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Middle Park Miocene Fossils from the USGS-Denver Collections: Vertebrates

Middle Park, Troublesome Formation, Miocene Horses:
Merychippus the grass-eater (as are all modern horses)
Anchitherium, Hypohippus, and Parahippus were leaf eaters

There are more than 150 vertebrate localities in Grand County. The range of horses is well represented. Other numerous vertebrate fossils in Middle Park include giant camels, rhinos, rodents, sheep-like oreodonts, as well as an occasional gomphothere elephant. Mammals adapt relatively quickly to climatic changes. Adaptive features within the Survey fossil collections demonstrate dietary and locomotion changes. The perissodactyls (horses) were abundant in Middle Park and their evolutionary adaptations to climate change are easy to follow.

diagram showing the evolution of the horse from the Eocene to present
Horse Family Tree (Click on image to see a larger view).

Generally the Cenozoic climate trend was cooling and drying. To most trends there are exceptions. Middle Park represents a unique period. Middle Park's rocks are within the Middle Miocene (17-12.5 Ma). Glen Izett dated the tuffs above and below the mammal localities (Izett, Glen A., 1968, Geology of the Hot Sulphur Springs quadrangle, Grand County, Colorado, USGS Professional Paper #586, pp 79). This is an interesting period of a climate trend reversal, called the Middle Miocene Climate Optimum. Climate warming and drying as indicated by Deep Sea Drilling Projects indicates subtropical conditions for Colorado's latitudes 37°- 41° (Ennyu, Atsuhito, 2003, Middle Miocene Climate Evolution in the Pacific Realm, PhD dissertation, Penn State University, pp 235). Trends in global climatic change are reflected in plants and animals by gradual and punctuated evolutionary adaptations.

Plants responded to the Cenozoic cooling and drying trend with a revolutionary new family-Grasses, and by the Miocene grasslands became a predominant geographic feature. Mammals responded with revolutionary adaptations in teeth and limbs. The horses are a classic example of natural selection and adaptation. Hyracotherium-Anchitherium (Eocene-Middle Miocene) horses were leaf eaters. Their teeth were low-crowned molars with small conules adapted for chewing soft leaves. The chewing action was nipping and crushing-simply mechanically up and down. Horses adapted quickly to the advent of the grasslands. The horses with low crowned teeth would have starved as their teeth were worn out prematurely by chewing the abrasive grasses. A completely revolutionary trend in teeth was required to cope with the grasses. The grass-eaters developed high-crowned, continuously growing teeth to compensate for the highly erosive dusty grasses. Teeth were reinforcement and hardened with cement and complex surficial folds. The chewing mechanics became a grinding motion, like two millstones. Middle Miocene horse Merychippus is represented by several species each reflecting varing degrees of teeth transformations. It is the first horse of the lineage of modern grass-eaters.

There was a period of several million years during the Middle Miocene that both grazing and browsing horses existed simultaneously within Colorado. Merychippus dominated the grasslands. The Middle Miocene warming trend likely afforded abundant leaf vegetation in the uplands for the Anchitherium, Hypohippus, and Parahippus browsing horses. We have noted a few Middle Park localities populated by both grazing and browsing horses.

photo of a leaf-eating Anchitherium skull and the grazing, grass-eating Merychippus skull showing a difference in teeth, and especially the continously growing molar on Merychippus

Dental adaptation to the dusty grasslands was recorded in the fossil record of Middle Park. Merichippus was the first "Modern" horse. (Click on image to see a larger view)

symbol for the vertebrate localities on the Middle Park Map A Story of Adaptation to Climate Change

vertebrate fossil locations near Middle Park Colorado

USGS-Denver Miocene Mammal Localities in the Troublesome Formation (~12.7 Ma)

  • 164 Grand County
  • 16 Routt County
  • 10 Moffat County (Brown's Park Formation equivalent to the Troublesome Fm)
A variety of mammals populated ancient Middle Park including rhinos, camels, horses, oreodonts, and rodents. A major climate change occurred during this epoch. The warming and drying trend promoted a revolution in the Plant Kingdom--the rise to dominance of grasslands. The mammals adapted to the new food source. Change was profoundly expressed within the family of horses. Their legs grew longer and feet adapted for speed on the grasslands. Teeth adapted to the dusty, silica-covered grasses. The old-style low-crowned teeth specialized for nibbling leaves gave way to high-crowned, continuous growing--grinding teeth specialized for grasses. Merychippus, found in numerous Middle Park localities, is a classic example of adaptation to environment.

Vertebrates from the Middle Park Collection are from the Miocene Troublesome Formation, approximately 12.7 Ma

fossil skull of Protolabis angustidens, an ancient camel Protolabis angustidens (camel)
fossil skull of Merychippus sejunctus, an ancient horse Merychippus sejunctus (horse)
fossil skull of Aphelops megalodus, an ancient rhino Aphelops megalodus (rhino)