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The Arlington Archosaur Site (AAS) is a fossil site that produces Cretaceous age fossils of dinosaurs and crocodyliforms. It was discovered in Arlington Texas by UT-Arlington (UTA) students Phil Kirchhoff and Bill Walker along with local residents Art and Olivia Sahlstein in 2003. The site was later reported by Phil Kirchhoff to Dr. Christopher R. Scotese, a professor of geology at the UTA Earth & Environmental Sciences Department and his geology graduate student Derek Main in the fall of 2003. Phil Kirchhoff and Bill Walker brought some of the fossils they found at the site to show to Derek Main for identification. Main identified the fossils as ornithopod dinosaur teeth & vertebrae, along with croc scutes & teeth. The report of dinosaur and croc fossils in Arlington led to the origin of the name for the site, as dinosaurs and crocs belong to the archosaurs; or the "ruling reptiles". The AAS became Main's graduate research at UTA (both his masters and doctorate). He and the sites discoverers agreed to work the AAS as a collaborative effort and contacted the land owner for permission to use the land. The exploration and excavation of the site was however delayed by several years due to refusal of the land owner to grant access to the land. The delay was unfortunate, and dissapointing. However a new land owner later granted access to the land in 2007 and in the years to follow, it became one of the largest urban excavations the State of Texas has ever seen.

The excavation of the Arlington Archosaur Site began in the spring of 2008 when a new land owner, the Huffines Group, granted land access to UTA. Once permission was given, students from the UTA Dinosaurs and Earth History classes worked with Derek Main and the sites discoverers, Art Sahlstein and Phil Kirchhoff, to explore the area for fossils. A full scale excavation began in the spring and continued on through the summer and fall with the help of volunteers from the Dallas Paleontological Society. With volunteer assistance from Dallas Paleo Society (DPS) members, UTA student diggers and local volunteers, the excavation of the site continued on through the fall of 2011. All excavated fossils were taken to UTA for curation and scientific study as part of Derek Main's graduate research. The excavation of the site continues, with UTA students excavating the site for class credit along with DPS members and local volunteers.

The AAS has produced numerous fossils from various animals, the most prominent of course being from dinosaurs. A skeleton of a large herbivorous dinosaur was excavated from the northern hillside at the site. The herbivorous dinosaur was an ornithopod, a large three toed "duck billed" hadrosauroid (hadrosaur, with some iguanodont characters). The fossils found at the site were primarily post cranial (everything other than the skull), although a small partial lower jaw (dentary) and shed teeth were found. The dinosaurs pelvis (hip), limb bones, shoulder and most of the vertebrae were excavated from the hillside (Main et al, 2011). The only duck billed dinosaur known from the Woodbine Formation is Protohadros byrdi. Protohadros was discovered in Woodbine Formation sediments in the North Texas town of Flower Mound by Gary Byrd in the early 1990's (Head, 1998). A complete skull was recovered from a roadside contruction site (FM 2499) and taken to the Shuler Museum of Paleontology at SMU for scientific study (Head, 1998). SMU graduate student Jason Head studied the dinosaur skull for his masters thesis and named it Protohadros, meaning primitive hadrosaur (Head, 1998). Hadrosaurs evolved from iguanodonts at some point during the Early to Mid-Cretaceous (110-90 million years ago). Protohadros is important to science as it is a transitional species, or "missing link", in the evolution of iguanodonts into hadrosaurs. The AAS hadrosauroid (duck billed dino) is possibly a new taxon, a new dinosaur that is closely related to Protohadros (Main et al, 2011). It is however still under study. Other dinosaur fossils found at the site belong to a carnivorous dinosaur, a theropod ("beast foot"). Theropod fossils are rare in the Woodbine Formation, and as yet a theropod has not been named. The AAS theropod is represented by a partial femur, a phalange, several vertebra, teeth and claws (Main et al, 2011).

Crocodile (crocodyliform) fossils are among the most common found at the Arlington Archosaur Site. Most of the croc fossils found at the AAS are scutes (osteoderms) and teeth. The only named taxon of croc from the Woodbine Formation is Woodbinesuchus; literally "Woodbine crocodile" (Lee, 1997). Woodbinesuchus was named by SMU graduate student Young Nam Lee in 1997 for fossils found in Fort Worth, a partial skull and post crania that included numerous osteoderms(Lee, 1997). The AAS has produced numerous croc fossils, some of which can be assigned to Woodbinesuchus, others however represent a new genus and species. Of interseting note was the discovery of a large new croc in 2009. In the summer of 2009, local student Austin Motheral discovered a large skeleton at the base of the AAS hill while using his fathers tractor to remove overburden from the site. His discovery led to a week long marathon dig that the crew called Crocorama. The Crocorama croc is currently under study (2012), but likely represents a new taxon of Cretaceous croc that is related to the goniopholids of the Late Jurassic and Early Cretaceous (Noto et al, 2012).

The AAS is unique in that it is a major excavation in the middle of a large metropolitan setting and in that it preserves so many fossils from different animals. In fact, it preserves a near complete coastal ecosystem. Aside from dinosaur and crocodyliform fossils, the AAS also preserves fossils from turtles, lungfish, fish and sharks. The fossil turtles are found in the same peat bed as the Crocorama croc, some of which have croc bite marks (Noto et al, 2012). The bite marks are attributed to a large croc and show evidence an interesting feeding behavior (Noto et al, 2012). The AAS lungfish was discovered by Brad Carter while surface collecting fossils at the site. Lungfish from the Cretaceous of North America are quite rare, and the AAS lungfish represents a new species (Main et al, 2012). Throughout the site, coprolites (fossil feces) have been found, representing nearly every animal. Thin sections cut of the coprolites have revealed interesting aspects on the animals diets, as some coprolite samples contained bone fragments. Within a peat bed at the base of the Dinosaur Quarry, numerous fossil logs and plant material were found in the 2008 & 2009 excavations. The carbonized logs varied in size, but the longest were 3-4 meters (9-12 ft) and represent evidence of an ancient mangrove, or coastal swamp (Main, 2009).

The AAS lies within the Cretaceous (Cenomanian; 95-100 million years of age) rocks of the Woodbine Formation. The Woodbine Formation was mapped by R. T. Hill (often referred to as the "Father of Texas Geology") and named by Hill for the small Texas town of Woodbine in 1905. The Woodbine Formation preserves ancient coastal environments from a time of higher sea level than the present. A time in which what is now known as the Gulf of Mexico extended north through Texas and on northward, into Canada and on to the Arctic. The geologic term for sea level change is eustasy; eustatic change or eustatic high stand. The Earth's sea level has changed numerous times, the Cretaceous eustatic high stand is but one example. Many of the sediments from the Woodbine preserve coastal, deltaic environments, or swamps from a Cretaceous eustatic high stand. The sediments range from sandstone to mudstone,siltsone, paleosols (ancient soils) and peat. The fossils from the AAS are found within the mudstones, paleosols and peat beds of an ancient delta plain. The AAS was once a coastal swamp from a delta that spilled sediment into the shallow waters of the Interior Seaway nearly 100 million years ago. For more on the Woodbine Formation online, see the Wikipedia page.

The Arlington Archosaur Site is both an ongoing research project and a source of Earth Science education and outreach to the North Texas community. To volunteer for a dig at the Arlington Archosaur Site, join the AAS Facebook Group or "like" the AAS Facebook fanpage for dig dates and information. There is also a webpage with information and photos of the digs: