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This week is Earth Science Week, with National Fossil Day on Wednesday. The Museum of Discovery is holding its second annual National Fossil Day event on Saturday, the 17th, between 10 am and 3 pm. So in honor of the week and in preparation for the museum event on Saturday, I thought I would briefly talk about what I consider the three most famous fossils found in Arkansas. You may notice this list is exclusively vertebrates. That is because of the rather large bias in popularity vertebrates have over invertebrates. Vertebrates are much less common in Arkansas than invertebrates, but they get almost all the press. Let me know in the comments section if you have any other contenders.
The first contender for Arkansas’s most famous fossil is Arkansaurus, the only dinosaur to have been found in the state. Found in 1972 in Sevier County, the only bones found comprised the front half of one foot. Despite considerable searching, nothing else has ever been found. The lack of diagnostic bones has made it impossible to determine exactly what kind of dinosaur it was. All that can really be said is that it is some kind of coelurosaur, a type of theropod, but not a tyrannosaurid, ornithomimid, or any other more derived form related to birds. We can also say it was a medium-sized dinosaur, meaning it wasn’t terribly small, as the front half of the foot measures just over two feet long. A statue was made by Vance Pleasant, which was recently seen at the Museum of Discovery as part of a dinosaur exhibit. How accurate is it? It’s a reasonable estimate based on what we know right now, which is not much, except that the real animal probably had some form of feathers not seen on the statue. The fossils are currently housed at the University of Arkansas at Fayetteville.
My vote for the Arkansas fossil that is more widely known outside the state better than inside it is Ozarcus, a primitive shark found by the paleontologists Royal and Gene Mapes in the Ozark Mountains near Leslie. The reason for the fame of this fossil is that it is the oldest known shark fossil that preserves the gill supports, known as the branchial basket. These normally do not preserve because they are made of cartilage, much like most of the rest of the shark skeleton. The gill supports here indicated that both sharks and osteichthyans, or bony fish, evolved from an ancestor that looked more like bony fish than it did the cartilaginous sharks, meaning that the original sharks were not primitive to bony fish, but possibly evolved after the appearance of bony fish. Due to this, Ozarcus got international coverage and became well known to paleontologists. The fossil currently resides at the American Museum of Natural History in New York.
The last contender for Arkansas’s most famous fossil is the Hazen Mammoth, the only mammoth known from the state. Found in 1965, it consisted of the skull, tusks, and some vertebra. There was a lot more of the skeleton found, but unfortunately, the bones were very soft and were severely damaged or destroyed before they could be collected. The bones were identified as Mammuthus columbi, or the Columbian Mammoth, a less hairy version of the more commonly known woolly mammoth, indicating warmer temperatures than found in areas in which the woolly mammoth is known. Even though only one mammoth has been found in Arkansas, upwards of two dozen mastodons have been found. Mastodons were smaller cousins of the mammoths and preferred forest habitats over the grassy plains in which the mammoths lived. This provides evidence that much like today, the state was mostly forested during the Pleistocene Period in which they lived. Today, the mammoth is a resident of the University of Arkansas at Fayetteville.
So what would you call the most famous fossil of Arkansas?
Welcome to the first of a series on Arkansas fossils. Arkansas is not generally known as a mecca for dinosaur lovers. Most of the dinosaurs in Arkansas are statues created by a man named Leo Cate, which have all the accuracy of the old plastic toys on which he based the statues, which is to say, not much (of course, he made them for enjoyment, not as anatomical models, so they serve their purpose). Nevertheless, dinosaurs are the first thing I get asked about when I give talks in schools, so I decided to start off with a discussion of our one and only dinosaur, called “Arkansaurus fridayi.”
Ordinarily, I would not delve into how a fossil was found here, but because Arkansaurus is unique and illustrative of how many fossils are brought to the attention of science, a brief synopsis of the story of how it was brought to the attention of science may be of interest. In August, 1972, Joe Friday was searching for a lost cow on his property near Lockesburg in Sevier County, when he found some bones eroding out of a shallow gravel pit. He showed them to a Mr. Zachry, whose son, Doy, happened to be a student at the University of Arkansas in Fayetteville. Doy showed the bones to Dr. James H. Quinn, a professor at UA, who identified them as part of the foot of a theropod dinosaur. He contacted the Arkansas Geological Survey and Dr. Quinn, Ben Clardy of the AGS, and Mr. Zachry went back to the site where they found the rest of the bones. Dr. Quinn presented the bones at a meeting of the Society of Vertebrate Paleontology where he discussed the bones with Dr. Edwin Colbert, a noted paleontologist who was an expert in dinosaurs and vertebrate evolution. They came to the conclusion that the bones probably came from some type of ornithomimid, a group of ostrich-like dinosaurs (the name literally means bird-mimic), one of which, named Gallimimus, was made famous in Jurassic Park. Despite further excavations, no additional bones have been found. Dr. Quinn never officially described the bones, publishing only an abstract for a regional meeting of the Geological Society of America in 1973. It remained for Rebecca Hunt-Foster, now a paleontologist for the Bureau of Land Management, to publish the official description 30 years later in the Proceedings Journal of the 2003 Arkansas Undergraduate Research Conference.
The first thing to know about this particular dinosaur is that “Arkansaurus fridayi” is not its real name. In fact, it doesn’t even have an official name. The reason for this is because all we have of it is part of one foot. Specifically, we have the metatarsals, a few phalangeal bones, and the unguals. In non-science speak, on humans, they would refer to the bones making up the front half of your foot. The metatarsals are the long bones the toes are attached to forming the front part of the arch, the phalanges are the toes, and the unguals are the bony cores of the claws. The pictures show the actual bones and a cast, in which the missing phalangeal bones have been restored. The real fossil has all the phalangeal bones connecting to the metatarsals and all the unguals, but a couple of the middle phalanges are missing. We have no ankle bones and nothing at all of the rest of the animal. With such little to go on, it has been difficult to determine exactly what kind of dinosaur it is, so no scientist has been comfortable giving it an official name yet. To add to the complications, not a whole lot of feet from theropod dinosaurs are known, so good comparison material is limited, and little is known about theropods in the southern United States to begin with. (Aside: dinosaurs are separated into two groups. The Ornithischia, which are comprised of the herbivorous, mostly four-footed dinosaurs; and the Saurischia, which include the giant, long-necked sauropods and the bipedal, mostly carnivorous theropods.)
So why only one foot? What happened to the rest of it? I’ll let Rebecca Hunt-Foster explain it, as she did an excellent job: “There are several possibilities that would explain the occurrence of a single foot at the Friday site. It is a possibility that the rest of the Friday specimen could be gravel on highway 24. Road crews could have cut into the Trinity Group (Ed. Note. The rock formation in which the bones were found) when excavating the Quaternary gravel that lies directly above it, when building the road in 1954. As another theory, the animal may have begun to decompose before its body was carried by water to the site of deposition. Consequentlly, bits and pieces could have been scavenged by predators in the Lower Cretaceous, resulting in only a single foot remaining for preservation. Finally, it is possible that the entire specimen was preserved but that most of the skeleton was lost to Pleistocene erosion.” So just think about that the next time you go driving down the road. What fossils might you be driving upon?
Even if we don’t know for sure what it is, we do have some clues and can narrow down, at least a little, what it might be. What we know for sure is that it is some kind of coelurosaur. That, unfortunately, doesn’t help us a lot because coelurosaurs cover everything from little compsognathids to giant tyrannosaurs to modern birds, known principally for having bigger brains than earlier theropods, slender feet with three toes, and many of them had feathers. It does tell us it is not closely related to dinosaurs like allosaurs and spinosaurs, nor to early theropods like ceratosaurs and Coelophysis. Dr. James Kirkland opined that it was similar to Nedcolbertia, a small coelurosaur found in Utah. The problem here is that no one knows much more about Nedcolbertia either and its relationships to other dinosaurs are unclear. Quinn and Colbert thought it may have been an ornithomimid, but closer inspection by Rebecca Hunt-Foster and comparison with known ornithomimids indicates this is unlikely. Right now, all that can really be said is that it is likely a small coelurosaur, but not a tyrannosaurid, ornithomimid, or advanced form more closely related to birds, which leaves a small group of poorly known coelurosaurs no one really knows what to do with.
Using these animals as a comparison, what can we say about what kind of animal “Arkansaurus” was? It was likely a fast runner with probably an omnivorous diet, eating smaller animals and supplementing its diet with plants. It would likely have stood somewhere between 2-4 meters (6.5-13 feet) tall. It would have looked something like an ostrich with long arms ending in hands with three functional fingers, with one of them being at least semi-opposable, and a jaw filled with small teeth. If it had feathers (which seems increasingly likely), the feathers would have looked more like fur than the large feathery plumage seen on ostriches today. It would also have had large eyes like ostriches, with excellent color vision, based on the fact that its nearest living relatives, crocodilians and birds, all see a broad spectrum of colors (even better than humans).
The rocks the bones were found in were part of what is called the Trinity Group. These rock layers (or strata) consist of layers of sand, clay, gravel, limestone, and gypsum laid down in the Early Cretaceous Period, roughly around 100-120 million years ago (what is known as the Albian and Aptian Ages). The rocks indicate that during the time the rocks were formed, the environment was a shallow marine coastal area not unlike south Texas near the Rio Grande or in the Persian Gulf. Our dinosaur would certainly not have been alone. There were other dinosaurs in the vicinity, we just know very little about them. Sauropods left thousands of tracks in the coastal sediment forming a massive trackway found in a Howard Country gypsum mine in 1983. Another trackway found in 2011 has tracks from sauropods such as Pleurocoelus and Paluxysaurus (which may or may not refer to the same species and may or may not also be called Sauroposeidon) as well as tracks from what was probably the giant theropod Acrocanthosaurus.
Most of the information and images in this post not directly linked to came from the following sources. Many thanks to Rebecca Hunt-Foster for clean pictures from her paper, which she also graciously supplied.
Hunt, ReBecca K., Daniel Chure, and Leo Carson Davis. “An Early Cretaceous Theropod Foot from Southwestern Arkansas.”Proceedings Journal of the Arkansas Undergraduate Research Conference 10 (2003): 87–103.
Braden, Angela K. The Arkansas Dinosaur “Arkansaurus fridayi”. Little Rock: Arkansas Geological Commission, 1998.
The top image is a Leo Cate T. rex. Photo by Debra Jane Seltzer, RoadsideArchitecture.com.
UPDATE: Arkansaurus has recently been named the Arkansas official state dinosaur, reviving interest in the fossil. It is currently being re-examined by Dr. Rebecca Hunt-Foster, with the hopes that new fossils and information that has come to light since her last publication will provide a more refined determination of its relationships.