Age of Dinosaurs in Iowa
by Brian J. Witzke
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.
The microscopic structure of a petrified
dinosaur bone fragment from Guthrie County, Iowa, shows a once-porous
network of vascular canals for blood vessels.
Photo by Brian Witzke.
In Iowa, a fragment of fossil bone was found by the author in 1982 in ancient river gravels of the Dakota Formation in Guthrie County. The microscopic structure of this fragment (shown above) revealed densely vascularized bone, indistinguishable from that seen in typical dinosaur bone. Although not terribly impressive by itself, the Guthrie County discovery confirms that dinosaur fossils indeed occur in the Dakota Formation of Iowa.
The river deposits of the
Dakota Formation were progressively flooded by marine waters as a vast interior
seaway encroached eastward later in the Cretaceous, and a succession of younger
shale and chalk deposits were laid down on the ancient sea bottom in western
Iowa. These strata, named the Graneros, Greenhorn, Carlile, and Niobrara
formations, have yielded bones of large marine reptiles known as plesiosaurs
from several localities in Iowa. In nearby areas, these same formations have
also produced fossils of other extinct reptile groups including mosasaurs (giant
sea lizards) and pterosaurs (large flying reptiles). Although plesiosaurs are
sometimes linked with dinosaurs in the public’s imagination, plesiosaurs
actually represent an unrelated group of extinct marine reptiles. Long- and
short-necked plesiosaurs plied the waters of the Cretaceous interior seaway with
large paddle-like flippers in search of fish and other prey. The marine shales
and chalks of this seaway also yield the remains of true dinosaurs. Dinosaurs
certainly did not live at sea, but occasionally a carcass would float out to
sea, decay, and settle to the bottom. The discovery of dinosaur bones and teeth
(of hadrosaurs and ankylosaurs) in marine strata from nearby Minnesota, South
Dakota, and Kansas certainly raises the possibility of similar discoveries in
Long after extinction of the
dinosaurs, huge continental glaciers advanced and retreated across Iowa during
the “Ice Age” (the last 2.5 million years). These advancing glaciers
incorporated large volumes of rock and other sediments as they moved across the
continent. Much of the glacial material was derived from erosion of the
Cretaceous strata that underlie large areas of the northern Great Plains.
Reworked and transported Cretaceous fossils, such as plesiosaur bones and shark
teeth, are sometimes found in the glacial tills and associated gravel deposits
in Iowa, especially in the western part of the state. Two dinosaur bones can now
be added to the list of Cretaceous fossils recovered from glacially derived
gravels in Iowa, and to date these discoveries represent the best dinosaur
fossils found in the state.
Charlie Gillette of Dickinson County picked up a dark-colored, 3-inch fossil bone from a load of landscaping gravel that came from a nearby gravel pit. When his uncle Jack Neuzil, a retired educator and dinosaur enthusiast, saw the bone he suspected that it could be a dinosaur vertebra. His suspicions were confirmed by a leading dinosaur paleontologist, and the discovery of Iowa’s first identifiable dinosaur bone was soon reported in the Des Moines Register (9/7/2000). The fossil is a tail vertebra from an unknown dinosaur, possibly a hadrosaur.
Photos by Paul VanDorpe.
Following this discovery, a
second dinosaur vertebra from Iowa has come to light thanks to Doris Michaelson
of Bellevue. Her father, John Holdefer, had a keen eye and was fascinated by the
rocks and fossils that he saw as a Materials Inspector for the Iowa Highway
Commission. Sometime in the mid-1930s he picked up a fossil bone from a conveyer
in a gravel pit near Akron in Plymouth County, and the bone was kept on a shelf
and occasionally used as a doorstop in the family’s home. In response to a
recent newspaper article about dinosaurs in Iowa, Mrs. Michaelson contacted the
Iowa Geological Survey and brought the bone in for identification. It is a
partially weathered 4-inch dinosaur vertebra, likely from an hadrosaur.
It’s only a matter of time
before some lucky searcher examining Iowa’s Cretaceous formations or Ice-Age
gravels finds the next dinosaur fossil from Iowa. So keep looking!