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.
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.
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.
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.
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.
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.
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.
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