Theropds make up the group of dinosaurs that contain the Tyrannosaurus rex, Velociraptors, and modern birds. Theropods are visually distinct from other dinosaur groups with their three-toed feet and the way they walk exclusively on their hind legs. The theropod body plan emerged in the Late Triassic with varieties like the Coelophysis, which was fairly small. By the Jurassic, giants like the Allosaurus became the dominant predators, while smaller theropods like the Archaeopteryx developed flight feathers and learned to fly. The largest theropod was the Tyrannosaurus rex who dominated the American West during the Cretaceous period.
During the age of the dinosaurs, theropods were the major predators of the land. Different species developed their own ways of hunting, some went solo like the T. rex and some in groups like the Compsognathus, but they all benefited from agile, dextrous body plans. For years, paleontologists believed that theropods were slow-moving with tails dragging on the ground– in reality, theropods were able to move with speed and agility, their long tails counterbalancing their heads and necks. Many had sharp teeth, but others like the Struthiomimus had a beak like modern theropods (birds).
The theropods are the only group of dinosaurs with living members. Thanks to their small size, mobility, and diverse diets, at least a few birds managed to survive the grueling years following the K-pg Impact. Since then, they have flourished as today, birds actually outnumber humans about 6 to 1.
Brittle Stars are echinoderms (related to sea stars, urchins, crinoids, etc.) which use their long, serpent-like arms to crawl across the ocean floor or swim through the water. They can look very similar to sea stars but can be differentiated by their distinct central body disk and thin flexible arms, which are often covered in spines. If attacked, Brittle Stars can defensively drop their arms to make their escape and live to regenerate them another day.
Stromatolites form when carpets of microbial life (usually cyanobacteria) are covered with a layer of sediment. That layer is then repeatedly covered by more cyanobacteria, which is covered by more sediment, and so on and so forth. When this structure fossilizes, we can see the layers formed in stone. Stromatolites can have different colors due to the type of minerals that made up the sediment. Sometimes Stromatolites are cut into slices so that those layers are visible, as pictured here.
Stromatolites have been forming for as long as cyanobacteria have existed, which is a really long time (about 3.5 billion years ago), and continue to do so in certain marine environments today.
Bryozoans are a type of filter-feeding colonial marine animal that has been living in the oceans since near the beginning of life on the planet. An individual Bryozoa is plant-like in appearance, but the colonies often form round, blobby masses that tether to various surfaces.
Bryozoan fossils can also look like tree branches and can be found all over the world, including here in Tennessee!
Nautiloids are an entire group of armored or shelled cephalopods that were prevalent during the Mesozoic Era, but still have some living members today. Many nautiloid species had long, conical shells that protected them from predators. Others resembled their cousins Ammonites with a curled, spiral shell. The only living members of the Nautiloid family are the nine living species of Nautiluses, which sport a spiral shell and live deep in the Pacific Ocean.
Crinoids are marine echinoderms (the group that contains urchins and starfish) that have been a part of Earth’s history for about 500 million years. They first developed during the Cambrian period, and some species still live in the oceans today. Despite appearing plant-like at times, crinoids are animals. Older crinoid species typically had a stem which anchored them to the ocean floor, a calyx or cup on top of that stem which contained its organs, and arms branching out from the calyx to help it filter feed. Crinoids have diversified a lot in the last half eon; some still anchor down in coral reefs, while some have evolved to swim around using their feathery arms for propulsion.
Fossils of crinoids can be found all over the world, even in Tennessee. Sometimes, you’ll hear these fossils called ‘Indian money’, and while Native Americans did not use crinoid fossils for currency, they were sometimes used as beads for jewelry. To find crinoid fossils, keep an eye out for something that looks like a stack of coins or a disk with the middle cut out –– that’s a fossil crinoid stem!
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 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 on hydrothermal vents.
Hyracodon, or the Running Rhino, was a small and graceful member of the rhino family. They functioned more like horses than their modern rhino relatives, 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. Hyracodon lived throughout North America and Asia, only going extinct from climate change and loss of habitat.
An ancestor of modern horses, Mesohippus was an important developmental stage in horses’ family line. They had not yet developed hooves and walked on three-toed feet. 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.
Canid species within the Mesocyon genus, sometimes called the “middle dog” or “short-faced dog”, were part of a now extinct genus of canids and are therefore not ancestors to modern dogs. However, it is believed the Mesocyon canids hunted in packs in open spaces like modern canids, likely relying on a strong sense of smell and sharp hearing and were about the size of a modern coyote. The major difference between Mesocyon lifestyle and modern canids is their diets: Mesocyons were carnivorous while modern dogs are omnivorous.
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.
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 that people have found. Oreodont 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.
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 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.
Leptictus dakotensis was a small omnivorous mammal that resembled a modern shrew, but it actually belonged to its own now extinct order called Leptictida. Leptictus 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.
Desmostylus herspersus was bit like a hippopotamus, living all along the North Pacific Rim. They were herbivores who ate aquatic vegetation like seaweed. The Desmostylus is not a direct ancestor to any current living species, but they are distantly related to manatees, dugongs, and elephants.
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