National Fossil Day   Explore Nature
National Park Service
US Department of the Interior

What's on the 2013 artwork?

The 2013 National Fossil Day artwork features a variety of marine invertebrates from the Paleozoic Era. The scene is an idealized representation of a seafloor from the Ordovician Period (between about 485 and 444 million years ago) near what is now Cincinnati, Ohio.

Along the seafloor, a eurypterid, also known as a sea scorpion, chases after a soon-to-be meal of trilobites. Bryozoan fronds and rugose, or horn, corals are present in the background. A gastropod (snail) is partially buried in the sand of the seafloor.

Click on the tabs below to learn more about the animals featured on the logo, as well as ancient seas and the importance of Paleozoic marine invertebrate fossils.

  • Eurypterids
  • Trilobites
  • Bryozoans
  • Rugose Corals
  • Gastropods
  • Paleozoic Seas
  • Useful Fossils
Cumberland Gap NHP Geologic Map
Silurian fossil of Eurypterus remipes from New York. This genus is less "spiney" than the genus depicted in the 2013 artwork. Photo by H. Zell (CC BY-SA 3.0).

Eurypterid Fossil Facts

  1. Eurypterids were fearsome aquatic predators of the Paleozoic. The 2011 National Fossil Day artwork featured a fearsome aquatic predator of the Mesozoic.
  2. Eurypterids were the largest arthropods, attaining sizes more than 8 feet long!
  3. They lived from about 460 to 270 million years ago (during the Ordovician through Permian). Eurypterids are now extinct.
  4. The state fossil of New York is a Silurian eurypterid: Eurypterus remipes (the first eurypterid ever discovered).


Eurypterids were fearsome predators of Paleozoic seas across the globe. They were also among the largest arthropods (same phylum as insects, spiders, and crustaceans), getting up to more than eight feet in length. Early forms were marine, later eurypterids lived in brackish or even fresh water. There are about 250 species of eurypterids, all of them are now extinct. They range in age from the Ordovician to the Permian but are most diverse in rocks of the Silurian and Devonian (see a time scale). In North America they are found primarily in New York, Pennsylvania, Indiana, Illinois, and Ohio. The eurypterid in the logo is based on the Ordovician genus Megalograptus.

Eurpyterids, sometimes called "sea scorpions," have a fittingly predatory appearance. The front set of appendages are chelicerae that look like (but are not homologous to) scorpion pincers. Behind the chelicerae are legs. The last pair of appendages are paddles (modified legs) for swimming. At the end of the segmented body is a telson that can be a spike, a paddle, or set of pincers. Although the telson looks similar to a scorpion's stinger, it is unlikely that eurypertids could sting. Nevertheless, you probably wouldn't want to tangle with one that was longer than you are tall! Eurypterids probably ate, or scavenged, just about anything they could catch or fit in their mouth. The trilobites shown in the logo may have been a common meal.

Word Origins

The name eurypterid is derived from the Greek word for "wide" or "broad" (eurys) and the Greek word for "wing" (pteryx). "Wide or broad wing" refers to the wide swimming paddles.

Flexycalymene retrorsa VFM photo
Ordovician trilobite Flexycalymene retrorsa from Ohio. (Virtual Fossil Museum photo).

Trilobite Fossil Facts

  1. Trilobites were incredibly diverse marine arthropods which primarily lived on the seafloor and fed on detritus.
  2. Trilobites are now extinct, although they existed throughout the Paleozoic and their fossils are found on every continent.
  3. Some trilobites could roll up into a tight "ball" to protect their vulnerable soft parts.
  4. Three states have trilobites as their official state fossil: Ohio (Isotelus from the Ordovician, Pennsylvania (Phacops rana from the Devonian, and Wisconsin (Calymene celebra from the Ordovician and Siluiran.


Trilobites were a spectacularly diverse group of marine invertebrates and the most well-known Paleozoic fossil. Fossils of more than 17,000 species have been discovered on every continent. Trilobites were almost exclusively benthic, meaning they lived on the seafloor. They fed on detritus, organic material that settled to the bottom, using their many legs to "kick" food particles toward their mouth. With thousands of species, trilobites attained many different sizes, from slightly bigger than microscopic to more than two feet long. They also had a wondrous array of ornamentation, from unadorned to covered in spikes and spines. View a sample here or here. The trilobite in the logo was based on the Ordovician genus Isotelus.

Trilobites are among the oldest Paleozoic marine invertebrates, appearing about 540 million years ago during the Cambrian. By the end of the Cambrian, about 50 million years later, they achieved their maximum diversity. Afterwards, they dwindled in diversity but not necessarily abundance during the rest of Paleozoic. The last few trilobite lineages were extinct by the end of the Paleozoic Era, 251 million years ago. All told, trilobites scuttled along the floors of Earth's oceans for around 290 million years. By comparison, dinosaurs lived for about half that amount of time and the human genus Homo has been around for not quite one percent of that span.

Word Origins

The term "trilobite" was derived from Greek words meaning "three-lobed" referring to the three longitudinal (parallel to the body's length) lobes of trilobites.

Bryozoan. R Port photo.
Bryozoans like this one, formed lacy fronds. Others encrusted hard surfaces. NPS photo by Rebecca Port.

Bryozoan Fossil Facts

  1. Bryozoans, or "moss animals", are sessile, colonial, marine animals that superficially resemble corals, but are from a completely different phylum.
  2. They are one of the latest-appearing phyla in the fossil record and are still extant today.
  3. Fossils are found on every continent and across North America.
  4. Bryozoan colonies encrust hard surfaces (even the bottoms of ships!) or form beautiful fronds.


Bryozoans appear similar to coral and are primarily sessile (immobile), colonial marine animals. However, bryozoans are in a completely different phylum than corals, and the bryozoans were among the last animal phyla to appear in the fossil record. Definitive bryozoan fossils appear in the Ordovician, although some recent discoveries may push their first appearance in the fossil record back to the upper Cambrian. Bryozoans (also called "ectoprocts") are still extant, with about 5000 living species today. There are a few times that number known from the fossil record.

Individual bryozoans, rarely more than a millimeter across, are called "zooids." They live in colonies ranging from a few to a few million zooids. Bryozoans feed on even smaller microorganisms like diatoms via a filter-feeding structure called a "lophophore." They secrete a rigid skeleton of calcium carbonate (limestone) that forms the framework for the colony. The colonies can encrust (hence their name) hard surfaces or form expansive frond-like structures. Upright frond-like colonies, such as those on the National Fossil Day artwork, were common during the Paleozoic. The bryozoans in the logo were based on the Ordovician and Silurian genus Chasmatopora.

Word Origins

The term bryozoan is derived from the Greek words for moss (bryon) and animal (zoös). "Moss animal" is a common name for bryozoans and refers to the moss-like appearance of encrusting species.

Rugose Coral from Indiana
Rugose coral Grewingkia canadensis from Indiana. Public domain image.

Rugose Coral Fossil Facts

  1. Rugose corals were abundant corals during the Middle Ordovician through the Late Permian. They are now extinct.
  2. Solitary rugosans are called "horn corals" and some were around 3 feet long.
  3. Although they look like modern corals, rugosans are a much more ancient group, going extinct before modern stony corals appear in the fossil record.
  4. No rugosan corals are state fossils, but other Paleozoic corals have been selected as the state rock of Michigan (Hexagonaria percarinata, the "Petoskey Stone," of Devonian age) and the state stone of West Virginia (Lithostrotionella, of Mississippian age).

Rugose Corals

Rugose corals, also called "horn corals," are an extinct group of Paleozoic corals that were abundant from the Middle Ordovician through the Late Permian. They reached their peak diversity during the Devonian and their fossils have been found on every continent except Antarctica. They are found across North America but are particularly common in New England and the Ohio River valley . Individual rugose coral polyps were stationary and constructed calcium carbonate (limestone) frameworks. Some rugosans were solitary while others formed colonies.

Solitary rugosans often were horn-shaped, giving the group its common name, and could reach sizes around three feet in length. Although none of their soft parts have been preserved, rugose coral polyps were likely similar to modern corals in that they had tentacles (perhaps even with stinging cells) and could capture food floating past. You could call them "microcarnivores"!

Word Origins

Rugosus is Latin for "wrinkled" or "shriveled," referring to the wall of the coral.

Ordovician snail. Humboldt State University photo.
Salpingostoma richmondense is one Ordovician gastropod from the Ohio River valley. Humboldt State University Natural History Museum photo.

Gastropod Fossil Facts

  1. Gastropods (snails) are the most diverse group represented on the National Fossil Day artwork. There are at least 62,000 living species of gastropods and tens of thousands in the fossil record.
  2. The fossil record of gastropods extends back to at least the Lower Cambrian and covers every continent and every subsequent period of geologic time.
  3. Although gastropods today inhabit nearly every habitat on earth, the first gastropods were marine.
  4. No state has selected a Paleozoic marine snail as a state fossil. Maryland's state fossil (Ecphora gardnerae gardnerae) is a predatory marine snail from the Cenozoic Era.


Of all the animals featured on the 2013 artwork, the one you are most likely to run into (well, outside of a rock shop or fossil museum, anyway) is the gastropod, or snail. Gastropods today are found in just about every aquatic and terrestrial habitat from equator to pole. There are at least 62,000 extant species, and perhaps as many as 150,000 gastropod species alive today. Tens of thousands of fossil gastropod species are known. The earliest gastropod fossils appear during the Lower Cambrian and were marine. Terrestrial gastropods do not appear in the fossil record until 200 million years later.

Although marine snails today have a variety of shell shapes and some complex ornamentation, early Paleozoic gastropods had simple mostly unadorned coiled shells, similar to the snail in the NFD artwork (loosely modeled after the Ordovician and Silurian genus Salpingostoma). Paleozoic snails fed in a variety of ways. Some were detritus feeders, others were active predators able to drill holes in the shells of other gastropods to reach the animal within!

Word Origins

Gastros is Greek for "stomach" and poda is Greek for "foot." The term "stomach foot" refers to the appearance that a snail is crawling on its stomach.

click to see maps throughout the Paleozoic Era...

During the early Paleozoic Era, seas covered much of North America. The red star indicates the location of the idealized seafloor featured in the 2013 artwork about 450 million years ago (mya). Paleogeographic map by Ron Blakey (Colorado Plateau Geosystems).

Paleozoic Seas

Throughout the Paleozoic Era, shallow seas covered much of North America, meaning marine invertebrates had extraordinary expanses of habitat. Indeed their fossils are found across the continent and span hundreds of millions of years of geologic time. They are often found in exceptionally fossil-rich deposits. The logo depicts only a few of the major groups of Paleozoic marine invertebrates. Other important groups include brachiopods, bivalves, ammonites (Mesozoic ammonites were included on 2011 National Fossil Day artwork), graptolites, and echinoderms.

Why so many marine invertebrate fossils? North America was near the equator during much of the Paleozoic, meaning many of the shallow seas were warm and teeming with life. Lots of living things leads to lots of fossils. Following the development and diversification of hard parts in the early Cambrian, invertebrate animals were much more likely to be preserved as fossils.

Grand Canyon NP Paleozoic Coquina
Marine invertebrates are fantastically diverse and immensely important to paleontology and modern society. These fossils are part of a limestone "coquina" from Grand Canyon National Park (Arizona). US Geological Survey photo.

Useful Fossils

Although they don't star in many feature length films, Paleozoic marine invertebrates were swimming, crawling, burrowing, building reefs, attacking prey, filter-feeding, stinging, exploiting new habitats, constructing elaborate shapes and shells, and evolving for hundreds of millions of years before terrestrial animal fossils appeared during the Devonian.

Paleozoic marine invertebrates are immensely important to the science of paleontology. Because many species of marine invertebrates were widespread and abundant, but only lived for a relatively short (geologically-speaking) period of time, they make excellent index fossils. Index fossils are used to determine the age of sedimentary rocks and correlate them across a continent or around the world. Massive reefs and other limestone deposits constructed or inhabited by marine invertebrates are important source rocks for petroleum as well as sources of immense quantities of building stones. Therefore, much of our modern society is fueled by, or built upon, fossil remains of Paleozoic marine invertebrates!

Go See Them!

Numerous National Fossil Day partner fossil sites and museums are excellent places to learn about the fossils on the 2013 artwork. For example, the first discovered eurypterid fossil (the state fossil of New York) and the largest complete eurypterid fossil are both at the Paleontological Research Institution's Museum of the Earth. Trilobites are featured on the logos of many NFD partners. You can see bryozoans and rugose corals at Falls of the Ohio State Park (Indiana). A variety of Paleozoic invertebrates can be discovered at Mineral Wells Fossil Park (Texas).

Overall, Paleozoic marine invertebrates are incredibly common fossils and can be found in just about every natural history museum and rock shop in the country. You can also see them in many building stones. For example, the Indiana Limestone is a popular building stone (see it on the National Mall and the Empire State Building) nationwide that is chock full of fossils. Rock outcrops of Paleozoic limestone are commonly fossiliferous.

2013 Paleozoic Partner feature articles:

| January: Fossils of the 2013 National Fossil Day Artwork | February: Paleontological Research Institution, Museum of the Earth | March: Falls of the Ohio State Park | April: Field Museum of Natural History, Mazon Creek Collection | May: Prehistoric Trackways National Monument | June: Cincinnati Museum Center | July: Glacier Bay National Park & Preserve | August: University of Michican Museum of Paleontology, Silica Formation Fossils | September: Yale Peabody Museum of Natural History, Beecher's Trilobite Bed | October: Guadalupe Mountains National Park | November: Utah Geological Survey, Millard County Cambrian Fossils | December: Denver Museum of Nature and Science, High-Altitude Mass Extinction |

Learn about more National Fossil Day artwork:

  • The NFD logo features a titanothere.
  • The 2011 artwork features a mosasaur and ammonites.
  • The 2012 artwork features a mammoth.
  • The 2013 artwork features Paleozoic marine invertebrates.
  • The 2014 artwork features Mesozoic ecosystems
  • The 2015 artwork features Cenozoic life and landscapes
  • The 2016 artwork features Pleistocene life and landscapes
  • Last updated: January 15, 2013