Prehistory refers to any time that predates human civilization and/or written records. So, when discussing prehistoric animals, we are looking at animals that existed before the start of state-level societies or writing. The history of earth is organized into four main categories that denote geological time. The broadest classification is an eon, and there are four main eons that encompass the entire history of Earth. Within an eon, there are eras, and within an era, there are periods, and within a period, there are epochs.
In this article, we will explore the different fauna (animals) that existed in the Pleistocene epoch (2.6 million years ago to 11.7 thousand years ago). This is a time that preceded the Holocene epoch, and is also referred to as the Ice Age. Some scholars have proposed that we have now entered a new epoch called the Anthropocene, because of the amount of human-caused changes to the world that have occurred. As you will learn shortly, humans came to North America during the late Pleistocene, and played a role in the evolutionary trajectory of the animals that lived during this time.
Fossils & Stratification
We are able to get a good estimate of when an animal lived by analyzing their fossils and their location in the sediment they were found in. Fossils are the preserved remains of plants and animals. Organisms can become fossilized in different ways. Some can be encased in resin where the tissues are preserved, and some can undergo permineralization where water carries minerals throughout the body of the organism, turning the body to stone. To learn more about fossils and their formation process, watch this video.
There are two main dating methods that are used to determine when the fossil was formed. Relative dating is when the fossil is compared to other things with a known date. For example, if a fossil was found in a specific rock formation with a known date, it can be inferred that the fossil has roughly the same age. Similarly, if a fossil is found along other fossils of a known age, the same type of inference can be made. Some fossils, called index fossils, refer to organisms that lived within a very specific time frame. They are often used when determining the age of another fossil: if a fossil with an unknown age is found in a site where the index fossils are also present, then the fossil with the unknown age is assumed to be of the same age as the index fossils.
Absolute dating uses radiometric dating methods. In radiometric dating, the known
decomposing rates of certain elements within a fossil or rock formation are used to determine the age of it. To learn more about radiocarbon dating methods, take a look at this video on carbon dating.
Prehistoric Animals of North America
Having contextualized prehistory and fossils, we can now dive into the North American animal fossil record. In this article, we’ll explore short-faced bears (Genus Arctodus), giant ground sloths (Genus Megatherium) and ancient horses (Equus scotti).
Short-Faced Bears
Short-faced bears (Genus Arctodus) were fast, long-distance roamers with a great sense of smell. When on its hindlegs, this carnivorous bear reached heights of twelve feet. It most likely hunted large herbivores like bison, deer, horses, and ground sloths.
Certain defining characteristics, like its speed and senses, can be deciphered from its anatomy. For example, the forward-pointing toes of short-faced bears indicate a special adaptation for efficient long-distance walking and running, unlike modern bears that have inward-facing toes. Also, the large nasal passage indicates a highly sensitive nose, capable of intaking large amounts of oxygen at once. When bone samples of these bears were tested, a high amount of certain nitrogen isotopes were found, which is commonly seen in exclusive carnivores.
These powerful hunters most likely went extinct due to a lack of food availability towards the end of the Ice Age, when the climate and habitat landscapes began to change. Their closest living relatives are the spectacled bears (Tremarctos ornatos) of South America.
Giant Ground Sloths
Giant ground sloths (Genus Megatherium) stood at an impressive height of almost 23 feet tall and lived in both North and South America during the Ice Age. Weighing close to four tons, their sheer size is what inspired their genus name, meaning “great beast”. Giant ground sloths are thought to have lived in coastal environments, have an herbivorous diet and are known as the largest bipedal mammals ever.
While there are several hypotheses as to how they became extinct, among the most common are that they were being over-hunted by humans and they had a tough time adapting to the changing climate towards the end of the Ice Age. Their closest living relatives are three-toed sloths (Genus Bradypus).
Ancient Horse
The ancient horse, Equus scotti, lived in the Ice Age and descended from a 55 million-year-old horse ancestor, Eohippus angustidens. It is most closely related to the modern zebra, and eventually became extinct due to predation by humans and competition with other herbivores.
The genus that the ancient horse belongs to, Equus, is the same genus that modern horses, zebras, and donkeys belong to. This genus is comprised of hoofed, herbivorous mammals with an odd number of toes. Hoofed animals are also called ungulates.
This genus spread throughout the world and while Equus scotti became extinct some 10,000 years ago, the genus lived on in parts of Europe and Asia and was domesticated. Domesticated horses were reintroduced into the Americas by Spanish colonizers towards the end of the 15th century. For a complete overview of horse evolution, watch this 10-minute video by PBS Eons.
Conclusion
North America was a completely different world during the Pleistocene epoch. It was a time of change, where the landscape formed into how we know it today. While these three examples of Ice Age animals barely scratched the surface, knowing about them may expand our perspective and knowledge on natural history here in North America.
Sources
Introduction:
Short-Faced Bears:
Giant Ground Sloths:
Ancient Horses:
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