Paleontology in Arkansas refers topaleontological research occurring within or conducted by people from theU.S. state ofArkansas. Thefossilrecord of Arkansas spans from theOrdovician to theEocene.[1] Nearly all of the state's fossils have come from ancientinvertebrate life.[1] During the earlyPaleozoic, much of Arkansas was covered by seawater. Thissea would come to be home to creatures includingArchimedes,brachiopods, andconodonts. This sea would begin its withdrawal during theCarboniferous, and by thePermian the entire state was dry land.Terrestrial conditions continued into theTriassic, but during theJurassic, another sea encroached into the state's southern half. During theCretaceous the state was still covered by seawater and home to marine invertebrates such asBelemnitella.[2] On land the state was home to long neckedsauropoddinosaurs, who left behind footprints andostrich dinosaurs such asArkansaurus.[3]
During theCenozoic the state's seas were inhabited by marine invertebrates andsharks, although the waters were gradually shrinking away. During theIce Age, the state's climate cooled. Localgrasslands andforests spread that were inhabited by creatures such asmammoths,mastodons, andgiant ground sloths.
NoPrecambrian fossils are known from Arkansas, so the state's fossil record does not begin until thePaleozoic. During the Paleozoic northern, central, and western Arkansas was covered by seawater.[4] During theOrdovician, the life inhabiting modern Arkansas was diverse. Remains of life from this period fossilized in theEast Lafferty Creek andCushman areas ofIndependence County.[5]Silurian Arkansas was also home to a great diversity of life. Remains from this period, like the Ordovician, were preserved in the Cushman area of Independence County.[5] During the ensuingDevonian period,conodonts andSporangites were preserved atCaddo Gap inMontgomery County.[5]
Carboniferous Arkansas was home toArchimedes and a variety ofbrachiopods. This fauna was preserved in theWest Fork area ofWashington County and theFayetteville area. TheHabberton area also preserves a diverse invertebrate fauna from this time period.[5] During the early part of the Carboniferous, theMississippian, Arkansas was home to a variety of marine invertebrates.[6] Later in the period, the northern region of the state was exposed as dry land by the gradual withdrawal of the sea. Rivers flowed across this area of the state.[4]Pennsylvanian Arkansas was home to theblastoidPentremites, the brachiopodsComposita andSpirifer, and other invertebrates.[7] The sea had completely vanished by the start of the Permian. With the final retreat of the sea, local sedimentation had stopped and begun being eroded away.[4]
Northern and central Arkansas was a terrestrial environment during theTriassic. However, by the earlyJurassic seawater covered the southern region of the state. Southern and northeastern Arkansas were likewise covered by the sea into theCretaceous. Local life left behind many fossils.[4] During theEarly Cretaceous, the region of Arkansas southeast of theOuachita Mountains was submerged by theGulf of Mexico.[8] The invertebrates of Arkansas's Cretaceous sea includedclams,echinoids,oysters, andsnails.[4] During the Cretaceous, Arkansas was home toBelemnitella,Exogyra,Ostrea,Turritella, and other marine invertebrates in theArkadelphia area ofClark County.[1]Fish of the same age left behind teeth in the area nearSaratoga inHempstead County. Other fossils in this area are similar to those of the preserved in contemporary deposits near Arkadelphia.[1] During the Cretaceous Arkansas was home to the sea turtleBothremys, which may have fed on ancientsnails.[9] During the Late Cretaceous the region now occupied by the Quachita Mountains of Arkansas may have attracted long neckedplesiosaurs from hundreds of miles away as a source ofgastroliths.[10] Other vertebrates includedsharks.[4]
Southwestern Arkansas was part of the coastline to theWestern Interior Seaway during theCretaceous.[11] Dinosaurs roamed the ancient shorelines here both as individuals and in groups. Manyfootprints left by coastalsauropods have been preserved in the southwestern part of the state.[8] The ornithomimosaurArkansaurus fridayi lived at this time, known only from remains of its foot discovered in 1972.[12]
The seas of southern and eastern Arkansas began to shrink during the earlyCenozoic. Sharks and oysters were still present in its sea waters.[4] During theEocene epoch, Arkansas was home to a marine invertebrate fauna that includedechinoderms,Ostrea, andTurritella. The echinoderm fossils were preserved in theBradford area ofJackson County. TheOstrea fossils were preserved in the Bradford area of Jackson County and in theForrest City area ofSt. Francis County. TheTurritella fossils were preserved in the Bradford area of Jackson County.[1] The sea gradually and inconsistently retreated toward the south, leaving the state altogether by the mid-to-lateTertiary. By this timerivers andswamps covered the southern half of the state. During theIce Age, the state's climate cooled down. Most of Arkansas was covered by grasslands and forests. These were inhabited by creatures includinggiant ground sloths,mammoths, andmastodons.[4]
In August 1972, J. B. Friday discovereddinosaur bones in aSevier Countygravel pit nearLockesburg.[8] In 1973 the fossils Friday discovered were being cleaned and compared to related by dinosaurs byUniversity of Arkansas professor James Harrison Quinn. In the course of his research, Quinn made hypothetical clay models of the missing bones in the animal's foot and duplicated the actual fossils with plaster cast in latex molds. Quinn published an abstract about the bones and nicknamed the animal "Arkansaurus fridayi".[13] This abstract was the first scientific mention of any dinosaur bones ever found in Arkansas.[12]
Agypsum quarry in southwestern Arkansas preserves dinosaur tracks, which have hindered the quarry operations. This is called the Briar Site. Some areas of the quarry were rough due to what appeared to be largepotholes, which frustrated the drivers of the excavation equipment. In 1983, however these "potholes" were recognized asdinosaur footprints. Jeff Pittman was responsible for the correct identification of the tracks.[14] At the time he was studying the quarry'ssedimentology. After he visited the Purgatoire tracksite inColorado he noticed that the localsauropod tracks resembled the "potholes" seen at the Briar site in Arkansas.[15] Pittman later performed an aerialsurvey and found evidence for 10 parallel sauropod trackways on a rock surface that had also been extensively "trampled".
The Briar quarry has two separate surfaces that each preserve thousands of dinosaur tracks. The operations of the quarry continue to both uncover and destroy dinosaur footprints. In 1989 Pittman successfully dated the dinosaur tracks of the Briar quarry as equal in age to the lowerGlen Rose Formation's megatracksites, which are more than 200 miles away.[16]
In the fall of 1995 senior paleontologist of the Utah Geological Survey (then withDinamation International),James Kirkland, examined the Arkansaurus foot and found it to be larger but otherwise nearly identical to a new species found two years prior inUtah rocks of Early Cretaceous age. Kirkland concurred that Arkansaurus was likely related toOrnithomimus. Future clues about Arkansaurus may come from a similar dinosaur discovery inMaryland that, as of 2007, has yet to be described.[17]
From 2001 to 2003, then University of Arkansas undergraduate, ReBecca Hunt, studied the fossils and presented on them at several meetings, including the 2002 Society of Vertebrate Paleontology Annual Meeting, the 2002 Geological Society of America Annual Meeting, and the 2003 Arkansas Undergraduate Research Conference. Hunt published a brief description of the fossils with the limited scientific research available at the time.[18]
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