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.
The Marrella splendens was a small arthropod that lived in the Cambrian oceans. The Marrella splendens had many thin appendages that served as both its means of movement and its means of respiration, as the upper section of its legs possessed gills. It also had two pairs of antennae, one set for helping maneuver around the bottom of the ocean floor and the other set to aid in propulsion.
Marrella splendens is heavily associated with the Burgess Shale fossil deposit in British Columbia, Canada as it is the most plentiful fossil found there.
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!
Anomalocaris, whose name means ‘weird shrimp’, were one of many odd marine species that came from the Cambrian Explosion. Anomalocaris species ranged in size, but the most famous Anomalocaris canadensis was about 2 feet long. They swam by propelling themselves with overlapping flaps running down either side of their bodies and latched onto prey with two barbed appendages by their mouths. The Anomalocaris is considered an apex predator of its time, largely feeding on soft-bodied animals like jellyfish and worms.
It was long believed that the Anomalocaris went extinct at the end of the Cambrian period, but some species like the one pictured here actually lasted into the Ordovician period.
Graptolites are a type of colonial marine animal that developed in the Cambrian oceans. A Graptolite colony is made of many singular zooids that asexually reproduce to make more zooids. The Graptolites of old formed somewhat complex structures that initially left paleontologists head scratching, as they often resembled a pair of thin saw blades connected at one end. They also often have a shiny appearance like graphite pencil markings (the name Graptolites means “writing in the rocks”). It was not until scientists were able to remove the fossil from the rock that they could learn more about its shape and what it was actually made of.
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.
Trilobites were marine arthropods that lived on earth for 300 million years. They were generally small, had a segmented exoskeleton, and crawled around on the bottom of the oceans floor. Most Trilobites had eyes or eye-like structures that were positioned atop their heads and ate detritus that had settled into the sediment (a little like a living Roomba).
Because Trilobites were around for so long and were so prolific, their fossils are found all over the world. The smallest species were mere millimeters long, and the largest was almost 30 inches long. They often died in odd positions, similar to a bug, which you can see demonstrated here.
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!
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.
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