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Exploring the Fossils of Arkansas USA

Arkansas is known for its diverse and complex heritage, geography, geology and fossil record that reflects the state's location at the intersection of several different and very large geological provinces. Arkansas is known for its hot springs, diamond mines but also for the Early Cretaceous Arkansaurus fridayi theropod known as the "Arkansas Lizard" the official state dinosaur.

Three toed dinosaur footprint track
Three toed dinosaur footprint track

Early French settlers and explorers named the first nation tribe of Quapaw or OO-GAQ-Pa translated as "people who live downstream" as the “Arkansaw”. Among the other first nation tribes included the Caddo, Chickasaw, Osage and Tunica. Among the oldest was the Plum Bayou who built religious centres of earthen platforms or mounds preserved at the Plum Bayou Mounds Archeologically State Park.

Arkansas became the 25th state to join the United States in June 1836 some 33 years after the land deal between the United States and France known as the Louisiana Purchase. Under the 3rd United States President Thomas Jefferson, an American Founding Father principal author of the Declaration of Independence, the U.S. acquired in 1803 approximately 827,000 square miles of land west of the Mississippi River for US$15 million nearly doubling the geographical size of the United States.

Arkansas is bordered by Missouri in the north, Tennessee and Mississippi in the east, Louisiana in the south, and Texas and Oklahoma in the west. Its eastern border is almost completely bordered by the Mississippi River.

Arkansas National Parks - Image by U.S. National Park Service
Arkansas National Parks - Image by U.S. National Park Service

One of the most prominent geological features of northern and western parts Arkansas is the Ozark Mountains. The Ozarks are a series of low, rugged hills that are composed primarily of sedimentary rock, including sandstone, limestone, and shale. The Ozark Mountain region rises in the northwest, and it’s a forested region and features Mammoth Spring State Park.

Waterfall in the Ozark Mountains - Image by Connor Wilkins
Ozark Mountains Waterfall - Image Connor Wilkins

The other province is the Arkansas River Valley, which runs through the centre of the state bounded by the Ouachita Mountains to the south and the Ozarks to the north and characterised by a mix of sedimentary and igneous rock formations.

The Arkansas River Valley is south of the Ozark Mountains and includes the large tributary of the Mississippi River known as the Arkansas River. This area also has Mount Magazine, Arkansas’s highest point at 2,753 feet within the Ozark-St. Francis National Forests.

The oldest rocks in Arkansas are Precambrian in age and are found in the Ouachita Mountains. These rocks are primarily composed of granite, gneiss, and schist, and are believed to have formed around 1.4 billion years ago.

The Ozark Mountains, which cover the northern part of the state, are primarily composed of Palaeozoic sedimentary rocks, such as sandstone, shale, and limestone. These rocks were formed during the Cambrian, Ordovician, and Silurian periods, and contain abundant fossils of marine organisms.

In the eastern part of the state, the Mississippi River Valley is underlain by unconsolidated sediments, including sand, silt, and clay. These sediments were deposited by the Mississippi River and its tributaries over the past several million years.

Rocks of Lake Ouachita, Arkansas, USA - Image by Tyler Bagot
Rocks of Lake Ouachita, Arkansas, USA - Image by Tyler Bagot

The southwestern part of the state is home to the Gulf Coastal Plain, which is a broad lowland that extends from the Gulf of Mexico to the Mississippi River. The Coastal Plain is primarily composed of sedimentary rocks, including sand, clay, and limestone, which were deposited during the Cretaceous and Tertiary periods.

Both bauxite and quartz are Arkansas’s state rock and mineral respectively. Bauxite is a type of lithified soil which is relatively low in silica and high in aluminium formed by the weathering of nepheline syenite under tropical conditions by a process of laterization. The states quartz was formed as hot waters percolated through fractured rock in the Ouachita Mountains some 245 million years ago.

Arkansas is known for its rich fossil record particularly in the Ozark Mountains. Fossils found in Arkansas span a wide range of geological time periods and include both marine and terrestrial organisms. Some of the notable fossils found in Arkansas include Trilobites, Brachiopods, Crinoids, Ammonites, colonial Graptolites and the remains of both sauropod and theropods dinosaurs from the Mesozoic era.

In addition to these fossils the State of Arkansas is also home to fossilised plants including ferns, cycads, and conifers that indicate an ancient forest once covered the state.

During the Early Cretaceous between 144 - 97 million years ago a shallow sea covered the south-eastern and south-western part of North America including modern day Arkansas. Research including the tracks of sauropods indicates that dinosaurs such as large sauropods and ankylosaurs also roamed the shoreline of this sea.

The first dinosaurs bones to be scientifically described in Arkansas was the remains of a right hind foot Arkansaurus fridayi a small, carnivorous bipedal ornithomimosaurian dinosaur found in rocks of the Trinity Formation exposed in the southwestern area of Arkansas. These dinosaur bones were found in Arkansas in 1972 by service station owner Joe B Friday on his land in Lockesburg in Sevier County.

The Arkansaurus fridayi was about the same size as an ostrich but had differences such as a long tail and clawed fingers. The dinosaur is estimated to have been about 1-meter tall and 3-meters long. It is believed to have been a type of coelurosaur, a group of small, bird-like dinosaurs.

The Trinity Formation was deposited in the Early Cretaceous Period, approximately 110 million years ago. At the time, sea level was higher and this part of Arkansas was a forested plain crossed by large rivers just north of the Gulf of Mexico.

No other dinosaurs have been named from the Trinity Formation in Arkansas, but rocks of a similar age in nearby parts of Texas and Oklahoma have produced fossils of the large raptor Deinonychus and iguanodont Tenontosaurus, found throughout most of North America. They also preserve the large carnivore Acrocanthosaurus and the long-necked Pleurocoelus.


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