Wyoming and Montana

The fish world of 400 million years ago

Allocryptaspis [a type of primitive fish] head plate

The Devonian Period was approximately 410 – 360 million years ago, and represents a time of radical change in history of life on earth. Complex vertebrate faunas had evolved by this time and expansive colonization of the land also occurred.

Anarthaspis [a type of primitive fish] head plate

The work done at the Dallas Museum of Natural History was to examine the ecology of these fossil vertebrate communities just prior to the major explosion of life onto land.

view from Beartooth Butte

One for the Ages

Lately I can’t help but notice the increased tendency for people to add the letters a, g, and e to the end of some words, regardless if there are perfectly acceptable versions of these words available without the need of the suffix. One particular example of adding this suffix that is annoying to me is when these letters are added to the word "sign" a good word in it own right, producing the new word "signage" (which by the way is not in the dictionary). This tendency is happening with such regularity, and in a variety of venues, that it almost seems as if some great letter warehouse exists that has a surplus of the letters a, g, e, and people everywhere are being required to reduce the surplus. Personally, I don’t get it.

Beartooth Butte, Wyoming, see map

A more traditional use of "age" is to refer to a period of existence. And with that, paleontology at the Dallas Museum of Natural History is gearing up for a new, multiyear project in a unit of time, the Devonian Period, that is so dominated by fossil fishes that it has been referred to as the "Age of Fishes." The Devonian is a period of time extending from approximately 360 to 410 million years ago and is well known for its abundance of strange and unusual fish. Although some of the ancestors of today’s fish can be found in the Devonian, such as sharks, lungfish and of course the most famous of living fossils, coelacanths, most of the fish of this time were odd looking things, many with heavy, bizarre bony armor on their heads. In addition to the weirdness of the appearance of these fish, the fish were diversified into a large variety of sizes and shapes. Devonian fish represent the first major radiation of vertebrate life.

pteraspid [a type of primitive fish] head plates

Of some importance to us as back-boned land dwellers is that historically paleontology has recognized that by the end of the Devonian vertebrate life had moved onto land. With the new discoveries being made around the world in other Devonian rocks, I am undertaking this new project, in rocks of Wyoming and Montana, to study the environments of fossil vertebrates in the earliest part of the period just before the big explosion of life onto land. This timeframe is about 400 million years ago. My colleagues and I will be closely examining the environments in which these fossils formed, the types of fossils preserved, and the geochemistry of both the fossils and the sediments surrounding the fossils. With some luck, this study will produce new insights into the ancient environments of this time, and why vertebrate life moved onto land. Because this project is ecological in nature, many different facets of the fossil localities will need to be studied during the course of the project.

welcome to grizzly bear country

And to get back to the tendency of adding "age" onto the end of some words, there is at least one acceptable word with this suffix: verbage. The Oxford English Dictionary defines verbage as a variant of verbiage, which means superabundant or superfluous. And before this column is accused of either, and because this project is only now beginning, this seems like a good place to end.

Notes on the Half Moon Canyon section of the Beartooth Butte Formation (Lower Devonian), Big Snowy Mountains, central Montana

Abstract

The Beartooth Butte Formation is a Lower Devonian rock unit that is widespread through much of Montana and Wyoming. Of these exposures, localities at the type section at Beartooth Butte and Cottonwood Canyon in the Bighorn Mountains (both in Wyoming) have received a great deal of attention from vertebrate paleontologists. This report better documents the occurrence of this rock unit at a third locality in the Big Snowy Mountains in Montana.

Introduction

The Lower Devonian Beartooth Butte Formation has long been a source of vertebrate remains (Bryant, 1932, 1933, 1934, 1935, Dorf, 1934a). Subsequent to these early reports, additional work by others has shown this rock unit continues to have great paleontological yield (Denison, 1966, 1968, 1970; Elliot and Ilyes, 1996; Elliot and Johnson, 1997).

worm burrows

Despite the suggestion that this unit is widespread (Sandberg, 1961), virtually all of the paleontological attention on this formation by vertebrate paleontologists has been focused on either the type section at Beartooth Butte in northern Wyoming or at the Cottonwood Canyon locality in the Bighorn Mountains of Wyoming. The purpose of this report is to highlight ongoing work in the Half Moon Canyon section of the Big Snowy Mountains (Figure 1) on the reputed Beartooth Butte Formation. As Sandberg (1961) originally reported, this unit crops out in this canyon for a distance of over 100 m and is approximately 20 m thick (Figures 1 and 2). Though the work on this section is not complete, this report highlights the complex depositional history of the Half Moon Canyon section and the paleontological potential that this section holds for expanding our knowledge of Lower Devonian vertebrates from the Beartooth Butte Formation.

Geologic Background



The Beartooth Butte Formation was originally described by Dorf (1934a), while exploring the Beartooth Plateau he noticed a reddish channel fill between the Ordovician Bighorn Dolomite and the overlying Upper Devonian Jefferson Limestone. Dorf, a paleobotanist, described the fossil plant material from the formation, all of which he attributed to being terrestrial (Dorf, 1934b). Later work in Cottonwood Canyon of the Bighorn Mountains revealed the first evidence that this unit was more widespread than previously thought (Blackstone and McGrew, 1954). Bryant (1932, 1933, 1934, 1935) described in detail the original collections made of the fossil fish remains from the Beartooth Butte type section. Subsequent work by others, most notably Denison (1966, 1968, 1970) and Elliot and colleagues (Elliot and Ilyes, 1996; Elliot and Johnson, 1997), have demonstrated a diverse fauna for this rock unit.

Cottonwood Canyon, Wyoming, see map

The Beartooth Butte Formation has traditionally been considered to be Lower Devonian, and more specifically Pragian in age (Johnson et al., 1989). However subquent work Elliot and Ilyes (1996), and Elliot and Johnson (1997) on the vertebrate material, and spore assemblages by Tanner (1983) strongly suggest that the Beartooth Butte locality and the Cottonwood Canyon locality are two different ages. The former locality being Emsian in age, while the latter is still being considered dominantly Pragian in age.

Dorf (1934a) originally considered the depositional setting for this rock unit to be an estuarine channel fill, an interpretation that remains favored by subsequent workers (Sandberg, 1961; Johnson et al., 1988).

Half Moon Canyon Section


generalized map of the area being studied

The Beartooth Butte Formation crops out on the east wall, near the head of Half Moon Canyon (Figure 2). The thickness of the Beartooth Butte Formation in Half Moon Canyon is approximately 20 m and though partly covered, is comprised of several distinct subunits (Figure 1). The schematic section presented in figure 1 is based on observations at the south end of the exposure.

view of Beartooth Butte Formation at Beartooth Butte

At the base of this section is 2.5 m thick limestone pebble conglomerate that is pinkish in color. Clasts are up to 4 cm along the long axis in outcrop. Overlying this unit is approximately 4-5 m of cover, comprised of debris of the formation from exposures up section. Overlying this cover is approximately .5 m of bioturbated micrite with conglomeratic lenses of limestone. These lenses are reddish light gray to light gray, while the micrite is maroon or maroonish gray. The bioturbation features are generally structure-less and are probably invertebrate burrows. These burrows are light colored and have diameters of only a few millimeters with vertical depths of at least 2 cm. The conglomeratic lenses can be up to the full thickness of this subunit and contain abundant bone fragments. Approximately .5 m of maroonish red silty limestone overlies this subunit. This silty limestone is finely bedded, with beds on the order of millimeters in thickness. Mudcracks and ripple marks are common.

pteraspid [a type of primitive fish] head plate

Overlying this unit is approximately 7 m of cover, again comprised of debris from up section within the Beartooth Butte Formation. Above this cover is a 15 cm thick micrite with pebbles. Also within this unit are small bone clasts. Overlying this coarse unit is a 70 cm thick, variably-colored micrite, ranging in color from buff to maroon, to gray, to green. This unit contains rare placoderm remains. On top of this unit is a one and a half meter thick, mixed unit (similar to that below) of bioturbated maroon or maroonish gray micrite with light colored invertebrate burrows and conglomerate and massive lenses of limestones. The conglomeratic lenses are commonly 5 cm thick while the massive limestone lenses are up to 60 cm thick. These lenses are reddish light gray to light gray. The top of the Beartooth Butte Formation in Half Moon Canyon is capped by approximately 2 m of maroonish red silty thinly bedded limestone. These beds are on the order of only a few millimeters thick and alternate between siltier and less silty beds. Ripple marks are uncommon, while mudcracks are common. Small, hemispherical depressions resembling raindrop impressions are rare. Bone fragments are also rare in this unit.

placoderm [a type of primitive fish] head plates

Capping the reddish Beartooth Butte Formation more continuous exposures of massive gray to grayish white micritic limestone. These exposures are the Jefferson Formation.

Discussion and Conclusion

Based on the presence of several different lithologies, as well as the presence of several different types of sedimentary structures, the Half Moon Canyon section of the Beartooth Butte Formation represents a complex depositional history for this sedimentary interval. By the presence of mudcracks and possible raindrop impressions, this history also includes periods of non-deposition. Further, there are several bone bearing horizons in this section.

Half Moon Canyon, Montana, see map

Stable carbon and oxygen isotope data from this section indicate that this exposure was deposited under more fresh water conditions than either the type section at Beartooth Butte, or at the section in Cottonwood Canyon (Poulson, written comm.). The presence of at least two bone horizons, combined with the isotope data indicating a more fresh-water setting than the other two well-known sites, suggests that this section has great potential for yielding new insights into the vertebrate paleontology of the Beartooth Butte Formation.

Acknowledgments

I thank Dr. Simon Poulson for providing the stable carbon and oxygen isotope data referred to in this report. I also thank Dr. David Elliott of discussions regarding his work on the Beartooth Butte Formation. Lastly I thank Jim Sheldon, Robin Strathy, Barbara Beasley, Rusty Dersch (all of the U.S.D.A. Forest Service), and Cathleen May (formerly of the U.S.D.A. Forest Service) for their respective logistical support for this project. This project was funded by M.O.U. Forest Service Agreement # 1102-0002-96-033 through the U.S.D.A. Forest Service.

References

Blackstone, D. L., and McGrew, P. O. 1954. New occurrence of Devonian rocks in north central Wyoming. In, Pryor Mountains-Northern Bighorn Basin, Montana, Billings Geological Society Guidebook, Fifth Annual Field Conference, Richards, P. W. (ed.), p. 38-43.

Bryant, W. L. 1932. Lower Devonian fishes of Beartooth Butte, Wyoming. American Philosophical Society Proceedings, v.71, p. 225-254.

Bryant, W. L. 1933. The fish fauna of Beartooth Butte, Wyoming: Part I. American Philosophical Society Proceedings, v. 72, p. 285-314.

Bryant, W. L. 1934. The fish fauna of Beartooth Butte, Wyoming: Parts II and III. American Philosophical Society Proceedings, v. 73, p. 127-162.

Bryant, W. L. 1935. Cryptaspis and other Lower Devonian fossil fishes from Beartooth Butte Formation, Wyoming. Proceedings of the American Philosophical Society, v. 75, p. 111-128.

Denison, R. H. 1966. Cardipeltis, an Early Devonian agnathan of the Order Heterostraci. Fieldiana: Geology, v. 16, p. 89-116.

Denison, R. H. 1968. Early Devonian lungfishes from Wyoming, Utah, and Idaho. Fieldiana: Geology, v. 17, p. 353-413.

Denison, R. H. 1970. A revised classification of Pteraspidae with description of new forms from Wyoming. Fieldiana: Geology, v. 20, p. 1-41.

Dorf, E. 1934a. Stratigraphy and paleontology of a new Devonian Formation at Beartooth Butte, Wyoming. Journal of Geology, v. 42, p. 720-737.

Dorf, E. 1934b. Lower Devonian flora from Beartooth Butte, Wyoming. Geological Society of America Bulletin, v. 45, p. 425-440.

Elliot, D. K., and Ilyes, R. R. 1996. Lower Devonian vertebrate biostratigraphy of the western United States. Modern Geology, v. 20, p. 253-262.

Elliot, D. K., and Johnson, H. G. 1997. Use of vertebrates to solve biostratigraphic problems: examples from the Lower and Middle Devonian of western North America. In, Paleozoic Sequence Stratigraphy, Biostratigraphy, and Biogeography: Studies in Honor of J. Granville ("Jess") Johnson. Klapper, G., Murphy, M. A., and Talent, J. A. (eds.), Geological Society of America Special Paper 321, p. 179-188.

Johnson, J. G., Sandberg, C. A, and Poole, F. G. 1988. Early and Middle Devonian paleogeography of western United States. In, Devonian of the World: Proceedings of the Second International Symposium on the Devonian System. McMillan, N. J., and Glass, D. J. (eds.). Canadian Society of Petroleum Geologists, v. 1, p. 161-182.

Johnson, J. G., and Sandberg, C. A. 1989. Devonian eustatic events in the western United States. In, Devonian of the World. McMillan, N. J., Embry, A. F., and Glass, D. J. (eds.) Canadian Society of Petroleum Geologists Memoir 14, v. 1, p. 161-182.

Sandberg, C. A. 1961. Widespread Beartooth Butte Formation of Early Devonian age in Montana and Wyoming and its paleographic significance. Bulletin of the American Association of Petroleum Geologists, v. 45, p. 1301-1309.

{full citation: Fiorillo, A. R. 1998. Notes on the Half Moon Canyon Section of the Beartooth Butte Formation (Lower Devonian), Big Snowy Mountains, central Montana. Northwest Geology, 28:15-19.}

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