Smithsonian National Museum of Natural History

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Carboniferous

Big Cedar Ridge, Wyoming: a 73 million-year-old preserved landscape

 

About 73 million years ago (late Campanian stage of the Cretaceous) a mudflow of volcanic ash suddenly entombed the entire landscape at what is now Big Cedar Ridge in central Wyoming. This unusual preservational event allows us to sample the plants and the soils they grew on very much the same way ecologists sample living vegetation. We can reconstruct the types of vegetation that grew on the Cretaceous landscape, and the local habitats that different kinds of plants preferred.

The blue-gray layer across the middle of the ridge is the volcanic tuff that preserved the plants. It is exposed at the surface across ~4 km.

A small palmetto (Sabalites sp.) in growth position. The zigzag pattern follows the plications of the original leaf.

A crew collecting fossils at Big Cedar Ridge. The packages on the ground are wrapped fossils.

At each of 100 sites along the outcrop we used grids like these to estimate the area of bedding plane covered by each type of plant that grew there.

To date fossil collections at 100 sites along Big Cedar Ridge have produced about 175 types of plants, many of them new to science. Because many of the plants don’t have formal scientific names yet, we use letter designations followed by a number: F=fern, CO=conifer, CY=cycad, M=monocot, DE=dicot with smooth-margined leaf, DN=dicot with toothed or lobed leaf. The most abundant plant overall was the palmetto, Sabalites sp., which covered almost 25% of the area of identified fossils. Several kinds of ferns were also very abundant, and altogether ferns made up half of the area of identified fossils. Although flowering plants with broad leaves (magnolia-relatives and true dicots) were highly diverse (>70 types), they were relatively rare, making up only ~12% of the area of identified fossils.

By analyzing the types of plants found at each of the sites and the characteristics of the soil they were growing in, we have been able to reconstruct three main kinds of vegetation that grew on the Big Cedar Ridge landscape 73 million years ago.

Fern Wetland

Fossil sites at the south end of the Big Cedar Ridge are dominated by ferns. Several types are especially common, including Hausmannia sp. (F11 - Dipteridaceae), several species of the fern family Gleicheniaceae (F8, F9, F10), and one fern in the family Matoniaceae (F17). Most of these ferns are restricted to the tropics and subtropics today. Soils at the south end of the outcrop are very high in organic matter, even forming a 10 cm thick coal in some places. Few flowering plants grew on this peaty soil, but one that was moderately common was a relative of living buttercups (DN9 - Ranunculaceae).

Reconstruction of the fern wetland vegetation at Big Cedar Ridge, showing cordate leaves of Hausmannia sp., blue-green pinnate leaves of the cycad Ctenis sp., and a specimen of F17 (Matoniaceae). (Mary Parrish reconstruction)

Click on the images below to see larger photos of the fossil used in this reconstruction:

Hausmannia

To learn more about how reconstructions are done, click here.

 

Palmetto Thicket

Fossil sites near the north end of Big Cedar Ridge were dominated by the palmetto Sabalites sp., the fern F2 (family unknown), and at a few sites by a relative of the living Norfolk Island pine, Araucarites sp. A fern called Sectilopteris sp. (F1) was also moderately common. The vegetation may have been something like the palmetto swamps that grow in the southern US today. The soil underneath these palmettos was moderately organic rich, but not peaty, like the soils in the fern wetland to the south.

Reconstruction of palmetto-dominated vegetation at Big Cedar Ridge. In addition to the palmetto, Sabalites sp., this painting shows the most abundant fern found in this area, F2 (family unknown). (Mary Parrish reconstruction)

 

Click on the images below to see larger photos of the fossil used in this reconstruction:

To learn more about how reconstructions are done, click here.

Streamside Thicket

There is one part of the Big Cedar Ridge outcrop where fossil sites are dominated by broad-leaved flowering plants. Unlike the fern wetland and the palmetto wetland, the soil here had little organic matter. A local increase in sandy sediments below the tuff, and a channel-shaped lens of unusually thick tuff, suggest that a stream channel traversed the landscape in this area. In short, this area had been disturbed by channel erosion and fresh sediment deposition just before the tuff that preserved the landscape was deposited. The dominant plant in this area was probably related to living gooseberries (DN14 – Saxifragales). Quite a few other dicots occur in this area: DE8 (probably Laurales), DN12 (probably Austrobaileyales), and DN21 (family unknown). Apparently they grew well on bare mineral soils that had recently been deposited. Because of the nearby stream channel the soil here was probably wet, just as it was across the rest of the Big Cedar Ridge landscape.

Reconstruction of streamside vegetation at Big Cedar Ridge. This was the only part of the landscape with abundant broad-leaved flowering plants. The lobed and toothed leaves are DN14 (probably the order Saxifragales). The smooth-margined cordate leaves in the lower right of the painting are DE8 (probably the order Laurales), and the slender leaves at the top are DN12 (probably the order Austrobaileyales). (Mary Parrish reconstruction)

Click on the images below to see larger photos of the fossil used in this reconstruction:

To learn more about how reconstructions are done, click here.

 

External Links

A brief description of an amateur fossil hunter's visit to Big Cedar Ridge:

 

A short description of Big Cedar Ridge from the Worland Chamber of Commerce:

 

Big Cedar Ridge has been designated as an area of critical environmental concern by the Bureau of Land Management, who manage the area. 260 acres of the outcrop are managed primarily for research, public education, and fossil interpretation, as well as hobby (noncommercial) collection of fossils.

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