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.

Nummulites gizahensis (Eocene)

Nummulites are a type of foraminiferan (a type of single-celled organism) that develop disc shaped tests (shells) made up of connected chambers in a spiral arrangement. This species, Nummulites gizehensis, lived during the Eocene epoch and dwelled at the bottom of a shallow ocean around what is now Northern Africa and Southern Europe. The fossils of nummulites can be seen in the very stones that make up the Pyramids of Giza, hence their name.

Nummulites have an ‘alternation of generations’ life cycle, making both the large, coin shaped specimens, and the small, lentil shaped specimens, the same species. The large, coin shaped forms reproduced asexually after reaching maturity, producing the smaller form. The small, lentil shaped forms reproduced sexually after reaching maturity, producing the larger form.

Harpactocarcinus punctulatus (Eocene)

Harpactocarcinus punctulatus was a type of mud crab whose incredibly well-preserved fossils can be found in Europe and North America. These crabs fossilize astoundingly well because of their hard carapaces, which make their remains look a bit like statues instead of fossils.

Basilosaurus (Eocene)

The Basilosaurus was an apex predator of the Eocene seas. It is known for its prominent and intimidating teeth, which it used to hunt fish, sharks, and likely marine mammals. The Basilosaurus was long and slender, ranging between about 50-65 feet full grown. An interesting feature of their anatomy was their vestigial hind legs, which were not even attached to the skeleton and completely useless to the whale itself. Those legs do however tell paleontologists more about whale evolution, proving that whales’ ancestors once walked on land before returning to the water. 

The group Brontotherium is a family of large megafauna of rhinoceros-like mammals. By the end of the Eocene, Brontotheres evolved to be very large (about 8 feet tall and 16 feet long) and would have traveled in herds. Like their distant relatives rhinoceroses, Brontotheres sported large horns; unlike rhinos, their horns were made entirely of bone instead of keratin. They were herbivorous and grazed on soft vegetation, which ultimately lead to their extinction at the end of the Eocene. During a shift in climate, the warm verdant lands of the Eocene dried out and became much colder, which effectively destroyed the Brontheres’ food source.

This group of animals goes by many names, but the two most popular are Brontothere which means “thunder beast” and Megacerops, meaning “large-horned face”. 

Hyracodon (Late Eocene – Early Miocene)

Hyracodon, or the Running Rhino, was a small and graceful member of the rhino family. They functioned more like horses than their modern rhino counterparts, as they were hornless with long, slender legs. They were about the size of a pony, and likely used their agility to flee from predators rather than fight them. Hyracodons lived throughout North America and Asia, only going extinct from climate change and loss of habitat.

Subhyracodon (Late Eocene – Early Oligocene)

The Subhyracodon was a hornless rhinocerotid whose fossils can be found largely in the Modern United States, especially in the Badlands. They were not as large as their modern relatives, and in fact were about the size of a tapir.

Mesohippus (Late Eocene - Oligocene)

An ancestor of modern horses, Mesohippus was an important developmental stage in horses’ family line. Mesohippus was a small, primitive horse that was about the size of a labrador retriever. These early horses were grazers that lived in North America and had to contend with predators like the cat-like nimvarids and other large carnivorous mammals called Hyenadons.

Ischyromys (Early Eocene – Early Oligocene)

Ischyromys was a small, tree climbing mammal and was among the first true rodents to ever exist. It shared many visual traits with modern squirrels, like its long tail and long incisors, but is actually more closely related to the Mountain Beaver, a small rodent that lives in the Pacific Northwest.

Leptomeryx (Eocene – Early Miocene)

Leptomeryx was a small quadruped that resembled a modern deer but was similar in size to a modern cat. Leptomeryx was a member of the artiodactyl order, the same as modern hoofed animals, and grazed on soft vegetation.

Merycoidodon (Early Eocene- Late Miocene)

Merycoidodon, or more commonly known as Oreodonts, were grazing mammals that resembled pigs or sheep. They are famous both for their teeth (the name Oreodont translates to “mountain teeth”) and the sheer number of their fossils people have found. Oreodonts populations were outlandishly abundant, so much so that their fossils are a common find in places like Badlands National Park. They moved in herds, which protected them from predators like saber-tooth cats. Despite their numbers, they were not adaptable to changes in food availability and went extinct as a result of major climate change and being outcompeted by other foraging species.

Poebrotherium wilsoni (Eocene – Oligocene)

The ancestors of modern camels originated in North America during a dry period of Eocene epoch, yet Poebrotherium were visually distinct from their modern relatives. These early camels were small, standing at only two feet, and weighed about 50 lbs. Eventually, camels would spread to Asia and Africa, and the North American populations would go extinct (approx. 10,000 to 13,000 years ago).

Archaeotherium mortoni (Eocene – Oligocene)

Archaeotherium were large, and likely aggressive beasts that once roamed North America and Eurasia. They stood about six foot tall and weighed around 2000 lbs. They had snouts and tusks, and fossil evidence suggests that they used these tusks to ram others of their species for dominance. Despite their common name and appearance, Hell Pigs were not true pigs but were more closely related to hippopotamuses and whales than modern pigs.

Lepictus dakotensis (Late Eocene – Early Oligocene)

Lepictus dakotensis was a small omnivorous mammal that resembled a modern shrew, but it actually belonged to its own now extinct order called Leptictida. Lepictus dakotensis is a good example of convergent evolution in that it exhibited a rodent-like body plan despite not being a true rodent. Its fossils are found in North America, predominately the American West.

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.