Fossil Highlands
Arkansas’ most ancient fossils, which date back more than 230 million years to the Paleozoic Era, are found in the Interior Highlands. This region includes the Ozark Plateaus, Arkansas Valley, and Ouachita Mountains of northwestern Arkansas. Here are found Arkansas’ oldest rocks, some dating back 435 million years or more to the Ordovician Period. Scattered sites yield fossils dating to the Ordovician or to the younger Silurian and Devonian periods.
An enormous bulge called the Ozark Dome has been around since the Cambrian Period, long before the Appalachians were formed. Any ancient seas that covered this dome would have been shallow — the type of habitat most marine animals prefer. Numerous caves indicate the abundance of limestone, which is easily dissolved by water, in the Ozarks.
One paleontologist says the Ozarks are “crawling with fossil marine invertebrates.” Crinoids, cephalopods (animals related to squids and octopuses), blastoids, bryozoans, and other animals once lived in shallow marine waters. Closer to shore were horn and colonial corals and algal communities, some still in life positions.
The Ouachita [WASH-eh-taw] Mountains, on the other hand, were never a dome. Rather, this region was a deep sea basin. About the same time the Atlantic Coast collided with Africa and Europe, the Gulf Coast collided with South America. A deep sea basin buckled as it was raised higher and higher. Since animals are not so abundant in the deep sea as in the shallows, the Ouachitas are not as famous for fossils as the Ozarks.
Between the Ozarks and the Ouachitas is the Arkansas River Valley. Here are exposed rocks that date back more than 300 million years to the Pennsylvanian Period. Considerable quantities of coal and natural gas have been recovered in this region. Fossils of both marine animals and land plants that lived during the Mississippian and Pennsylvanian periods are found in both the Arkansas River Valley and the northern highlands.
Fossil Lowlands
The West Gulf Coastal Plain covers most of southern Arkansas. In Arkansas, it can be divided into two parts. Farthest from the sea is an area where the sediments date back more than sixty-five million years to the Cretaceous Period, when dinosaurs still stalked the land. Arkansas’ best fossil sites are here. People find the remains of mosasaurs, sharks, turtles, crocodiles, and fishes. More common are snails, bivalves (such as clams), sponges, and worm tubes.
Between this area and the sea is an area of younger Cenozoic Era sediments. Cenozoic fossils include shark and ray teeth, bryozoans, and bivalves.
The most recent epoch of the Cenozoic was the Pleistocene, or Ice Age, which ended less than 20,000 years ago. Fossils of Pleistocene animals are scattered across Arkansas. One good place to look for such fossils is along the Mississippi River Plain in eastern Arkansas. In fact, it covers about one third of the state. How can it be so big?
For millions of years, the Mississippi River has flowed into the sea, often changing course. The Mississippi River Plain represents some of the Mississippi River’s ancient routes. Along these routes, the river eroded hills and deposited mud and other sediments, patiently leveling the land.
The Ice Age in Arkansas
No Ice Age glaciers reached Arkansas. Yet eastern Arkansas is a souvenir of the Ice Age. The northern glaciers ground boulders and rocks into gravel and dust. During warmer interglacial periods, the great glaciers melted. The Mississippi and Ohio river systems carried tremendous quantities of sediment into Arkansas. The slow-moving Mississippi River also deposited many millions of tons of glacial sediments across its meandering path. Of course, the Mississippi River is still depositing sediments.
You won’t find complete skeletons along the Mississippi River. Instead, there are scattered bones and teeth of such animals as mastodons, along with petrified wood.
An Arkansas State Fossil?
Paleontologist John McFarland once asked other Arkansas geologists what fossil they thought best represents Arkansas. People who worked for the oil and gas industry thought tiny microfossils, such as forams and conodonts, should be adopted. They are important to Arkansas’ economy, because they help geologists locate oil and gas.
Other people think Arkansas should adopt a plant, to represent the coal deposits in the Arkansas River Valley and the lignites in south Arkansas.
Some consider the dinosaur Arkansaurus fridayi Arkansas’ “unofficial state fossil.” However, only bones from the animal’s right foot have been found, though a cast has been made of the animal.
Dr. John Thurmon thinks Rayonnoceras would be a good state fossil. It is a straight-shelled nautiloid found in Carboniferous deposits of north Arkansas. Some Rayonnoceras fossils are several feet long.
Dr. McFarland thinks a goniatite might be a good choice. Goniatite fossils are common in northern Arkansas. Arkanites relictus is a goniatite named for Arkansas. It is found in the Hale Formation, which dates to the Early Pennsylvanian Period.
If you remember what a nautiloid is, then you have a pretty good idea what a goniatite is. Like nautiloids, squids, and octopuses, goniatites are cephalopods. They are members of a larger group of cephalopods called ammonoids.
Like nautiloids, ammonoids were protected by hard shells. Their shells ranged from coiled to straight. Some ammonoids swam in the sea, while others lived on the seafloor.
The animals that lived inside ammonoid shells were probably more similar to squids and octopuses than to nautiloids, however. Like squids and octopuses, ammonoids had an ink sac, for example.
Ammonoids are classified into three groups by the sutures that hold the segments of their shells together. With their zigzag sutures, goniatites were the first ammonoids. They were established by the Middle Devonian Period and survived for 170 million years. At the end of the Permian Period, after the Appalachian Mountains had been raised, they suddenly declined. Goniatites survived through the Triassic Period before becoming extinct.
Ceratites, which also had simple sutures, exploded after the goniatites crashed. But the ceratites themselves became extinct at the end of the Triassic.
The third type of ammonoid, ammonites also appeared in the Triassic Period. These ammonoids had very elaborate and beautiful sutures, sometimes seeming to be the creation of an artist. They thrived through the Jurassic and Cretaceous periods when dinosaurs ruled the land.
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