All of us at the SI dinosaur site. Photo by B. Lee Drake
(Click on the following link to listen to an audio version of this blog … Wyoming fossils
One of the remarkable things about Wyoming is the presence of fossils from a vast range of the geological time periods of the Earth. As I mentioned in last week’s blog, the eastern side of the Big Horn Basin in the northcentral part of the state is particularly rich in fossils. This fossil abundance is due to the area’s long history of very productive marine and terrestrial ecosystems, and its steady and almost continuous rate of sedimentary rock formation. Also, the local geological activity and energetic uplifting of these fossil-ladened rocks into the active, weathering interface of the Earth’s surface is critical. Without this uplifting and subsequent erosion, the fossils would remain part of the deep rock layers and would never be seen! The combination of these three features has made Wyoming, paraphrasing the words of Marian Murray (author of Hunting for Fossils, 1974, Collier Books), “the place for all of the great museum of world to send their fossil collectors as often as possible!”
In particular, Wyoming is a treasure trove of dinosaur fossils!
Mold and castfossils of a trilobite. Photo by R. Wellner, U. Houston Pressbook.
Fossils are very rare things. The bodies of most living organisms rapidly decompose when they die, and their nutrients and energy molecules cycle back into the tissues of other living creatures. Those creatures will then, in their time, suffer the same “recycling” fate at the ends of their lives. This “circle of life” is part of biological existence! Very rarely, though, the remains of an organism bypass this biotic recycling process, and some bones, or skin, or eggs or feces, or shells, or stems or leaves get sealed away from the consumers, scavengers and decomposers and become slowly incorporated into encasing sedimentary rock. These very small numbers of plants and animals, then, can provide a glimpse into the ecosystems that existed often many millions or even hundreds of millions of years ago!
Standing on a ridge up over the Big Horn River just outside of Thermopolis, Wyoming, it is very hard to believe that this dry, rugged, rocky sagebrush and juniper steppe was once a lush, marshy, tropical coastal plain, and it is equally hard to believe that this high, dry plateau was, in another time period, the mucky bottom of a great, shallow sea. The movement of the Earth’s continents and the rise and fall of the Earth’s surface repeatedly moved “Wyoming” over a wide range of ecological extremes!
Ammonite fossil. Photo by J. St. John. Wikimedia Commons
During the marine phases of these oscillations ocean-dwelling invertebrates, fish, reptiles and dinosaurs lived and died and a very few of them fell down into the soft, forming sediments of the seabed and were sealed away from decay. These few, though, accumulated over vast numbers of years and generated extremely rich fossil deposits of trilobites, brachiopods, ammonites, snails, clams, ichthyosaurs, plesiosaurs, bony fish and mosasaurs. There are places on these rocky plains where you can’t move an inch without stepping on a crust of these fossils!
During the terrestrial phases of our Wyoming, great lumbering dinosaurs lived and died and a very few fell into sealing muds of swamps, rivers or lakes. Their remains, then, were preserved and transformed into fossils. Again, the individual events of fossil formation were rare, but the vast time frame over which these “rare” events occurred allowed great numbers of fossils to accumulate. These fossils included allosaurs, apatosaurs, diplodocus, stegosaurs, ankylosaurs, triceratops and tyrannosaurs. There are places on these ridges where leg and skull bones of these dinosaurs protrude up and out of their encasing surface rocks!
Brachiopod fossil. Photo by Dwerganpaartje. Wikimedia Commons.
Wyoming of the Precambrian Era (4.6 billion years ago to 541 million years ago) sat at the bottom of a shallow sea. Stromatolite-forming bacteria grew in abundance along with the earliest forms of multicellular life. This covering sea persisted during the transition into the Paleozoic Era and the rapid diversification of complex life forms in the first Paleozoic period, the Cambrian (541 to 485 million years ago). During this “Cambrian explosion” almost all of the phyla of Animalia came into existence! Fossils from these early Paleozoic, Wyoming rocks include abundant trilobites and brachiopods.
In the Silurian Period of the Paleozoic (444 to 414 million years ago) the sea receded and left Wyoming high and dry. The older and now exposed rocks eroded away and little new rock was deposited. The sea then returned in the Devonian Period (419 to 359 million years ago) and persisted through the Permian Period (299 to 252 million years ago) laying down a steadily accumulating mass of sediments that formed great layers of sandstone, limestone and shale. Fossilized materials trapped within and in between these sedimentary rocks included the rich marine fauna of the Devonian seas and also the economically important coal, natural gas and petroleum deposits from the Mississippian Period (358 to 323 million years ago) and Pennsylvanian Period (323 to 298 million years ago).
Ichthyosaur fossil. Photo by J. St. John. Wikimedia Commons.
In the Mesozoic Era (252 to 66 million year ago) there were great fluctuations in the seas over Wyoming and increasingly terrestrial ecosystems predominated. In the Triassic Period (252 to 201 million years ago) changing sea levels generated coastal plains in which dinosaur fossils and footprints were preserved. The Triassic red sandstones and abundant mudstones in rivers and wetlands formed. These mudstones were very important sites for the entrapment of dinosaur bones. Fossils of ammonites, snails and clams accumulated in the shallow sediments along with the skeletons of phytosaurs (crocodile-like vertebrates). In the Jurassic Period, extensive sand dunes surrounded the shrinking inland sea. Fossils of ichthyosaurs and plesiosaurs (great aquatic reptiles) and terrestrial dinosaurs like allosaurs, apatosaurs, stegosaurs and diplodocus abound in these Jurassic rocks.
At the end of the Mesozoic, the Western Interior Seaway formed along what would become the eastern edge of the future Rocky Mountains. This great seaway split the North American continent in two and connected the Gulf of Mexico with the Arctic Ocean. Sea levels continued to rise and fall generating cycles in which plesiosaurs, mosasaurs, sharks and bony fish were trapped and fossilized along with numerous bivalves and other marine species. Early species of birds and pterosaurs were also abundant on the shores of the seaway.
In the third period of the Mesozoic (the Cretaceous Period (145 to 66 million years ago)), sea levels continued to rise and fall but the onset of the Laramide Orogeny (about 80 million years ago) and its uplifting of the Rocky Mountains pushed the entire Wyoming Plateau well about sea level, and the sea disappeared and has not been able to return.
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Wyoming Dinosuar Center (overview). Photo by Wyoming Dinosaur Center, Wikimedia Commons
The Wyoming Dinosaur Center is located just across the Big Horn River from the town center of Thermopolis. The Center was established in 1995 and currently houses many of the 10,000 plus fossilized bones excavated from the ridges and hillsides of the nearby Warm Springs Ranch. The Center has on display 50 mounted skeletons (some real, some replicas) that include the 115’ long, diplodocid sauropod dinosaur Supersaurus viviana. This immense, “long-necked” dinosaur was excavated from Douglas, Wyoming and donated to the Center. Skeletons of T. rex, Triceratops, several Hadrosaurs, Stegosaurus, Allosaurus and a 90% complete Camarasaurus that was excavated by Center personnel from the Warm Springs Ranch are also on display at the Center. There are also marine reptiles, flying reptiles, turtles and a rich display of Devonian fish and invertebrates. The Center is an immersion into the rich, fossil history of Wyoming!
Thermopolis Archaeopterus. Photo by incidencematrix, Wikimedia Commons
There is also a specimen of Archaeopteryx on display at the Center. Archaeopteryx is acknowledged to be the transition fossil between dinosaurs and birds! There are only eleven Archaeopteryx fossils in existence, and this is the only Archaeopteryx specimen that is found outside of Europe!
Out on the Warm Springs Ranch there are a number of active excavation sites that the public can tour under the supervision of
Allosaurus skeleton. Photo by T. Evanson, Wikimedia Commons
Center personnel. One of the most remarkable of these excavation locations is the site called “Something Interesting” (or just “SI”). This site is one of the few fossil dig sites on Earth where dinosaur trace fossils (footprints and body indentations) can be found along with corresponding skeletal remains. The footprints and the body indentations tell a story of a juvenile Camarasaurus (about 30 feet long) being scavenged by a number of Allosaurs. The teeth and claw marks on the bones and the abundance of shed Allosaur teeth (over 100!) clearly illustrate the feeding frenzy that went on back on the muddy shores of an alkaline lake back in the late Jurassic (maybe 150 million years ago). This site is a frozen tableau of an ancient ecological system. The footprints and the debris of the “meal” were quickly covered by the thick lake shore muds and then sealed away to be preserved in the forming mudstone.
Standing in the middle of this staggeringly old, killing/scavenging field was a very profound experience. I want to thank Dr. B. Lee Drake for taking us out to the site and for interpretating it for us!