Temporal range: Campanian
|A model of the species on display at the Bollinger County Museum of Natural History|
Baird & Horner, 1979
Hypsibema missouriensis (
The species is estimated to have had around 1,000 small teeth,   weighed 3–4 short tons (2.7–3.6 t)  (or around as much as an elephant today), stood 10 feet (3.0 m) tall at its back, and stretched about 30–35 feet (9.1–10.7 m) from head to tail.   H. missouriensis lived in what is now southeast Missouri during the Campanian age   of the Late Cretaceous period. It was not a carnivorous species, its teeth were more serrated than other hadrosaurs, an indicator that the vegetation of Missouri at the time was very coarse or tough.  
Paleontologist Charles Whitney Gilmore and geologist Dan R. Stewart described the caudal vertebrae retrieved from Missouri in a 1945 Journal of Paleontology report, writing, "Caudal vertebrae amphicoelus; centra longer than wide; ends having concave central areas decorated with radiating ridges and depressions surrounded by a flattened peripheral border; chevron facets only on posterior ends." Of the thirteen adult tail bones, twelve appeared to be consecutive, and the smallest centrum was 69 millimetres (2.7 in) long. 
Remains of Hypsibema missouriensis were first discovered in Bollinger County, Missouri by members of the Chronister family while they were digging a cistern, and were subsequently collected by Stewart,   later nicknamed "Dinosaur Dan."  In 1942, Stewart, of the Missouri Geological Survey, had been examining clay near Glen Allen when he came upon a boy who led him to the family at work digging.   According to Stewart, property owner Lulu Chronister had found several "unusual" bones while digging and had saved them. They had been found about 8 feet (2.4 m) deep in the Chronisters' well, which had an overall depth of 24 feet (7.3 m), "imbedded in a black plastic clay."  Stewart reported his discovery to the Smithsonian Institution, which bought the remains—thirteen vertebrae of a dinosaur's tail—from Chronister for US$50, which was later used to purchase a cow.   Two other bones, of unknown type, were also recovered from the site, while one additional vertebrae had been given by Lulu Chronister to a friend.  At the Smithsonian, the bones were analyzed but the species from which they originated was incorrectly identified.  
The site where the bones were found was largely untouched by paleontologists until around 1990, when excavations restarted. Remains of other dinosaurs, fish, turtles, and plants have also been found, including teeth belonging to a member of the Tyrannosauroidea.    Other parts of H. missouriensis, including dental remains  and part of a jaw, have also been found.  The variety of faunal remnants found at the Chronister site suggest that a large body of water once existed close to the area. 
Guy Darrough, a paleontologist from St. Louis, Missouri currently working at the dig site, said it was "pretty much a miracle" that dinosaur bones were found in Missouri, because the state's soft soil has resulted in the deterioration of most prehistoric remains.   However, some of the remains found have been damaged by erosion and other processes.  While much of Missouri lies upon rocks from the Paleozoic or Precambrian eras, the Chronister site is situated over Mesozoic rock.  Stewart, who found the bones after being assigned to study the origins of clay in the southeastern portion of the Ozarks, was able to conclude that part of the region lies upon deposits from the Upper Cretaceous period, although much of the sediment from that time period has eroded away. 
The Chronister family dug the well (which they ultimately abandoned after it was unable to provide enough water) just southwest of their farmhouse, atop a body of limestone. The farmhouse was located near the bottom of a steep valley, sitting atop the remains of a terrace. The layer of clay in which the bones were found was described by Stewart as being 9 feet (2.7 m) thick, situated below 7 feet (2.1 m) of yellow-brown clay and gravel at the surface, and above a dense mass of limestone. 
The Chronister dig site is located atop the Ripley Formation.  The land around the Chronister home and dig site sits atop sandstone from the Lower Ordovician Period or Canadian Epoch. It is located in a section of the Ozarks affected by erosion and filled with chert and sandstone debris. In addition, the region has been impacted by frequent faulting, leading to the combination of rocks from different geologic periods. As a result, it is difficult to create an accurate geologic map of the area. 
Gilmore, at the Smithsonian, along with Stewart, first described the species as a sauropod in the January 1945 issue of the Journal of Paleontology,  a classification made in error and without positive evidence.   Gilmore only deemed the species a sauropod by process of elimination; when he was left with the possibilities of Hadrosauridae and Sauropoda, he dismissed the former, saying, "The more elongate centra of the Chronister specimen, with the possible exception of Hypsibema crassicauda Cope, and the presence of chevron facets only on the posterior end appear sufficient to show that these vertebral centra do not pertain to a member of the Hadrosauridae." 
The species, first called Neosaurus missouriensis, was renamed to Parrosaurus missouriensis later that year by Gilmore and Stewart  because the name "Neosaurus" was preoccupied.   However, Gilmore died soon after, and the bones were left untouched for several decades. 
Parrosaurus missouriensis was once again moved in 1979, to the genus Hypsibema, this time by Donald Baird and John R. Horner.   In the late 1970s, Bruce L. Stinchcomb, a geologist, traveled to the Chronister site after reading about Gilmore's report in the 1950s. He was able to purchase the property from a member of the Chronister family,  and in the 1980s, test excavations were performed by Stinchcomb, David Parris, and Barbara Grandstaff, leading them to conclude that H. missouriensis was actually a hadrosaur rather than a sauropod.   Thomas Holtz has suggested reverting to Parrosaurus for this species. 
On January 21, 2004,  a bill was introduced in the Missouri House of Representatives by State Representatives Rod Jetton  and Jason Crowell.  Jetton had originally proposed the hadrosaur as the state dinosaur, but was not specific enough, so the House Conservation and Natural Resources Committee settled on Hypsibema missouriensis.  The bill was then sent to the 92nd Missouri General Assembly.  It passed the Missouri House of Representatives on March 8, 2004 with a vote of 147–4,  the Missouri Senate on May 14, 2004 with a vote of 34–0,  and was approved by then-governor Bob Holden on July 9, 2004.   The bill, House Bill 1209, went into effect August 28, 2004.   Missouri became the sixth U.S. state to have designated an official state dinosaur, following Colorado, Maryland, New Jersey, Texas, and Wyoming, as well as the District of Columbia. 
In 2005, representatives from Bollinger County businesses and local government officials met in an effort to generate more revenue, and came up with a dinosaur-centered tourism campaign. Some businesses contributed to the creation of a billboard along Interstate 55 that would advertise, "Bollinger County, Home of the Missouri Dinosaur."  The Bollinger County Museum of Natural History, which displays some of the bones found,  has said their exhibit on the species has attracted tourists from other parts of the United States,  and the museum says the designation of H. missouriensis as the state dinosaur resulted in a tripling of visitors. 
In March 2008, construction on a full-size model of a H. missouriensis was completed and placed on display at the museum. Jetton, then Speaker of the Missouri House of Representatives, sponsored a dinner event for state legislators to celebrate the completion of the exhibit on March 7, 2008.  The two-year project was directed by Darrough, who was also in charge of excavations at the Chronister excavation site, and is the only permanent museum exhibit to feature the species. At the opening of the exhibit, Jetton mentioned that he hoped the dig site would become part of a state park one day.  Currently, excavation is being conducted by the Missouri Ozark Dinosaur Project.   The site has been covered to prevent water from flowing over dig material.   The Chronister dig site near Glen Allen, currently under private ownership  by Stinchcomb,  is the only location in Missouri where dinosaur bones have been found.  
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