Rocky Mountain Geology 37.1

Microbial taphonomic processes in the fossilization of insects and plants in the late Eocene Florissant Formation, Colorado

NEAL R. O’BRIEN, HERBERT W. MEYER, KIMBERLY REILLY, ANGELENA M. ROSS AND STACEY MAGUIRE

Keywords: Florissant, diatoms, insect fossils, plant fossils, mucus, taphonomy

The remarkably well-preserved fossils in the Florissant Formation (Eocene) at Florissant, Colorado occur in a lacustrine shale consisting of numerous couplets of millimeter-thin, alternating laminae of diatomite and volcanic debris of ash and smectite clay. The results of this study indicate the importance of microbial processes in fossilization. The plant and insect fossils are found within the diatomite layers and are covered with a mucus film produced by diatoms. The laminated shale sequence represents cyclic sedimentation during which ash-clay sedimentation alternated with intervals of abundant diatom growth (blooming?). During each proliferation of diatoms, a mat of diatom mucus formed. Additional mucus came from secretions of bacteria lying on the diatom mat. Bodies of organisms blown or washed into the lake became entangled in the sticky mucus mat that later settled to the lake bottom. The mucus retarded decomposition of the plants and insect bodies while sinking through the water column and during the interval before burial on the lake floor. The fossil-containing diatomite layer was then buried under an ash-clay layer. The organisms escaped decomposition and were fossilized mainly because of the protective mucus covering.

A new insect and plant Lagerstätte from a Tertiary lake deposit along the Canyon Ferry Reservoir, southwestern Montana

EMILY A. COBABE, KEVIN R. CHAMBERLAIN, MICHAEL A. IVIE, AND J. JOSEPH GIERSCH

Keywords: Insects, Lagerst‰tte, Oligocene, paleobotany, Renova, Montana

A diverse Oligocene insect and plant Lagerst‰tte has been identified recently from paper shales in a sequence of fine-grained deposits exposed on the west side of Canyon Ferry Reservoir, southeast of Helena, Montana. Limited excavation and preparation thus far has led to the discovery of at least 37 families of insects and more than 40 taxa of plants (leaves, seeds, and stems). Preservation of the insect fossils is outstanding, with color patterns, feeding and reproductive structures commonly present. Quality of the preservation compares favorably with that of other Tertiary insect and plant Lagerst‰tten, such as Green River, Florissant, and Republic. The geologic affinities of the site are not certain, although the deposit has been regarded broadly as Oligocene, based on the mammalian biostratigraphy and an ash-flow tuff stratigraphically well below the insect horizon that has a U-Pb radiometric date of 32.0 ± 0.1 Ma.

Paleobiology of middle Eocene plant-insect associations from the Pacific Northwest: A preliminary report

Conrad C. Labandeira

Keywords: plant-insect associations, leaf mining, galling, bud feeding, oviposition, leaf rolling, component community, plant hosts, Eocene, Republic Flora, Klondike Mountain Formation, paleobiogeography, extinction

Well-preserved, middle Eocene floras from the Pacific Northwest, particularly the graben-fill deposit at Republic, Washington, offer an unique window for examination of some of the earliest documented, modern-aspect, plant-insect associations in the fossil record. Of special interest is fossil plant damage attributable to leaf mining and galling taxa that is very similar or identical to modern damage on the same host genera, indicating the geologically long-term persistence of some associations. Other types of middle Eocene, insect-mediated damage appear generalized and are not assignable to known types of modern associations, owing in part to imperfect taxonomic resolution of plant host, insect herbivore, or more likely, extinction. This preliminary report records the taxonomically diverse and morphologically detailed wealth of preserved foliar material in a Lagerst‰tte, which is important for addressing the macroevolutionary tempo of plant-insect associations. This relevance includes the prevalence of long-term associations, extinction of plant host and interacting insect herbivores, and the occurrence of host-switching at Cenozoic time scales for taxa presently confined to eastern Asia and North America.

Intraspecific dental variability in cf. Coryphodon anthracoideus (Mammalia: Pantodonta) from Roehler’s Coryphodon Catastrophe Quarry, Washakie Basin, Wyoming

Elizabeth M. McGee

Keywords: Coryphodon anthracoideus, Eocene, intraspecific variation, Washakie Basin, Wyoming

Roehler's Coryphodon Catastrophe Quarry (Wasatch Formation, lower Eocene) in the Washakie Basin, Wyoming is a paucispecific assemblage containing a minimum of 10 individuals from a single population of cf. Coryphodon anthracoideus. The RCCQ Coryphodon assemblage provides a relatively rare opportunity to assess intraspecific variation in a genus that has historically been difficult to classify due to variation from specimen to specimen and species to species. Variation in metric features of the dentition (i.e., length, width, and molar area) in the RCCQ Coryphodon is within the range observed in comparable sexually dimorphic extant species. The RCCQ Coryphodon assemblage is also compared to another paucispecific, single population assemblage of Coryphodon molestus from New Mexico. Intraspecific metric variation characterizing these two assemblages is also comparable. In both assemblages, overall intraspecific metric variation is most pronounced in the third molar, suggesting sexual dimorphism as the source of variation.

Structural and seismic-reflection evidence for development of the Simpson Ridge anticline and separation of the Hanna and Carbon Basins, Carbon County, Wyoming

Brian P. Kraatz

Keywords: basement-involved uplifts, out-of-the-basin faulting, Laramide orogeny, seismic-reflection profiling, Hanna Basin, Carbon Basin, Wyoming

The eastern border of the Hanna Basin, Wyoming, is defined by Simpson Ridge anticline, a Laramide structure that separates the Hanna Basin from the more easterly Carbon Basin. New geologic mapping along with interpretation of well logs and seismic-reflection data suggest that this structural feature was created through a combination of thick- and thin-skinned deformation during the contractional Laramide orogeny. Major west-directed, basement-involved faulting occurs beneath Simpson Ridge, suggesting that this structural feature is not related to the east-vergent Elk Mountain anticline. Although major basement-involved thrusts underlie Simpson Ridge, the development of the anticline also involved thin-skinned, out-of-the-basin thrusting related to the deeply rooted basement faults by a trishear deformational zone. An unconformity between the CretaceousPaleocene Ferris Formation and Paleocene Hanna Formation marks initiation of fault-controlled uplift along the Simpson Ridge anticline. On the eastern side of Simpson Ridge, the Hanna Formation was faulted westward onto the Lewis Shale, thereby yielding a younger-on-older structural relationship. Palynological data suggest that the age of the Hanna Formation in the southwestern Carbon Basin is late Paleocene. These data thus provide a maximum age for later deformation along Simpson Ridge anticline and for the consequent definition of the western edge of Carbon Basin.

Katharine Fowler-Billings: Pioneering woman field geologist trained in the Rocky Mountains

Carol D. Frost

Keywords: History of geology, biography, Precambrian, anorthosite, Fowler-Billings (Katharine), women in geology, geologists