An Introduction to the Brule: Awareness of Sheepcamelpig

When we last left Badlands National Park, we had barely begun to scratch the surface, having covered only the Chadron Formation. While respectable in its own right, the Chadron is only a very small percentage of the park. The bulk of Badlands NP (literally) is formed by the Brule Formation – a complex, fossiliferous formation that has been awed for centuries.

Putting the “Land” in Badlands

As I mentioned previously, the Chadron Formation is the lowermost formation in the park, forming “haystack” buttes. The Brule, in contrast, is responsible for the archetypical badlands. The picture above, from the park’s own website shows the cliff-forming Brule in all its glory, with prominent geographic features replete with funny names – castles, fingers, spires, hoodoos, and so on. (You can also see the color banding representative of the paleosols found throughout the formation, a topic worthy of its own blog post in the future…) The dramatic change between the landforms of the Chadron and Brule is very symbolic, as there was a significant change in paleoenvironment recorded in their respective depositional histories.

The transition from the Chadron to the Brule represents the transition from the Eocene (55.8 – 33.9 Ma) to the Oligocene (33.9 – 23 Ma), a period of upheaval (again, literally) in North America and also in the area of the future badlands. To the northwest, the uplift of the Black Hills had peaked, and fluvial systems were carrying the eroded sediment through the area with a vengeance (Retallack, 1983). In addition, in the later stages of the Oligocene, enormous volcanic eruptions in Nevada and Utah were causing vast ashfall deposits on the plains immediately to the west, many of which were picked up and transported by wind into southwest South Dakota (Larson and Evanoff, 1998). In addition, a gradual drying trend afflicted North America during the Oligocene, turning the subtropical environments of the area into savannah-like grasslands. As a result, the Brule is actually subdivided into two members: the lower Scenic member, dominated by fluvially deposited mudstones representing a subhumid environment, and the upper Poleside member, dominated by eolian (wind-deposited) siltstones representing a semiarid environment, both of which are rich in volcaniclastics (Evanoff et al., 2010). The presence of the volcaniclastics makes the rocks of the Brule harder and more difficult to erode than those of the Chadron, allowing the creation of sharp, distinct badlands formations.

Ecologically, the fauna were experiencing some upheaval (not literally, this time) of their own. The extinction of brontotheres at the end of the Eocene marked the beginning of the downfall of the once-dominant perissodactyls at the hands of diverse artiodactyls (Scott and Jepsen, 1940). Which is not to say that perissodactyls were reduced to the role of savannah wallflowers – “rhinos”* experienced their “culminating point” in North America at this time, and the beginning of the infamous evolution of horses began around the same time as well (Scott and Jepsen, 1940). But the ecosystems were clearly becoming dominated by artiodactyls, not the least of which are…

*The quotation marks are intentional. For now, let’s not worry about exactly what that terms means…

Oreodonts: dominant, diverse, damned confusing

Arguably the enigmatic taxa of the Badlands NP, and concurrently the Brule Formation, are the oreodonts (image at left from here), a diverse group of controversial affinity. Two taxa, the small, bizarre Leptauchenia, and larger Merycoidodon, are the most common forms in the park. They have no modern representatives, so are commonly described as “pig-like” or “sheep-like.” As it turns out, they share some features with possible camel ancestors, so “camel-sheep-pig” may actually be more fitting, though no more helpful. They are allegedly the most common large mammals in the Brule Formation “by far,” with Merycoidodon distinguished as “the most common of all badlands fossils” (Prothero and Whittlesey, 1998). In fact, Hayden (1857) originally referred to the Scenic member the “Turtle and Oreodon* beds,” and the Poleside member as the “Leptauchenia beds**.” Furthermore, oreodont fossils are abundant and obvious enough in Badlands NP that they are the most commonly reported fossils by visitors, and not by a small margin (personal observation).

* Again, not a typo. We’ll get to that in a few minutes…

** For the record, naming formations or members by common fossils was not an atypical practice by early geologists at all. This is a thoroughly bad idea for a number of reasons, but unfortunately they didn’t let that stop them, so now we have to deal with the results.

They are also a very diverse group, with an incredible variety in body forms. The aforementioned Leptauchenia, for example, has a markedly unusual skull that caused many earlier researchers to assume it led a lifestyle similar to modern beavers (see Scott and Jepsen, 1940, the source of the images at left). Unfortunately, this diversity has led to a lot of oversplitting (inappropriate naming of new taxa), with a lot of confusing terminology – for example, the name “Oreodon” is the above paragraph is actually an older name for Merycoidodon, an oreodont, and a large number of oreodonts are now classified under the group Merycoidodontidae (still with me?). Robert Carroll’s exhaustive “Vertebrate Paleontology and Evolution” (1988), in fact, doesn’t even list the term “oreodont,” as it is apparently not a valid name for the group. Not surprisingly, the group as a whole is currently undergoing a lot of professional revision (Benton, Miller, Weiler, etc, personal communication, 2010). Regardless, oreodonts are still a fascinating group, and there is much more to say about them, but I will leave that for a future post…

No, you didn't really see this. Keep scrolling.

REFS

Carroll, R.L. 1988. Vertebrate Paleontology and Evolution. WH Freeman and Company, New York, 698 p.

Evanoff, E.E., Terry, D.O., Jr., Benton, R.C., and Minkler, H. 2010. Field guide to the geology of the White River Group in the North Unit of Badlands National Park: a guide for the field trip: recent advances in understanding the geologic history of the White River Badlands, 24-25 April 2010: GSA Rocky Mountain Section Meeting, 21-23 April 2010, Rapid City, SD, USA.

Hayden, F.V. 1857. Notes on the geology of the Mauvaises Terres of White River, Nebraska: Proceedings of the Academy of Natural Sciences of Philadelphia, v. 9, p. 151-158.

Larson, E.E., and Evanoff, E. 1998. Tephrostratigraphy and sources of the tuffs of the White River sequence, in Terry, D.O., Jr., LaGarry, H.E., and Hunt, R.M., eds., Depositional environments, lithostratigraphy, and biostratigraphy of the White River and Arikaree Groups (Late Eocene to Early Miocene, North America): Geological Society of America Special Paper 325, p. 1-14).

Prothero, D. R., and Whittlesey, K.E. 1998. Magnetic stratigraphy and biostratigraphy of the Orellan and Whitneyan land mammal "ages" in the White River Group, in Terry, D.O., Jr., LaGarry, H.E., and Hunt, R.M., Jr., eds, Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River and Arikaree Groups (Late Eocene to Early Miocene, North America): Geological Society of America, Special Paper 325, p. 39-61.

Retallack, G.J. 1983. Late Eocene and Oligocene paleosols from Badlands National Park, South Dakota: Geological Society of America Special Paper 193, 82 p.

Scott, W.B., and Jepsen, G.L. 1940. The mammalian fauna of the White River Oligocene: Part IV, Artiodactyla: Transactions of the American Philosophical Society, New Series, v. 28, no.4, p. 363-746.