The Cenozoic Era
66 million years ago - Present
The Cenozoic era is the span of time lasting from the end of the Cretaceous period 66 million years ago all the way up to the present day. Often this time period is referred to as the “Age of Mammals” since many of the new ecological niches which opened up after the extinction of the non-avian dinosaurs were taken over by mammals. Despite this, mammals were not the only animals to radiate into new roles over the course of the Cenozoic. Many types of crocodylomorphs, birds, snakes and turtles grew to gigantic sizes at various points during this era. Much like the Mesozoic era, the Cenozoic is divided into three periods; the Paleogene period, The Neogene period, and the Quaternary period. The Paleogene was the first period of the Cenozoic and this span of time saw the first appearances of many of the large mammal groups we are used to seeing today; including the first hoofed mammals and whales, the first primates, and the first carnivorans. The Paleogene period began as a very warm moment in geologic history but as this period gave way to the Neogene, the Earth had started to grow colder. The Neogene saw the expansion of grassland habitats. Much of the Midwest would have been covered in wide savannahs during the Neogene and rhinos, tapirs, camels and early horses would have been abundant in these new open environments. The Quaternary period is the last period in the Cenozoic and is the geologic period which includes the present day. This time period was characterized by the advance and retreat of huge continental ice sheets which at various times in the last 2.5 million years had managed to cover nearly the entire Midwest. Animals like mammoths, mastodons, giant sloths and bison would have been common in North America during this period. Humans arrived in the Midwest towards the end of the Quaternary and their arrival is thought to have been one of the reasons for the extinction of the large land mammals which used to be found in this region.
The Paleogene Period:
66 - 23 million years ago
In the immediate aftermath of the end-Cretaceous mass extinction most of the animals who had managed to survive the effects of the asteroid impact were small generalists or seed eaters. With most of the large animals gone, mammals, crocodylomorphs, squamates and birds were able to move into new niches and grew rapidly to much larger sizes. Rocks which date back to the Paleogene period are found in parts of the upper Great Plains, particularly in North Dakota. Some of the earliest large Cenozoic animals to appear in the fossil record are found in these deposits, such as the strange pantodont mammal Titanoides. The Paleogene is divided into three epochs; the Paleocene, the Eocene and the Oligocene. For most of the Paleocene and Eocene epoch the climate of North America was generally warm and humid. Huge rainforests spanned across the entire continent and large crocodilians and other cold-sensitive reptiles were found at very high latitudes in the northern hemisphere. Over the course of the Eocene and Oligocene epochs the Earth’s climate began to cool and become more dry. By the end of the Paleogene period the large rainforests had fragmented and were increasingly divided by larger expanses of more open environments. Many different groups of animals crossed over repeatedly between Eurasia and North America during the Paleogene, including early horse ancestors and several groups of carnivorous hoofed mammals. Camels first appeared in North America during the Paleogene and they would become one of the most abundant and diverse mammal groups endemic to the continent during the next period in geologic time; the Neogene period.
(ABOVE) A scene set in a dense rainforest during the Eocene epoch of the Paleogene period. A pair of large rhino-like uintatheres stand beside a small stream. These forest are home to many types of birds and hoofed carnivorous mammals.
(ABOVE) A giant Megacerops titanothere with a forked nasal horn moves in to protect its young calf from a pair of cat-like sabertoothed nimravids. Fossils of Megacerops are often found in parts of the White River badlands in South Dakota
Paleogene Animals
The Paleogene Period
23 million years ago
The Neogene Period
The Neogene Period:
23 - 2.58 million years ago
The Neogene period is divided into two epochs; the Miocene epoch (23-5.3 million years ago) and the Pliocene epoch (5.3-2.58 million years ago). In many parts of North America, grasslands took over the landscape during the Neogene period. Although the first grasses found in the fossil record date all the way back to the Cretaceous period, these plants became much more abundant in the Neogene as forests began to retreat as a result of a cooling climate. In broad terms the Midwest was still warmer during the Neogene period than it is today. Many parts of the Great Plains were covered in savannah-like habitats which were home to relatives of elephants, ancient rhinos, horses, tapirs, dogs, camels and early cats. Some of the best preserved fossils from this time period are found within deposits linked to the Ogallala formation/group which today extends from Texas to South Dakota. The fossils found in these deposits show us that Neogene life in North America would have been just as diverse as the biotas of today’s African savannahs. A particularly notable series of sites in Nebraska, the Ashfall Fossil Beds, preserve the remains of an entire herd of Teleoceras rhinos, as well as the fossils of dozens of other types of animals, preserved in fine ash along the edge of what was once a watering hole. These animals were victims of the ash generated by an ancient eruption of the Yellowstone supervolcano which formed an ash plume extending across most of North America. The western parts of North America seem to have been very volcanically active over the course of much of the Cenozoic and many Neogene fossil localities in the western US are linked to the deposition of ash as a result of these catastrophic eruptions. Despite this, none of these volcanic events seem to have resulted in large-scale mass extinction events. Towards the end of the Neogene period, North and South America became connected by the isthmus of Panama. This allowed many types of animals to move between the continents. North America got its first armadillos and sloths, while South America would receive its first carnivorans, horses, proboscideans and deer.
(ABOVE) A representation of the landscape of central North America during the Miocene epoch. The warmer climate which characterized this moment in the Neogene period, along with the spread of prairie grasses, led to the formation of savannah-like grassland environments in parts of Kansas, Nebraska, Oklahoma, South Dakota and Texas.
Neogene Animals
The Neogene Period
2.58 million years ago
The Quaternary Period
The Quaternary Period:
2.58 million years ago - Present
The Quaternary period is the final and shortest period of the Cenozoic era. It is also the period of geologic time which includes the present day. The span of time within the Quaternary period between 2.58 million years ago and 11,700 is known as the Pleistocene epoch, while the length of time from 11,700 years ago to the present is known as the Holocene epoch. The Pleistocene saw large ice sheets advance and retreat across the Midwest over the course of many successive warm and cold cycles lasting tens of thousands of years. At their largest extent, these ice sheets extended as far south as Kansas City. The presence of such large volumes of ice had huge impacts on the world’s geography. Sea levels were very low during much of the Pleistocene and many parts of the world which are currently covered by shallow seas would have been coastal lowlands at various times throughout this epoch. The movement of ice sheets also reshaped the landscape of the Midwest, scouring the surface of the land, changing the drainage patterns of rivers, and even creating the conditions necessary to form the Great Lakes. North America was also home to many unique forms of large animals during the Pleistocene. These extinct Pleistocene animals, often collectively known as “ice age megafauna”, included mammoths, mastodons, deer, bison, ground sloths, camels and tortoises. Many of these large land animals became extinct towards the end of the Pleistocene, possibly as a result of increased hunting from humans who arrived in North America towards the end of the Pleistocene epoch.
Our Pleistocene Field Guide
(ABOVE) Forest and wetland animals found in the Midwest during the Pleistocene epoch; (1) Cervalces, (2) Castoroides, (3) a Mastodon
We have an online field guide about all the different kinds of animals who lived in the Midwest during the Pleistocene epoch! Click the link to learn more!
Extinct Pleistocene Animals
(BELOW) The skyline of Kansas City as it appears today at the meeting point of the Kansas and Missouri rivers. The path followed by the Missouri River today was shaped by the presence of large ice sheets in this region around 700,000 years ago which pushed the river further south. The advance of these ice sheets also caused many rivers in the area to flow from east to west instead of north to south.
