are defined by divisions between chronostratigraphic units. Chronostratigraphic units represent all of the rocks deposited over the course of a particular time interval. These units are based on the relative ages of different rock deposits. In cases where certain fossils are known to be very common in rocks from a particular chronostratigraphic unit, the appearance or disappearance of these index fossils can be used to define the start and end of a particular unit of time. For instance, the Jurassic system (a chronostratigraphic unit) contains all rocks deposited during the Jurassic period (a geochronologic unit) and the start of the Jurassic is defined by the first appearance in the fossil record of the ammonite Psiloceras spelae tirolicum.
By looking at each of the units of time on the geologic timescale, we can get a general sense of what was happening on Earth at a particular time in the history of the planet. For example, rocks from the Campanian stage of the Cretaceous period found in Kansas preserve the remains of marine animals living in warm shallow waters. Similarly, rocks from the early Permian period in the eastern part of the state were formed 215 million years earlier, when that part of the world was covered in coastal rainforests filled with giant amphibians and millipedes.
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(66 million years ago - Present): After the extinction at the end of the Cretaceous period mammals become some of the largest animals found on Earth. The planet's climate slowly cools until large-scale glaciation covers many parts of the Midwest in ice sheets during the Pleistocene epoch.
(538.8 - 252 million years ago): Life diversifies in the world's oceans and begins to move onto land. The first trees, insects, reptiles, amphibians and fish appear during this era. At various times during the Paleozoic the Midwest is covered in shallow seas or in huge rainforests filled with giant scale trees.
(252 - 66 million years ago): Reptiles begin to take over new niches in the water, on land and in the skies. The first marine reptiles, pterosaurs, crocodylomorphs and dinosaurs appear during this era. Mammals and flowering plants also get their start during the Mesozoic. The Great Plains are covered in a shallow seaway during the second half of the Cretaceous period.
(4.56 billion years ago - 538.8 million years ago): The Earth and Moon are formed and the crust of the planet hardens into solid rock. The first oceans spread across the world and land is fairly rare. Life appears in the fossil record for the first time and by the end of the Proterozoic era, simple animals can be found living in the shallow seas of the Midwest.
How To Read The Geologic Timescale
The geologic timescale is composed of several different kinds units used to categorize spans of time. The largest units are eons, which usually span across billions of years or hundreds of millions of years. Each eon contains multiple eras, which are units of time lasting hundreds of millions or tens of millions of years. Eras are further broken down into periods, lasting tens of millions of years. Geologic periods are divided into units called epochs and ages which can last anywhere from millions of years to tens of thousands of years.
Eons
Eons are the largest units of geologic time. The history of the Earth is separated into four eons; the Hadean eon (4.65 billion years ago - 4 billion years ago), the Archaean eon (4 billion years ago - 2.5 billion years ago), the Proterozoic eon (2.5 billion years ago - 538.8 million years ago) and the Phanerozoic eon (538.8 million years ago - present). The Phanerozoic is the eon in which we are currently living. The name "Phanerozoic" means "visible life" since this era begins around the time that complex multicellular life began to proliferate. The entire evolutionary histories of mammals, reptiles, fish, insects, arachnids and land plants fall within the Phanerozoic eon.
Eons are divided into smaller units called eras. Within the Phanerozoic eon there are three eras. The first of these is the Paleozoic era (538.8 - 252 million years ago) during which complex animal life diversified and the first vertebrates appeared and spread onto land. The Paleozoic contains six geologic periods; the Cambrian period, the Ordovician period, the Silurian period, the Devonian period, the Carboniferous period and the Permian period. The Paleozoic era was followed by the Mesozoic era (252 - 66 million years ago) which is often called the "age of reptiles". This is the span of time during which dinosaurs, birds, mammals and flowering plants appear in the fossil record. This era is divided into three periods; the Triassic period, the Jurassic period and the Cretaceous period. The Mesozoic era transitions over to the Cenozoic era at the end of the Cretaceous period 66 million years ago when an asteroid impact resulted in a mass extinction event. The Cenozoic era (66 million years ago - present) is the current era of geologic time. This span of time saw mammals move into many of the niches left behind after the extinction of the non-avian dinosaurs. It is divided into three periods; the Paleogene period, the Neogene period and the Quaternary period. The Quaternary period began 2.58 million years ago and is the time period which includes the present day.
Eras and Periods
Epochs, Ages and Stages
Periods can further be divided into smaller units of time called epochs. An epoch usually lasts anywhere from tens of millions of years to tens of thousand of years. The current period of geologic time; the Quaternary period, is divided into the Pleistocene epoch (2.58 million years ago - 11,700 years ago) and the Holocene epoch (11,700 years ago - present). Mesozoic time periods are divided into "early", "middle" and "late" epochs, i.e. the Upper Cretaceous epoch or the Early Jurassic epoch.
By putting all of this information together we can say that, for instance, we are currently in the Holocene epoch of the Quaternary period of the Cenozoic era of the Phanerozoic eon! Tyrannosaurus rex, on the other hand, lived in the Late Cretaceous epoch of the Cretaceous period of the Mesozoic era of the Phanerozoic eon.
Some time periods and epochs are so long that it becomes useful to further divide them into units called ages. These ages are also often referred to using their chronostratigraphic equivalent unit term "stages". These ages often span a few million years. The Cretaceous period, for instance, is divided into two epochs; the Late Cretaceous and the Early Cretaceous. The Late Cretaceous is composed of six ages/stages; the Maastrichtian, the Campanian, the Santonian, the Coniacian, the Turonian and the Cenomanian. The Early Cretaceous epoch also has six ages/stages; the Albian, the Aptian, the Barremian, the Hauterivian, the Valanginian and the Berriasian.
Many simplified representations of the geologic timescale like the one below will show the periods of the Phanerozoic (Cambrian- Quaternary) but will not show the periods of the previous three eons. The 12 phanerozoic periods can also be listed with their 1-2 letter abbreviations. These spans of time also have their own associated colors.
Eons don't have universally accepted abbreviations so they are shown here as Had. (Hadean), Arch. (Archaean) and Prot. (Proterozoic).
Abbreviations
Cambrian period - Ꞓ
Ordovician period - O
Silurian period - S
Devonian period - D
Carboniferous period - Ꞓ
Permian period - P
Triassic period - T
Jurassic period - J
Cretaceous period - K
Paleogene period - Pg
Neogene period - Ng
Quaternary period - Q
The Early Earth
4.56 billion years ago - 538.8 million years ago
For the vast majority of Earth's history there were no plants, no animals, no fungi. For billions of years the entire planet was only home to simple single-celled organisms such as archaea. These first living things were closely tied to the presence of liquid water and may have gotten their start near hydrothermal vents or in warm mineral-rich geothermal springs. Plants first appeared as simple algae-like photosynthetic organisms sometime during the Proterozoic eon. It wouldn't be until the very end of the Proterozoic that organisms we might recognize as animals first appeared. The span of time previously known as the "Pre-Cambrian", now divided into the Hadean, Archaean and Proterozoic eons, comprises more than 70 percent of the history of the planet. It includes the earliest days of the Earth when our planet was still a ball of molten rock floating in space, as well as later periods of extreme cold when the planet would have nearly been covered in ice from pole to pole
The Hadean eon is the first of four eons which make up the history of the Earth. This first span of around 750 million years saw the planet begin to coalesce from smaller pieces of metal and rock orbiting around the sun. Early on it its history, Earth is thought to have collided with another proto-planet called Theia, which resulted in the formation of the moon. The outer crust of the planet cooled into solid rock during the Hadean eon and the Earth developed its first atmosphere, which was composed of large amounts of methane, water vapor and ammonia. As The Earth cooled, some of this water vapor condensed to form the first oceans. Asteroids and comets rained down on the planet's surface bringing water, gasses, metals and other materials along with them. The end of the Hadean saw a particularly intense series of collisions during a time period called the Late Heavy Bombardment during which asteroid and comet impact became much more common. The first life forms on Earth may have appeared towards the end of the Hadean or during the first half of the next eon, the Archaean. During this eon the Earth was almost completely covered in a deep ocean, with only a few rare islands poking above the surface of the water. Cyanobacteria first appeared during this eon. Some of these ancient organisms are known from ancient fossils, known as stromatolites, which are formed from the cemented and layered remains of some types of microbial mats. The atmosphere was still oxygen-poor and was composed of large amounts of methane gas. The next eon, the Proterozoic, is the longest eon in the history of the Earth. Oxygen became an important component of the atmosphere and the world's oceans during the early part of this eon. A second oxygenation event later in the Proterozoic caused a series of glaciation events which may have covered nearly the entire planet in ice. Life became increasingly complex during the Proterozoic. The first microscopic plants appeared in the oceans during this eon, as did the first eukaryotic single-celled organisms. Some of the first groups of true animals appeared at the very end of the Proterozoic, including ancestors of sponges and jellyfishes.
(ABOVE) A scene set during the late Archaean eon showing a cluster of stromatolites in the foreground. The rising levels of oxygen in the atmosphere have turned the sky a pale shade of blue while in the distance a few scattered volcanic islands stand in the middle of a vast global ocean