Lepisosteidae (Early Cretaceous – Present)

Gar fish are long-bodied freshwater fish who possess armored scales along their bodies. Gars belonging to Lepisosteidae first developed in the early Cretaceous, but ancestors to these Gar first show up in the fossil record during the Jurassic Period. Gars are often called ‘living fossils’ as they have gone virtually unchanged over the last 100 million years. There is plenty of fossil evidence for the Gar in the Hell Creek Formation; their armored scales fossilize very well and appear a bit like shards of obsidian among the dirt.

(Early Devonian – Present)

Lungfish are long, eel-shaped fish that can breathe air. While some species of lungfish possess both lungs and gills, they have an advantage over gill-exclusive fish types because they are able to survive in oxygen poor waters and withstand droughts, which may explain why they have managed to stay relatively unchanged over the last 400 million years. Lungfish are considered a “missing link” in the evolution of mammals, reptiles, amphibians, and birds as they are the closest relative to early tetrapods (or all vertebrate animals with four limbs).

Brachiopods (Cambrian – Present)

Brachiopods are aquatic soft-bodied invertebrates who typically live inside a symmetrical two-halved shell. Though similar to bivalves in design, Brachiopods are deceptively unique, belonging to their own phylum, Brachiopoda. Brachiopods are generally stationary and are typically anchored in one spot, relying on filter feeding.

Bivalves are aquatic soft-bodied invertebrates who typically live inside a vertically symmetrical two-halved shell. They are similar to brachiopods, who are symmetrical both laterally and vertically, but the two are not actually very closely related. Today, bivalves vastly outnumber brachiopods, but during earlier periods, the inverse was true.

Bivalves are more mobile than they look, often burying themselves in sediment with the help of a muscular organ called a ‘foot’, and some can even swim by clapping their shell open and closed. Their shells have historically been used as jewelry, tools, and even money in certain cultures.

Gastropods (Late Cambrian – Present)

Gastropods are a class of soft-bodied invertebrates and are members of the phylum Mollusca, including things like snails and slugs. Gastropods have been around since almost the beginning of the fossil record, developing all the way back in the oceans of the Cambrian Period (485- 541 million years ago). Gastropod’s bodies are simple but highly adaptable, and have diversified far past marine living. Today there are species in the oceans, in rivers, on land, and even hydrothermal vents.

Coelodonta antiquitatis (Pleistocene)

In the cold temperatures of the Pleistocene epoch, Woolly Rhinos kept warm with a layer of fur, similar to the Woolly Mammoth. The Woolly Rhinoceros lived in Eurasia, but unlike the Mammoth, never spread to the Americas. They had two horns, the foremost horn longer than the hindmost. They were similar in size to modern Rhinoceroses, and grazed on grasses and other plants.

Mammuthus primigenius (Late Miocene- Holocene)

Woolly Mammoths were large, furry, herbivorous members of the family Elephantidae, and were relatives to Mastodons and the modern African and Asian Elephants. Their thick fur protected them from the extreme colds of the Ice Age, while modern elephants need to stay mostly naked to protect from high heat. Different species of Woolly Mammoths lived during the Pleistocene, making their homes in both Eurasia and the Americas. Mammoths went extinct from a combination of rising global temperatures and human hunting practices.

Megalonyx jeffersoni (Late Miocene- Late Pleistocene)

The Giant Ground Sloths lived across North and South America. Unlike their modern relatives, Giant Ground Sloths did not live in trees, and instead stuck mostly to grasslands where they ate plants like yucca and agave. There were many different species of the Giant Ground Sloths, the largest reaching up to 20 feet in length. Megalonyx jeffersoni was a smaller species, but still reached nearly 10 feet in length and weighed about 2200 lbs.

Alligator mississippiensis (Pliocene – Present)

Crocodilians have been around for around 80 million years and have successfully survived several mass extinction events relatively unchanged. The modern American Alligator first developed during the Pliocene period (23-5 million years ago) and has changed very little since then. Their tough scales, efficient hunting techniques, and relatively low demand for food makes them incredibly good at surviving whatever comes their way. Today, American Alligators are found in the American Southeast, predominately in Florida and Louisiana.

Glyptodon (Pliocene- early Holocene)

Glyptodons were herbivores who protected themselves with bony armor plating, similar to their modern relative, the armadillo. However, Glyptodons were much larger than armadillos (about the size of a small car) and defended themselves with their spiked club tails.

Hemiauchenia macrocephalus (Eocene- late Pleistocene)

A small, sheep-sized member of the camelids family which contains modern camels, llamas, and alpacas. Like modern camels, Hemiauchenia macrocephalus was able to survive in dry conditions across the Great Plains region of the modern United States and some places in South America. Hemiauchenia macrocephalus was likely an ancestor to the modern South American llamas.

Bison antiquis (Late Pleistocene – Early Holocene)

Similar to the modern bison (Bison bison) but substantially larger (about 25% larger to be exact). Bison antiquis is extinct but is likely an ancestor to the modern bison still dwelling on the plains of North America.