Age of Dinosaurs in Iowa

Sunday, August 1, 2010

Hadrosaurs, commonly known as duck-billed dinosaurs, occupied subtropical environments in the coastal lowlands of the central United States during the Cretaceous Period, about 100 million years ago.

Did dinosaurs once live in Iowa? The simple and unqualified answer is “Yes, without a doubt!” But the actual evidence for dinosaurs in Iowa is limited to only a few fossils. Dinosaur fossils have been found in several states adjoining Iowa (Nebraska, Minnesota, Missouri, South Dakota), and wandering dinosaurs would have been unimpeded by these artificial boundaries.

Fortuitous circumstances are needed to preserve dinosaur remains in ancient sedimentary environments. Following the death of an animal, the bones need to be buried within the sediments and protected from chemical and mechanical destruction. Many great dinosaur discoveries are associated with sediments of ancient river systems, where dinosaur bones may be preserved within floodplain and river channel deposits. The discovery of fossil bones is aided by a careful understanding of a region’s geology (where to look), considerable patience (keep looking), and a significant measure of good fortune.

Dinosaurs were the dominant land animals for about 170 million years of earth history, spanning the Late Triassic, Jurassic, and Cretaceous periods. This “Age of Dinosaurs” came to a close 65 million years ago when a mass extinction ended the dinosaurs’ reign. (Almost all paleontologists consider birds to be descendents of small carnivorous dinosaurs, so a segment of the dinosaur pedigree survived this extinction.) Sedimentary deposits from the Age of Dinosaurs cover extensive portions of Iowa and have real potential to yield dinosaur fossils. The Jurassic Fort Dodge Formation was deposited at the same time as strata in the American West that have produced remarkable dinosaur fossils, but no Jurassic dinosaur fossils have yet been found in Iowa. The Cretaceous formations are more widespread in Iowa, and these same formations have produced dinosaur fossils at scattered localities in the central United States.

Iowa’s oldest Cretaceous sediments, the Dakota Formation, were deposited in ancient river systems that drained westward to an interior seaway during the middle part of the Cretaceous period, about 95 to 100 million years ago, a time of global “greenhouse” warming. Floodplains and coastal lowlands were covered with lush subtropical vegetation at that time, providing suitable habitats for dinosaurs. The first dinosaur fossil found in the Dakota Formation, a portion of a leg bone (femur), was collected in 1928 from the Missouri River bluffs near Decatur, Nebraska. This locality lies only about one mile from the Iowa border. Although this fragmentary fossil has not been assigned to a particular dinosaur species, its features are sufficient to identify it as a large ornithopod, a highly successful group of generally bipedal plant-eating dinosaurs. The proportions of this leg bone, when compared with other ornithopods, indicate a dinosaur that was about 32 feet long. This Dakota fossil likely represents an early hadrosaur. Hadrosaurs are a well-known family of “duck-billed” ornithopod dinosaurs that comprise the most abundant and diverse group of Late Cretaceous dinosaurs in North America.

Other dinosaur fossils have been uncovered from the Dakota Formation in nearby northern Kansas, eastern Nebraska, and Minnesota. A family of heavily armored ankylosaurian dinosaurs, the nodosaurids, is represented by partial skeletons of a ten-foot-long creature known as Silvisaurus. Additional hadrosaur bones have been found in Minnesota. Three-toed fossil footprints of ornithopod dinosaurs have been discovered recently in Dakota strata.

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