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Cenozoic fossil beds (blue circles in Banner) are strewn across the western United States and Florida. The Nacimiento Formation at Angel Peak in New Mexico and the Bug Creek Anthills in Montana contains early Paleocene mammals. Early Eocene fossils are at Fossil Butte National Monument in Wyoming (Figure 12-1). John Day National Monument in Oregon documents the Middle Eocene to the Late Miocene (44 to 7 Ma). Late Eocene plant fossils are at Florissant Fossil Beds National Monument in Colorado and Badlands National Park in South Dakota. Miocene mammals are at Agate Fossil Beds in Nebraska. Pliocene (5.3 – 2.6 Ma) fossils are in the Hagerman Fossil Beds National Monument in Idaho (3.5 Ma), and Pleistocene (2.6 Ma – 10 ka) fossils such as terror bird Titanis walleri are in Florida. You can look up the many National Park Service paleontological sites in the US at https://www.nps.gov/subjects/fossils/fossil-parks-list.htm,
The North American Land Mammal Ages scale (NALMA) provides a geologic timescale based on land mammal fossils in the Cenozoic Era. Thus, scientists determine the ages (positions in time) of geologic layers in different regions based on their fossils. They also use radiometric dating to date or constrain the range of dates for geologic formations.
Paleontologists call the Paleocene the dark epoch. It is difficult to find land vertebrate fossils in the Paleocene because only a few animals survived the End Cretaceous extinction, and most of them were small, especially in the Early Paleocene (https://youtu.be/5NnA10MBaWU). The Puercan Stage (NALMA chronology) is just above the K-Pg boundary. Purgatorius (Figure 12-4) was an early Primatomorpha in the Puercan Stage (64.85 Ma) in Montana (Bug Creek fauna). It was a possible predecessor of the plesiadapiforms.
Figure 12‑4. Purgatorius. Credit: Patrick Lynch. ©. Authorized for any purpose, with attribution.
Figure 12‑5. Teeth from Protungulatam. Natural History Museum of London. Credit: Ghedoghedo. Used here per CC BY-SA 3.0.
Protungulatam (64.85 Ma) (Figure 12‑5) was also in the Bug Creek fauna. Different groups of paleontologists place it at different positions in the tree of placental mammal evolution. It is a controversial fossil.
The Early Paleocene Nacimiento formation (65.7 to 61 Ma) lies exposed at Angel Peak (Figure 12‑6) in New Mexico. It formed in an environment of swamps and lakes in hot and humid conditions. Fossils in the Nacimiento formation include mollusks, fish, lizards, multituberculate mammals, marsupial mammals, plesiadapiforms (related to primates), carnivorans, mesonychids (carnivorous ungulates), and condylarths (possible ungulates and other unclassified mammals).
Figure 12‑6. Nacimiento formation at Angel Peak (elevation 6,988 feet) in New Mexico. Credit: BLM New Mexico. Public domain.
Because there were few known fossils in the Paleocene, pioneering 19th century geologist Charles Lyell thought that the beginning of modern animal life was in Eocene formations, which he named the Eocene, “dawn.” The Eocene lasted from 56 to 33 Ma. The Early Eocene Green River formation (53.5 – 43.5 Ma) at Fossil Butte (Figure 12‑7) formed in a giant lake and a subtropical forest adjacent to the lake (https://youtu.be/WDVbe38lI2s). At the time it was at a low elevation because the Laramide Orogeny had not uplifted the area of the Rocky Mountains by the Early Eocene.
Figure 12‑7. Exposure of the Green River Formation at Fossil Lake in Fossil Buttes National Monument. Credit: Magicpiano. Used here per CC BY-SA 4.0.
Figure 12‑8. Diplomystus dentatus from the Green River Formation in the Split Fish layer. Credit: Seth Sorensen. Used here per CC BY-SA 3.0.
There are many fossil fish, crocodiles, turtles, and a few snakes in this formation. Scientists reveal fossils by breaking apart the thin limestone shale layers. Freshwater lakes typically deposit calcium carbonate on the lake floor and form a muck. Annual (or more frequent) light and dark changes form varves, which enable scientists to track species evolution over time. Periodic orange volcanic ash layers enable scientists to date these layers with radiometric dating. Some layers have well preserved fossils because of low oxygen, rapid deposit of calcium carbonate, or lack of predation in those years. Scientists break apart these layers in Fossil Lake to expose the well-preserved fossils. A common fish in Fossil Lake was Diplomystus (Figure 12-8). This fish went extinct 30 Ma.
Figure 12‑9. Knightia eocaena, 25 cm length. Credit: National Park Service.
The most common fossil fish in Fossil Lake was Knightia, (Figure 12-9). Giant turtles, 1.7 m long, also inhabited Fossil Lake. One of the most amazing discoveries was an early fossilized horse, which was the size of a dog, protorohippus (Figure 12-10). It died in the lake along with the fish.
Figure 12‑10. Protorohippus. A dog-sized horse from the early Eocene at Fossil Butte. National Park Service.
Figure 12‑11. Timeline of geologic formations at John Day National Monument. Credit: NPS
The John Day National Monument in Oregon has exposed strata from the Middle Eocene (44 Ma) to the Late Miocene (7 Ma) (Figure 12‑1). It is the Cenozoic version of the fossil record of the Paleozoic and Mesozoic on the Colorado Plateau. Its 2,200 meters of the stratigraphic column (Figure 12‑11) stretches across the National Monument and is exposed at different locations along the John Day River. Thomas Condon discovered many important Cenozoic fossils at John Day in the 1800s. Condon was originally a minister, then the State Geologist of Oregon, and finally a professor of geology at the University of Oregon. One of his most significant discoveries was the three-toed horse Miohippus, Condon also established the Thomas Condon Paleontology Center at the University of Oregon. Click the following image to watch the PBS video on Thomas Condon.
The two earliest fossil formations at John Day are the Nut Beds Formation (44 Ma) and the Mammal Quarry (40 Ma) in the Clarno Unit, which are Middle and Late Eocene formations, respectively. During the Middle Eocene, the climate of Oregon was still hot and wet, but it cooled and dried by the time of the Mammal Quarry. The Nut Beds Formation (44 Ma) in the Clarno Unit formed in a semitropical forest in a hot and wet climate (Figure 12‑12) with annual rainfall of 3 meters. Mammals at the Nut Beds Formation included creodonts such as the carnivorous cougar-like Patriofelis and the Perissodactyls Orohippus and Hyrachus. The Hancock Mammal Quarry (Figure 12‑13) contains fossils from 5 million years later in the Late Eocene with a cooler and drier climate (39 Ma).
Figure 12‑12. Clarno Nut Beds Formation reconstruction (44 Ma) at John Day National Monument. It formed in a Middle Eocene semitropical forest. Credit: NPS
Figure 12‑13. Hancock Mammal Quarry reconstruction (40 Ma) at John Day National Monument. It formed in the Late Eocene with a cooler and drier climate than the Middle Eocene. Credit: NPS
Temperatures continued to drop during the Late Eocene and into the Oligocene. The Bridge Creek formation (33 Ma) has fossils from hardwood forests (Figure 12‑14). The global temperature had dropped by 3 to 6 0C.
Figure 12‑14. Bridge Creek Formation reconstruction (33 Ma) at John Day National Monument. It formed in the Early Oligocene when there were hardwood forests. Credit: NPS
The Turtle Cove Member (29 Ma) of the Sheep Rock Formation at John Day formed four million years after the Bridge Creek Formation in the Oligocene (34 - 23 Ma). The Turtle Cove fauna includes the three-toed horse Miohippus, burrowing animals, several carnivores, oreodonts, and beavers (Figure 12‑15). The climate was cooling and there were more open areas as grasses began to evolve.
Figure 12‑15. Turtle Cove Member reconstruction in the (29 Ma) Oligocene. It formed as grasslands began to evolve and cover much of the landscape. Credit: NPS
The Mascall Formation (15 Ma,) formed after a period of intense volcanic activity in the Miocene (23 - 5.33 Ma). The landscape had broad basins and lakes and forests had returned (Figure 12‑16). Grasslands led to the proliferation of ruminants.
Figure 12‑16. Mascall Formation reconstruction (15 Ma). It formed during a period with basins, lakes, and forests. Credit: National Park Service.
The Rattlesnake Formation (7 Ma) is the youngest formation at John Day. It formed when there were only grasslands in the region (Figure 12‑17). The fossils in this formation lie in sedimentary rock below a mesa formed from ash and other debris from a volcanic explosion.
Figure 12‑17. Rattlesnake Formation (7 Ma) in Late Miocene. It formed when the area was covered by grasslands, and streams meandered through the plain. Credit: National Park Service.
The Hagerman Valley in Idaho provides the next snapshot of the evolution of mammals. It formed 4 Ma during the Pliocene (5.33 - 2.58 Ma). The flooding of historic Lake Bonneville eroded sediments and exposed the formation. Fossils were preserved in the rivers and lakes of the region. These sedimentary layers. Many types of animals were preserved, some still existing but many eliminated. The most famous fossil was the Hagerman horse (Equus simplicidens), which is the oldest fossil of a horse with a single toe. Prior to this, horses had three toes. There are many large mammals, carnivores, and amphibians. Tons of fossil material is contained in the Smithsonian Institution in Washington DC.
The next Pliocene fossils are in Florida. As the climate cooled during the Oligocene and Miocene, water was tied up in glaciers near the poles and sea level dropped. With a drop in sea level and land uplift, Florida became a land area during the Late Miocene, Pliocene (5.3 – 2.6 Ma) and Pleistocene (2.6 Ma – 10,000). Pliocene fossils included many ancient versions of modern mammals in North America as well as camelids, dugongs, saber toothed cats, mastodons, and tapirs.
Figure 12‑18. Titanis walleri, Florida Museum of Natural History. Credit: Amanda. Used here per CC BY 2.0.
Titanis walleri (Waller’s terror bird) was an enormous carnivorous bird (Figure 12-18) that lived during the Pliocene and Pleistocene. It migrated from South America when the continents of North and South America connected between 5 Ma and 3 Ma. Pleistocene mammal fossils in Florida also include “short-faced bears, saber-toothed cats, glyptodonts, mammoths, mastodons, giant ground sloths (from South America), and wolves.” [1] The onset of cold during the Ice Ages unfortunately eliminated the Waller’s terror bird approximately 2 million years ago. There were many large megafauna such as mammoths and mastodons that did well in the Pleistocene; however, climate change and human impacts in the Late Pleistocene eliminated the giant megafauna.
References
References
[1] Wikipedia. Paleontology in Florida.
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National Park Service map of fossil parks in the United States. Blue circles represent locations with Cenozoic fossils. Credit: National Park Service.
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