Camel Butte

Camel Butte is an extremely rich fossil locality in Fallon County, Montana. This site is important because it represents the time of one of the most severe extinction events in Earth’s history, at the end of the Mesozoic Era—the Age of Dinosaurs—which saw the demise of the giant dinosaurs, among them Triceratops and Tyrannosaurus rex. Dated to about 65 million years ago, the Camel Butte deposits span the end of the Cretaceous Period and the start of the Tertiary Period (known as the K–T boundary), helping us to better understand not only the extinction of the giant dinosaurs, but also which animal lineages survived.

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The Camel Butte site in Montana, discovered in 2006 by Yale Peabody Museum Curatorial Affiliate  Brian Roach.

The Extinction Debate

There has been much debate about the extinction of the giant dinosaurs: Was it a catastrophic event caused by a meteorite? A gradual extinction caused by global warming? Or something else?
Proponents of the “catastrophic meteorite” hypothesis argue that dinosaurs were very diverse to the very end of the Cretaceous Period. Those who argued for the “gradual” hypothesis proposed that the large dinosaurs were dwindling by the end of the Cretaceous, or even that they were already extinct. A so-called three meter gap—a zone of rock representing the end of the Cretaceous that seemed to have no vertebrate fossils—seemed to support the gradual hypothesis. However, in 2010 Yale anthropology graduate student Stephen Chester found a dinosaur brow horn at Camel Butte that helped to fill this gap. The discovery showed that at least some dinosaurs were not extinct before the K–T boundary, but were alive and well right up to the end of the Mesozoic Era.

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This brow horn, from either Triceratops or Torosaurus, represents one of the last giant dinosaurs to roam Earth before these animals became extinct.

Yale Geology and Geophysics graduate students Rachel Racicot and Allison Hsiang (left) screenwash for microvertebrate fossils.

Screenwashing efforts at Camel Butte allow the recovery of microvertebrate fossils, like this tiny mammal tooth.

The Beginning of the Age of Mammals

Although the K–T boundary is known mainly for the disappearance of the giant dinosaurs, it is also the time of one of the most significant events in the evolution of mammals. After the K–T extinction event mammals underwent an adaptive radiation (an increase in diversity) in the early Tertiary and began to fill more ecological niches. Primitive marsupials and a successful group of rodent-like mammals with very specialized teeth called multi-tuberculates were the dominant mammals species in the Late Cretaceous. But many multi-tuberculates, and almost all of the marsupials, became extinct at the K–T boundary. New types of mammals, particularly species of hoofed mammals called condylarths, first appeared in the early Tertiary.



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Yale Peabody Museum Curatorial Affiliate Brian Roach collects vertebrate fossils below the K-T boundary at Camel Butte.

Our understanding of the recovery of mammals in the earliest Tertiary has been limited, because we have very few K–T boundary fossil sites. Camel Butte contains fossils of mammals and other vertebrates very close to the K–T boundary, probably from within the first 100,000 years after the extinction event. Yale paleontologists have collected more than 150 mammal fossils at Camel Butte, including several nearly complete jaw fragments from condylarths and multi-tuberculates, hundreds of other vertebrate fossils (including crocodiles, turtles and fish) and at least one new species of condylarth. Future expeditions and ongoing research will bring more insights about the evolution of mammals and other vertebrates after the Age of Dinosaurs.


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Yale Geology and Geophysics graduate student Tyler Lyson carefully excavates the brow horn core and collects rock samples (right) to be analyzed back at the lab to determine the exact placement of the K-T boundary.

Brow Horns

The significance of the highly weathered brow horn found within the three meter gap is not the quality of the bone, which is quite poor, but rather that it is very close to the K–T boundary. This implies that at least some giant dinosaurs were thriving right up through the end of the Cretaceous.


Photograph Above:

Yale Anthropology graduate student Stephen Chester finds a ceratopsian (horned dinosaur) brow horn very close to the K-T boundary,