Southwestern Wyoming Research Articles

Redescription of the Type Species for the Genus ‡Notogoneus (Teleostei: Gonorynchidae) Based on New, Well-Preserved Material

Abstract †Notogoneus osculus Cope, the type species for the geographically widespread Upper Cretaceous to Upper Oligocene genus †Notogoneus (Gonorynchidae), is redescribed in detail based on much new material. This species is known from the Early Eocene Fossil Butte Member of the Green River Formation, southwestern Wyoming. It is geologically the youngest known species of the family Gonorynchidae in North America, although the family is known from Oligocene and Miocene lacustrine deposits of Europe and Australia and is extant today in tropical marine environments of the Pacific (Gonorynchus spp.). In this paper we also correct a number of mistakes in previously published morphological descriptions of †N. osculus (e.g., in skull and caudal skeleton) based on a large, new sample of well-preserved, well-prepared specimens. Accurate description of this species is important because it is the type for the genus and it is also by far the best preserved of all known species of †Notogoneus. In addition, it may she...

Awakening an Old Field—A Case Study of a Refracturing Program in the Greater Green River Basin

Summary Restimulation of wells completed in the Frontier Formation, a low-permeability (e.g., 0.01–0.05 md) gas sand, has proved to be successful in the Fontenelle Field. The field is east of the Fon-tenelle reservoir, on the LaBarge Platform in the western part of the Greater Green River Basin in southwestern Wyoming. In this area the majority of hydrocarbon reserves in the Second Frontier are produced from the massive shoreface sandstones locally known as the Second Bench of the Second Frontier (Kf22b). To date, Berco Resources, LLC has increased reserves in the Fontenelle Field by restimulating four wells producing from the Frontier Formation with massive hydraulic fracturing treatments. Because of the success of these treatments, another operator in an offset area to the north has also successfully implemented a Frontier refracturing program. This paper summarizes the refracturing program and the success which Berco Resources, LLC has realized. The candidate selection process is examined, as well as operational considerations when restimulating old wells. The stimulation treatments and fluid systems are also investigated. Daily/monthly production data consisting of gas, condensate, and water rates along with well head pressures are evaluated for all cases with fractured well type curves, the pseudoradial flow specialty plot (Δp/q vs. log t), and a production simulator comparing actual versus modeled production rates. Finally, the impact of the refrac-turing treatments on economic return is presented. The success of the refracture treatments has led to a new drilling/completion program in the Fontenelle Field in 2006 by an operator that recently acquired theBerco Resources, LLC properties.

Using extant patterns of dental variation to identify species in the primate fossil record: a case study of middle Eocene Omomys from the Bridger Basin, southwestern Wyoming

Patterns of extant primate dental variation provide important data for interpreting taxonomic boundaries in fossil forms. Here I use dental data from several well-known living primates (as well as data from selected Eocene forms) to evaluate dental variation in Middle Eocene Omomys, the first North American fossil primate identified by paleontologists. Measurements were collected from a sample of 148 omomyid dental specimens recovered from Bridger B localities in the Bridger Basin, Wyoming. Most of these specimens have not previously been described. Nonmetric traits were also scored for this sample. Lower molar coefficients of variation range from 4.01 for M2 length (n = 80) to 6.73 for M3 talonid width (n = 57). All of the nonmetric traits scored exhibit less than 100% presence in the overall sample, including traits previously described as representative of Omomys (e.g., P4 metaconids present in 91%, n = 55; M2 pericones present in 80%, n = 15). Dental traits also vary in a set of spatially restricted localities from the same fossil horizon and in a separate, single fossil locality (DMNH 868, P4 metaconids present in 67%, n = 6). An increasing frequency in several premolar traits across time in these more restricted samples suggests an anagenetic change in Bridger B Omomys. However, this degree of morphological variability is consistent with that seen in extant primate species from single locations. Metric variation in this sample is comparable to that seen in other Eocene primates, such as new data presented here for the omomyid Arapahovius gazini from the Washakie Basin, southern Wyoming. Omomys metric variation is also comparable to that found in several samples of well-known extant primates from single localities (e.g., ring-tailed lemurs and gray-brown mouse lemurs). These metric data also correspond to the patterns of variability described in previously published studies of Omomyscarteri. In sum, a single species interpretation (O. carteri) for this new Bridger B Omomys sample from southern Wyoming is affirmed, and this study illustrates the usefulness of dental data from extant primates for evaluating primate fossil samples.

Tracking the Archean-Proterozoic suture zone in the northeastern Great Basin, Nevada and Utah

It is important to know whether major mining districts in north-central Nevada are underlain by crust of the Archean Wyoming craton, known to contain major orogenic gold deposits or, alternatively, by accreted crust of the Paleoproterozoic Mojave province. Determining the location and orientation of the Archean-Proterozoic suture zone between these provinces is also important because it may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. The suture zone is exposed in northeastern Utah and southwestern Wyoming and exhibits a southwest strike. In the Great Basin, the suture zone strike is poorly constrained because it is largely concealed below a Neoproterozoic–Paleozoic miogeocline and Cenozoic basin fill. Two-dimensional resistivity modeling of three regional north-south magnetotelluric sounding profiles in western Utah, north-central Nevada, and northeastern Nevada, and one east-west profile in northeastern Nevada, reveals a deeply penetrating (>10 km depth), broad (tens of kilometers) conductor (1–20 ohm-meters) that may be the Archean-Pro-terozoic suture zone, which formed during Early Proterozoic rifting of the continent and subsequent Proterozoic accretion. This major crustal conductor changes strike direction from southwest in Utah to northwest in eastern Nevada, where it broadens to ∼100 km width that correlates with early Paleozoic rifting of the continent. Our results suggest that the major gold belts may be over-isolated blocks of Archean crust, so Phanerozoic mineral deposits in this region may be produced, at least in part, from recycled Archean gold. Future mineral exploration to the east may yield large gold tonnages.

A Basinwide Perspective on Entrainment of Fish in Irrigation Canals

AbstractEntrainment in irrigation canals has been considered a major source of mortality for some fish populations. However, the magnitude of fish loss from entrainment is usually not evaluated from a basinwide perspective, thus precluding an assessment of population‐level consequences. We incorporated such a perspective in evaluating entrainment losses of Bonneville cutthroat trout Oncorhynchus clarkii utah and brown trout Salmo trutta in a southwestern Wyoming river basin that is used for irrigating agricultural crops. We estimated the number of Bonneville cutthroat trout and brown trout (>150 mm TL) entrained in irrigation canals and then compared these numbers to population estimates within the river main stem and across the entire basin. We also examined previous studies to determine annual mortality rates for assessment of realized losses to irrigation canals. Small percentages of the Bonneville cutthroat trout (1.2‐3.3%) and brown trout (0.4‐1.2%) populations within the basin were entrained in 2004 and 2005. These values were much lower than the total annual mortality rates for inland riverine cutthroat trout O. clarkii (57.4%) and brown trout (56.4%) in the mountain west region of the United States. Examining the number of entrained fish in context with the basinwide population and total annual mortality allows evaluation of the significance of fish entrainment losses to irrigation canals.

Habitat Selection of Rocky Mountain Elk in a Nonforested Environment

Abstract: Recent expansions by Rocky Mountain elk (Cervus elaphus) into nonforested habitats across the Intermountain West have required managers to reconsider the traditional paradigms of forage and cover as they relate to managing elk and their habitats. We examined seasonal habitat selection patterns of a hunted elk population in a nonforested high‐desert region of southwestern Wyoming, USA. We used 35,246 global positioning system locations collected from 33 adult female elk to model probability of use as a function of 6 habitat variables: slope, aspect, elevation, habitat diversity, distance to shrub cover, and distance to road. We developed resource selection probability functions for individual elk, and then we averaged the coefficients to estimate population‐level models for summer and winter periods. We used the population‐level models to generate predictive maps by assigning pixels across the study area to 1 of 4 use categories (i.e., high, medium‐high, medium‐low, or low), based on quartiles of the predictions. Model coefficients and predictive maps indicated that elk selected for summer habitats characterized by higher elevations in areas of high vegetative diversity, close to shrub cover, northerly aspects, moderate slopes, and away from roads. Winter habitat selection patterns were similar, except elk shifted to areas with lower elevations and southerly aspects. We validated predictive maps by using 528 locations collected from an independent sample of radiomarked elk (n = 55) and calculating the proportion of locations that occurred in each of the 4 use categories. Together, the high‐ and medium‐high use categories of the summer and winter predictive maps contained 92% and 74% of summer and winter elk locations, respectively. Our population‐level models and associated predictive maps were successful in predicting winter and summer habitat use by elk in a nonforested environment. In the absence of forest cover, elk seemed to rely on a combination of shrubs, topography, and low human disturbance to meet their thermal and hiding cover requirements.

Covenant Field: A major oil discovery in the Sevier thrust belt of central Utah

The overthrust belt of western North America extends from British Columbia to southern Nevada. Large anticlines lie on thrust sheets along the length of this overthrust trend. Many oil and gas fields have been discovered on these structures in Canada, but significant hydrocarbon production in the United States has been primarily limited to a productive salient in southwestern Wyoming and northeastern Utah (Figure 1). On 23 December 2003, Wolverine Gas and Oil-Kings Meadow Ranches well 17-1 drilled into a 487-ft oil column in the Navajo Sandstone in Sevier County, Utah. This new significant oil reservoir is 146 miles southwest of the nearest thrust belt production at Pineview Field in Summit County, Utah. The discovery of what we call Covenant Field is not only a major discovery for Wolverine Gas and Oil but could open up a new Mississippian-sourced oil and gas province in central Utah.

Stimulation Effectiveness in Multilayered, Tight Gas Reservoirs: Pinedale Anticline Area

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 100574, "Using Reservoir Modeling To Evaluate Stimulation Effectiveness in Multilayered ‘Tight’ Gas Reservoirs: A Case History in The Pinedale Anticline Area," by S.K. Schubarth, SPE, Schubarth Inc.; J.P. Spivey, SPE, Phoenix Reservoir Software LLC; and P.T. Huckabee, SPE, Shell E&P Co., prepared for the 2006 SPE Gas Technology Symposium, Calgary, 15–17 May. Low-permeability, or "tight," gas reservoirs are being developed at an ever-increasing rate in the U.S. Currently, there are several tight gas plays in the U.S. where multiple intervals are commingled for economic viability. The Pinedale anticline completions pose a complex problem in determining the best stimulation method because as many as 22 separate stimulation treatments are placed in as many as 70 discrete sands over a 6,000-ft-thick interval. Evaluation is complicated by permeability variations that exceed two orders of magnitude and pore-pressure gradients that range from 0.22 to 0.83 psi/ft. Introduction Development of the Pinedale anticline in southwestern Wyoming has continued at an aggressive pace over the past several years. Massive hydraulic-fracturing (MHF) treatments are the only way to stimulate production from the tight gas sands present in this area to economically acceptable levels. Generally, each well requires between 14 and 22 MHF treatments to produce effectively from all potential pay intervals. Often, more than 2 million lbm of proppant is used per well, representing a high investment cost to the operator. The ability to evaluate the incremental production associated with the use of one proppant vs. another can have a significant effect on field profitability. Data Analysis Production and completion information from 11 wells with 172 fracture-treated intervals was included in this analysis. Production information was updated, and all 11 wells have at least two production logs, with one early in the well life. A detailed production history also was available for all wells for use in the reservoir-simulation evaluation. Six different proppants were used in the fracture treatments pumped. Reservoir Simulation. For single-layer tight gas reservoirs, fracture and reservoir properties usually are estimated by analyzing production data. Methods for such single-layer analysis include advanced decline-curve analysis with constant-terminal-pressure type curves, type-curve matching with constant-terminal-rate type curves, and automatic history matching with a single-layer analytical reservoir simulator. Single-layer analysis methods may provide estimates of in-situ permeability to gas, fracture half-length, fracture conductivity, and drainage area, which are used in evaluating the success of a fracture treatment, selecting restimulation candidates, optimizing future fracture treatments, forecasting future performance, and estimating reserves.

Lunulichnus Tuberosus Ichnogen. and Ichnosp. Nov. from the Early Eocene Wasatch Formation, Fossil Butte National Monument, Wyoming: An Arthropod-Constructed Trace Fossil Associated with Alluvial Firmgrounds

A new trace fossil, Lunulichnus tuberosus, is described from fluvial deposits of the Wasatch Formation (early Eocene) at Fossil Butte National Monument, southwestern Wyoming, USA. L. tuberosus are straight, vertical to obliquely oriented, unlined cylindrical burrows with pronounced crescent-shaped wall sculptings. In situ examples of these trace fossils are most commonly preserved as sand-filled casts emanating from the erosional bases of fluvial channel sandstone bodies into underlying floodplain mudstone/siltstone beds. L. tuberosus is interpreted as the dwelling trace of a stream-dwelling decapod crustacean. Excellent preservation of fine detail, particularly their diagnostic crescent-shaped wall sculptings, support the hypothesis that L. tuberosus were excavated in firm substrata subjacent to fluvial erosional surfaces. As such, they are interpreted as constituents of alluvial Glossifungites trace fossil assemblages.

OIL SHALE RESEARCH IS MOVING FORWARD

THE BUSH ADMINISTRATION IS pressing ahead with plans to extract oil from vast underground beds of shale rock in the Rocky Mountains, a project that Congress hopes will eventually yield at least 800 billion barrels of recoverable crude. The push, mandated by the Energy Policy Act of 2005, comes more than two decades after the last boom in oil shale busted, leaving western Colorado's economy reeling for years. Soaring energy prices are behind t he revived interest in trying to t ap t he large reserves of oil buried in shale beneath huge stretches of federal land in northwestern Colorado, eastern Utah, and southwestern Wyoming. Oil shale generally refers to any sedimentary rock that contains solid bituminous materials that are released as petroleumlike liquids when t h e rock is heated in a process called retorting. The vast deposits of oil shale in the western U.S. have been known for a century, but attempts to ...

Linkage of Sevier thrusting episodes and Late Cretaceous foreland basin megasequences across southern Wyoming (USA)

ABSTRACTDeposition and subsidence analysis, coupled with previous structural studies of the Sevier thrust belt, provide a means of reconstructing the detailed kinematic history of depositional response to episodic thrusting in the Cordilleran foreland basin of southern Wyoming, western interior USA. The Upper Cretaceous basin fill is divided into five megasequences bounded by unconformities. The Sevier thrust belt in northern Utah and southwestern Wyoming deformed in an eastward progression of episodic thrusting. Three major episodes of displacement on the Willard‐Meade, Crawford and ‘early’ Absaroka thrusts occurred from Aptian to early Campanian, and the thrust wedge gradually became supercritically tapered. The Frontier Formation conglomerate, Echo Canyon and Weber Canyon Conglomerates and Little Muddy Creek Conglomerate were deposited in response to these major thrusting events. Corresponding to these proximal conglomerates within the thrust belt, Megasequences 1, 2 and 3 were developed in the distal foreland of southern Wyoming. Two‐dimensional (2‐D) subsidence analyses show that the basin was divided into foredeep, forebulge and backbulge depozones. Foredeep subsidence in Megasequences 1, 2 and 3, resulting from Willard‐Meade, Crawford and ‘early’ Absaroka thrust loading, were confined to a narrow zone in the western part of the basin. Subsidence in the broad region east of the forebulge was dominantly controlled by sediment loading and inferred dynamic subsidence. Individual subsidence curves are characterized by three stages from rapid to slow. Controlled by relationships between accommodation and sediment supply, the basin was filled with retrogradational shales during periods of rapid subsidence, followed by progradational coarse clastic wedges during periods of slow subsidence. During middle Campanian time (ca. 78.5–73.4 Ma), the thrust wedge was stalled because of wedge‐top erosion and became subcritical, and the foredeep zone eroded and rebounded because of isostasy. The eroded sediments were transported far from the thrust belt, and constitute Megasequence 4 that was mostly composed of fluvial and coastal plain depositional systems. Subsidence rates were very slow, because of post‐thrusting rebound, and the resulting 2‐D subsidence was lenticular in an east–west direction. During late Campanian to early Maastrichtian time, widespread deposits of coarse sediment (the Hams Fork Conglomerate) aggraded the top of the thrust wedge after it stalled, prior to initiation of ‘late’ Absaroka thrusting. Meanwhile Megasequence 5 was deposited in the Wyoming foreland under the influence of both the intraforeland Wind River basement uplift and the Sevier thrust belt.

Tularemia Transmitted by Insect Bites: Wyoming, 2001–2003

Tularemia is a zoonotic disease caused by Francisella tularensis, a fastidious, gram-negative coccobacillus that infects vertebrates, especially rabbits and rodents. In humans, tularemia is classified into six major syndromes: ulceroglandular (the most common form), glandular, typhoidal, oculoglandular, oropharyngeal, and pneumonic. The case-fatality rate among humans can reach 30%-60% in untreated typhoidal cases. Although bites from ticks and handling infected animals are considered the most common modes of tularemia transmission in the United States, the disease also is spread through ingestion of contaminated food or water, inhalation, and insect bites. During 2001-2003, Wyoming experienced an increase in reported human cases of tularemia. This report describes the subsequent investigation by the Wyoming Department of Health (WDH), which indicated that 1) insect bites (particularly from deerflies and other horseflies) were the most commonly reported likely mode of transmission, and 2) the increase in cases was geographically and temporally associated with an outbreak of tularemia among rabbits in southwestern Wyoming. To obtain a timely diagnosis and provide information on appropriate preventive measures, health-care providers and public health officials should have knowledge of the local epidemiology of tularemia, particularly regarding modes of transmission and resultant clinical syndromes.

GREATER SAGE-GROUSE NESTING HABITAT SELECTION AND SUCCESS IN WYOMING

Nesting habitat degradation and its negative effect on nesting success might contribute to the recent population and distributional declines of greater sage-grouse (Centrocercus urophasianus) throughout North America. We used radiotelemetry to locate greater sage-grouse nests in 7 different areas of central and southwestern Wyoming between 1994 and 2002; we studied each area for 2 to 4 years. Using binary logistic regression, we compared microsite vegetal data collected at nests (n = 457) and random (n = 563) sites and successful (n = 211) and unsuccessful (n = 238) nests to test hypotheses concerning greater sage-grouse nesting habitat selection and vegetal conditions associated with nesting success. We used Akaike's Information Criterion (AICc) and model averaging to make inference about the weighted support for the importance of individual habitat variables through the comparison of sets of competing models. Selected nest sites were located in areas with increased total shrub canopy cover (relative importance [RI] = 1.00), residual grass cover (RI = 0.47), and residual grass height (RI = 0.77) compared to random sites. Successful nests had increased residual grass cover (RI = 0.43) and height (RI = 0.48) relative to unsuccessful nests. Additionally, annual nest success rates (i.e., above vs. below our study's average) were related to the preceding year's spring (Apr–May; RI = 0.44) and winter–early spring (Jan–Jun) precipitation (RI = 0.32). Correct classification rates for weighted average models that we derived through the 3 comparisons were between 60 and 70%, suggesting the variables adequately differentiated between plot types. However, high model selection uncertainty (i.e., the total number of models included in the sets of AICc-selected models) suggested that nest site selection and nesting success may be influenced by factors not considered in the modeling process. Management strategies that protect dense sagebrush stands and enhance residual grass cover and height within those stands should be used to maintain nesting habitat and increase nesting success of greater sage-grouse.

TAENIODONTA OF THE WASHAKIE FORMATION, SOUTHWESTERN WYOMING

Six specimens of the rare taeniodont, †Stylinodon mirus, are recorded from the Lower part of the Adobe Town Member (TWkA1), Washakie Formation, Late Bridgerian–Early Uintan Land Mammal Age of the Washakie Basin. They are described and compared with other similarly aged specimens from other western North American basins. †S. mirus is the last of the taeniodonts, one of the early lines in the mammalian adaptive radiation that followed the demise of the dinosaurs. The most important of them is the articulated partial skeleton, FMNH PM 3895. It shows much new anatomical detail and it exhibits three notable specializations that provide a firm basis for speculation about its habitus. †Stylinodon inexplicatus Schoch and Lucas, 1981a, is shown to be a juvenile of †S. mirus, and is thereby put into synonymy with that species.

Pictures by the Seedskadee: A Preliminary Analysis of the Biographic Rock Art of the Green River Basin, Southwestern, Wyoming

The Green River Basin of southwestern Wyoming has only recently been recognized as containing a rich rock art record closely related to Plains styles and traditions. Preliminary recording in 2002 shows that Biographic rock art is a major component at six Green River Basin sites. The Biographic petroglyphs there represent four different styles that relate to both Northern Plains and Colorado Plateau rock art, reflecting the importance of the Green River as a traders’ rendezvous and travel route throughout the Historic period. An extensive Biographic composition at the La Barge Bluffs site is the finest realisticb composition currently known in Plains rock art.

Weed Invasion Susceptibility Prediction (WISP) Model for Use with Geographic Information Systems

The Weed Invasion Susceptibility Prediction (WISP) model was developed as an extension of the ArcView Geographic Information System to predict potential risk of invasion by individual weed species in rangelands. Existence potential was determined by comparing growth requirements of each weed species with respect to nine site characteristics obtained from geographic data layers: distance from water and disturbance sources, elevation, annual precipitation, soil texture and pH, aspect, slope, and land cover. Disturbance is important for predicting weed infestations; an innovative aspect of the WISP model is that we use data layers for transportation and energy development as an estimate of disturbance. Data from weed surveys conducted at the Jack Morrow Hills Study Area in southwestern Wyoming, USA, were used to test WISP model predictions for the occurrence of Hyoscyamus niger (black henbane), Cardaria draba (hoary cress), and Lepidium latifolium (perennial pepperweed). Data acquired from the U.S. Department of Interior, Bureau of Land Management were used to test model predictions for the occurrence of Euphorbia esula (leafy spurge) and Centaurea maculosa (spotted knapweed) for four counties in northwestern Wyoming, USA. The model accuracy, based on false-negative errors where a species was present but was predicted not to occur, averaged 89% for the five weed species. The WISP model allows land managers to predict more easily and accurately the potential for weed invasions in order to prioritize areas for detailed monitoring.

Ceratopsid dinosaurs from the Upper Cretaceous Almond Formation of southwestern Wyoming

The 1937 American Museum of Natural History (AMNH)–Sinclair Oil Company joint expedition to the late Campanian and early Maastrichtian Almond Formation of the Rock Springs Uplift, southern Wyoming, recovered two ceratopsid cranial specimens. AMNH 3652, a partial skull lacking the frill, is characterized by elongate, procurving postorbital horns and a deep rostrum. Although the specimen cannot be identified to the generic or specific level, it appears to be closely related to the clade of chasmosaurine ceratopsids including Anchiceratops, Arrhinoceratops, Diceratops, Torosaurus , and Triceratops , exclusive of Pentaceratops and Chasmosaurus . AMNH 3656, a frill fragment, is characterized by large, triangular marginal processes and an average thickness in excess of 40 mm. This specimen is similar to previously reported frills from Anchiceratops . The ceratopsid specimens from the Almond Formation are significant because they represent two early occurrences of chasmosaurine ceratopsids as well as a unique occurrence in southern Wyoming. Present address: Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794, U.S.A. email: afarke{at}ic.sunysb.edu

Late Quaternary stratigraphy and geochronology of the western Killpecker Dunes, Wyoming, USA

New stratigraphic and geochronologic data from the Killpecker Dunes in southwestern Wyoming facilitate a more precise understanding of the dune field’s history. Prior investigations suggested that evidence for late Pleistocene eolian activity in the dune field was lacking. However, luminescence ages from eolian sand of ∼15,000 yr, as well as Folsom (12,950–11,950 cal yr B.P.) and Agate Basin (12,600–10,700 cal yr) artifacts overlying eolian sand, indicate the dune field existed at least during the latest Pleistocene, with initial eolian sedimentation probably occurring under a dry periglacial climate. The period between ∼13,000 and 8900 cal yr B.P. was characterized by relatively slow eolian sedimentation concomitant with soil formation. Erosion occurred between ∼8182 and 6600 cal yr B.P. on the upwind region of the dune field, followed by relative stability and soil formation between ∼5900 and 2700 cal yr B.P. The first of at least two latest Holocene episodes of eolian sedimentation occurred between ∼2000 and 1500 yr, followed by a brief (∼500 yr) episode of soil formation; a second episode of sedimentation, occurring by at least ∼700 yr, may coincide with a hypothesized Medieval warm period. Recent stabilization of the western Killpecker Dunes likely occurred during the Little Ice Age (∼350–100 yr B.P.). The eolian chronology of the western Killpecker Dunes correlates reasonably well with those of other major dune fields in the Wyoming Basin, suggesting that dune field reactivation resulted primarily due to departures toward aridity during the late Quaternary. Similar to dune fields on the central Great Plains, dune fields in the Wyoming Basin have been active under a periglacial climate during the late Pleistocene, as well as under near-modern conditions during the latest Holocene.

A new species of cf. Dilophodon (Mammalia; Perissodactyla) from the early Bridgerian of southwestern Wyoming

ABSTRACT A new species of the diminutive ceratomorph similar to Dilophodon is reported from the latest early Eocene at Desertion Point in the Little Muddy area, Green River Basin, Wyoming. The Little Muddy specimens are assigned to cf. Dilophodon on the basis of their small size, simple premolars, absence of a first lower premolar resulting in a long post-canine diastema, and absence of a hypoconulid on the m3. Cf. Dilophodon destitutus, sp. nov. is the only helaletid known from the latest Early Eocene of the southwestern Green River Basin. It is considerably smaller than other species of Dilophodon. Its premolars are longer and narrower than in Dilophodon leotanus. The lophids on the lower molars are simpler and less posteriorly deflected than in either D. leotanus or Dilophodon minusculus. Dilophodon was previously known only from the middle Eocene (“Bridger C,” Bridgerian zone Br3) of southwestern Wyoming, and the late Eocene (Uintan, zone Ui3) of Wyoming, northern Utah, Montana, and possibly California. The specimens described here extend the chronologic range of Dilophodon to include the latest early Eocene (lower “Bridger A”; Bridgerian zone Brlb) of the Green River Basin, southwestern Wyoming.

Distribution of Phoma sclerotioides and incidence of brown root rot of alfalfa in Wyoming, U.S.A.

Brown root rot of alfalfa, caused by Phoma sclerotioides, has been reported from Alberta and Saskatchewan, Canada, and was the cause of widespread “winterkill” of alfalfa in southwestern Wyoming, U.S.A., in 1996. Affected plants exhibit characteristic dark, sunken, necrotic lesions on upper tap and lateral roots. Surveys of irrigated fields were conducted in nine counties to determine distribution of the pathogen within Wyoming. Symptomatic as well as asymptomatic root-tissue isolations demonstrated that P. sclerotioides was present in most counties surveyed, indicating its distribution may be statewide. Disease incidence was assessed in two fields where P. sclerotioides occurred. Results indicate that the pathogen is uniformly distributed within infested fields.