White River Group Research Articles

Characterization of Modern Turtle Death Sites for Comparison with Late Eocene and Early Oligocene Turtle Sites

Taphonomic studies of extant turtles are useful for interpreting the taphonomy of fossil turtles. In order to provide modern comparators for fossil turtle sites, we have characterized two modern turtle (Chrysemys picta) deathsites; one in northwestern Nebraska and one in southwestern South Dakota. During these studies we characterized carapace position (up or down), presence or absence of non-shell elements, relative spatial position of the turtles and presence of shell disturbances (lesions). In the Trunk Butte site (Nebraska), six turtles were in carapace up position, five were in carapace down and one was not determinable. In addition, seven of those turtles contained some non-shell elements and five had indeterminable non-shell element status. At the Buffalo Gap site (South Dakota), four were carapace down, two were carapace up and three were not determinable for that character. Six of these had non-shell elements associated with them and three had no non-shell elements. The occurrence of turtles in the carapace down position suggests either the presence of medium to large scavengers able to overturn a turtle or that the turtle died while in the water and overturned while sinking. The spatial distributions of turtles in the two modern sites were also plotted and compared to that of two Whitney (Brule Formation, White River Group) and one Chadron (White River Group) site to address the hypothesis that the fossil assemblages were associated with small ponds. The results suggest that the fossil turtle sites were not the result of death events associated with small ponds, nullifying that hypothesis.

Isotopic evidence for the diversity of late Quaternary loess in Nebraska: Glaciogenic and nonglaciogenic sources

Pb isotope compositions of detrital K-feldspars and U-Pb ages of detrital zircons are used as indicators for determining the sources of Peoria Loess deposited during the last glacial period (late Wisconsin, ca. 25–14 ka) in Nebraska and western Iowa. Our new data indicate that only loess adjacent to the Platte River has Pb isotopic characteristics suggesting derivation from this river. Most Peoria Loess in central Nebraska (up to 20 m thick) is non-glaciogenic, on the basis of Pb isotope ratios in K-feldspars and the presence of 34-Ma detrital zircons. These isotopic characteristics suggest derivation primarily from the Oligocene White River Group in southern South Dakota, western Nebraska, southeastern Wyoming, and northeastern Colorado. The occurrence of 10–25 Ma detrital zircons suggests additional minor contributions of silt from the Oligocene-Miocene Arikaree Group and Miocene Ogallala Group. Isotopic data from detrital K-feldspar and zircon grains from Peoria Loess deposits in eastern Nebraska and western Iowa suggest that the immediate source of this loess was alluvium of the Missouri River. We conclude that this silt probably is of glaciogenic origin, primarily derived from outwash from the western margin of the Laurentide Ice Sheet. Identification of the White River Group as the main provenance of Peoria Loess of central Nebraska and the Missouri River valley as the immediate source of western Iowa Peoria Loess indicates that paleowind directions during the late Wisconsin were primarily from the northwest and west, in agreement with earlier studies of particle size and loess thickness variation. In addition, the results are in agreement with recent simulations of non-glaciogenic dust sources from linked climate-vegetation modeling, suggesting dry, windy, and minimally vegetated areas in parts of the Great Plains during the last glacial period.

The Chuska erg: Paleogeomorphic and paleoclimatic implications of an Oligocene sand sea on the Colorado Plateau

Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern New Mexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic field of western New Mexico-Arizona (as much as 300 m thick). 40 Ar/ 39 Ar ages of intercalated volcanic rocks indicate eolian deposition in these areas was approximately synchronous, with eolian accumulation beginning regionally at ca. 33.5 Ma and ending at ca. 27 Ma. Probable eolian sandstone of Oligocene age 483 m thick is also present in the subsurface of the Albuquerque Basin of the Rio Grande rift. The beginning of eolian deposition on the Colorado Plateau corresponds closely to the beginning of eolian (loessic) deposition in the White River Group of the Great Plains and major Oi1 glaciation in Antarctica, suggesting possible global paleoclimatic control. Successions of Oligocene eolian sandstone on the Colorado Plateau are thicker than all of the better known Upper Paleozoic-Mesozoic eolianites in the region, except the Jurassic Navajo Sandstone. We suggest that the widely separated Oligocene eolianites in the Colorado Plateau region were probably originally continuous, and thus are erosional remnants of an extensive (∼140,000 km 2 ), regional sand sea (the Chuska erg). This interpretation is based on: (1) comparison with thickness trends of older eolianites in the Colorado Plateau region, (2) evaluation of regional topographic gradients of modern ergs, and (3) hydrologic modeling of a 300- to 400-m–thick zone of saturation that existed during eolian deposition in the Chuska Mountains. The Chuska erg represents the final episode of Paleogene aggradation on the central and southern Colorado Plateau. Aggradation was driven primarily by trapping of fluvial sediments on the plateau by development of major volcanic fields along the eastern plateau margin. These volcanic fields blocked earlier Laramide drainages that had previously transported sediments eastward off the plateau. Following a shift to widespread eolian deposition at ca. 33.5 Ma, constructional volcanic topography induced eolian accumulation upwind of developing volcanic fields. Stratal accumulation rates (not decompacted) of eolian deposits were ∼28–82 m/m.y. The reconstructed top of the Chuska erg would lie at a present-day elevation of ∼3000 m or more, and provides a datum for assessing subsequent erosion on the Colorado Plateau. Major exhumation (≥1230 m) occurred during the late Oligocene and early Miocene, following the end of Chuska deposition and prior to the onset of Bidahochi Formation deposition at ca. 16 Ma on the south-central part of the plateau. The Bidahochi Formation attained a thickness of ∼250 m by ca. 6 Ma, followed by ∼520 m of late Miocene and younger erosion in the valley of the Little Colorado River. The depth of late Oligocene-early Miocene (ca. 26–16 Ma) exhumation of the central and southern Colorado Plateau thus was more than twice that of the late Miocene-Holocene (ca. 6–0 Ma). The timing of initial deep erosion in the Colorado Plateau-Southern Rocky Mountains region suggests the beginning of major epeirogenic rock uplift occurred during post-Laramide magmatism.

Skeletochronology: A Method For Determining The Individual Age and Growth of Modern and Fossil Tortoises (Reptilia: Testudines)

Skeletochronology is a method used to estimate the individual ages of animals by counting lines of arrested growth (LAGs) within skeletal tissues. This method was applied to evaluate the ages of modern gopher tortoises, Gopherus polyphemus, from north central Florida and fossil tortoise species, Gopherus laticuneus and Stylemys nebrascensis, from the White River Group in northwestern Nebraska. Different skeletal elements were tested for growth lines and the humerus was determined to be the most useful bone for analysis. LAGS in Gopherus polyphemus were correlated with alternative age estimations based on scute annuli counts, carapace lengths, and plastron lengths. While age estimates were similar in younger individuals, the alternative aging techniques did not accurately reflect the ages of older individuals. Scute wear, sexual dimorphism, and decreased growth with age are factors contributing to this discrepancy. Similarly, LAGS were found in the humeri of Gopherus laticuneus and Stylemys nebrascensis. Fossil specimens range in age from 0 years (hatchling) to over 40 years old. Fossil samples provide insight into changing ecological conditions during the Eocene-Oligocene Transition. This critical period appears to show a shift from a Stylemys nebrascensis to Gopherus laticuneus dominant ecosystem.

Magnetostratigraphy and paleontology of Wagner Quarry, (late Oligocene, early Arikareean) basal Arikareean Group of the Pine Ridge Region, Dawes County, Nebraska

Mammalian fossils (the Wagner Quarry local fauna) from the basal Arikaree Group (Late:Oligocene) near Chadron, Dawes County, Nebraska, are described. It is the first large mammal concentration described from the Pine Ridge Arikaree Group. Twenty-seven species of mammals are present: 1 marsupial (Herpetotherium), 2 insectivores (Proscalops, Ocajila), 9 rodents (Downsimus; Aywoodia, Cedromus savannae n. sp., Nototamias, Agnotocastor, Palaeocastor, Proheteromys, Leidymys, Geringia), 3 lagomorphs (Palaeolagus {2 sp.], Megalagus), 3 carnivores (Paradaphoenus, Canidae, Nimravus), 3 perissodaciyls (Miohippus, Diceratherium [2 sp.]), and, 6 artiodaciyls (Entelodontidae, Desmatochoerinae, Leptauchenia, Anthracotheriidae, Pseudolabis, Nanotragulus). Faunal correlation, with the Ridgeview local fauna and the radibisotopically constrained faunas of the Wildcat Ridge indicates an early Arikareean (Arl, 30-28 Ma, Tedford et al. 2004) North American Land Mammal age for the fauna. The first paleomagnetic study of the Pine Ridge basal Arikaree Group (Wagner Quatry section) shows that polarity signals are not the same, as basal Arikaree Group sediments (Gering Formation) in the Wildcat Ridge region. Correlation of the Wagner Quarry section with Chron 10n (28.25- 28.9 Ma) places it older than the Gering Formation (Chron 9r, 27.95-28.25 Ma) and younger than the lowermost White River Group "Brown Silistone" (Chron 10r-11n, 28.9-30.08 Ma) of Nebraska. These sedimbnts could represent time not recorded in the Wildcat Ridge. Cedromus (Sciuridae), Oligospermophilus (Sciuridae), and Alwoodia (A lodontidae) are newly recognized in the earliest Arikareean (Arl) of the central Great Plains.

Paleoenvironmental implications of size, carapace position, and incidence of non-shell elements in White River turtles

In this study, 314 fossil turtles of Nebraska's White River Group were used to gain insights into the regional paleoecology of the late Eocene and Oligocene. We collected taphonomic information about the size, the associated fossils, carapace position (up vs down), the presence of non-shell elements, and the presence of pre-burial weathering. A plot of turtle size vs strata reveals that turtle size decreased dramatically in the Whitney (ca. 31 Ma), and then increased again by the time the Arikaree was deposited. These size fluctuations appear to reflect the major cooling and warming trends at the beginning and end, respectively, of the Oligocene. Analyses of carapace position (up vs down), pre-burial weathering, and non-shell elements indicate several modes of death and preservation among the different turtle populations, including scavenging, burial by flood, and prolonged cold or drought. One 10 m interval in the Whitney Member, near the 31 Ma Lower Whitney Ash, contains a large number of small, completely articulated turtles preserved in mostly upright position. This indicates an expiration and subsequent preservation environment free of scavenging or high energy disturbances that might overturn a turtle.

Deformation structures and an alteration zone linked to deposition of volcanogenic sulphate in an ancient playa (Oligocene of Nebraska, USA)

AbstractHistoric, sulphur‐rich volcanic eruptions have altered global climate for as much as five years, and much larger events are known from the geologic record. At Scotts Bluff, Nebraska, Early Oligocene strata of the lower Arikaree Group contain a tephra bed with abundant calcite pseudomorphs after gypsum. Previous work has shown sulphate from the pseudomorphs in this tephra bears a high 17O anomaly indicative of oxidation of sulphur gases by ozone or hydrogen peroxide in the atmosphere. Possible sources of the tephra were caldera eruptions at about 28 Ma in the San Juan volcanic field of south‐western Colorado (∼500 km SW of the study site) and the eastern Great Basin (∼1000 km WSW). The present sedimentological study shows that tephra and volcanogenic sulphate were deposited and preserved within a small, surface‐discharging playa that developed on the irregular upper surface of aeolian siltstones of the subjacent White River Group. Sulphate solutions (including perhaps sulphuric acid) percolated downward within the vadose zone, dissolving early formed smectite cement within underlying volcaniclastic sandstones, reddening these rocks along an irregular alteration front. Preserved fine‐scale stratification within the sandstones precludes the possibility that reddening took place during pedogenesis. Displacive growth of gypsum at the playa centre folded tephra beds and forced tephra into underlying sandstones, forming elongate cones. The large mass fraction of gypsum (now replaced by calcite) in the playa sediments suggests a huge, long‐distance delivery of sulphate aerosols. Some of the sulphate and tephra may have come from the same eruption, or the fine‐grained tephra may simply have aided preservation of dry‐fog sulphate derived from an unrelated, effusive eruption of lava.

MIOCENE MAMMALIAN FAUNAS, OGALLALA GROUP, PAWNEE BUTTES AREA, WELD COUNTY, COLORADO

Abstract The Pawnee Buttes of northeastern Colorado lie within spectacular outcrops of Oligocene (White River Group) and Miocene (Ogallala Group) strata that form part of the Chalk Bluffs, an 150 mile long escarpment near the state line. These deposits were some of the earliest High Plains units to be explored by vertebrate paleontologists (Marsh, 1870; Cope, 1873). Consequently their fossil remains yielded the first information on North American faunas of mid- and late Tertiary time. Many parties subsequently exploited these deposits; their work, and his own investigations, were monographed by Galbreath in 1953. The present paper reviews the literature, a substantial part of the collections, as well as later stratigraphic work to present a revised litho- and biostratigraphy focused on the Pawnee Buttes area of eastern Weld County, Colorado. The oldest post-White River Group unit is the Martin Canyon Formation, the type locality of which is in adjacent Logan County. It contains an early Hemingfordian faun...

A New Method for Finding Small Vertebrate Fossils: Ultraviolet Light At Night

Vertebrate fossils from many different formations fluoresce when exposed to ultraviolet (UV) light. In this paper field observations and controlled experiments in the Chadron Formation (White River Group, late Eocene) of Wyoming are used to assess the utility of searching for fossils at night using ultraviolet light. The results indicate that, especially for very small teeth and egg‐shell fragments, searching with ultraviolet light at night can result in significantly more specimens than searching during daylight hours. This method has the potential to increase sample sizes for small vertebrate specimens that are often overlooked when using standard collecting techniques.

Using phytolith assemblages to reconstruct the origin and spread of grass-dominated habitats in the great plains of North America during the late Eocene to early Miocene

This paper outlines a general approach for analyzing siliceous microfossils from plants (phytoliths) in geologic sediments and uses this method to reconstruct the evolution of grasslands in the central Great Plains during the late Tertiary. Phytolith assemblage analysis is widely employed for reconstructing vegetation types in Pleistocene–Holocene paleoecology and archaeology, but no standardized methodology is currently available. Published phytolith studies are often system-specific and tailored after specific modern analogues. As a result, they vary in extraction technique, classification scheme, and method of inference. For ancient phytolith assemblages whose modern analogues are not well understood, a more general analytic approach is necessary. This analytic approach includes studying all size fractions (2–250 mm), using a classification scheme based on the available literature and a comprehensive modern phytolith reference collection, and comparing relative frequencies through time of all diagnostic phytoliths to interpret changes in habitat structure. This general method was applied to late Eocene to early Miocene sediment samples from northwestern Nebraska to deduce vegetation history and the results were compared to those obtained using a more specific technique [Strömberg, Paleogeogr. Paleoclimatol. Paleoecol. 177 (2002) 59]. In both studies, phytoliths were extracted by means of heavy liquid separation from the following lithostratigraphic units: (1) the late Eocene Chadron Formation and the early Oligocene Brule Formation (White River Group); (2) the late Oligocene–early Miocene Monroe Creek Formation, the early Miocene Harrison Formation and Anderson Ranch Formation (Arikaree Group); and (3) the early Miocene Runningwater Formation and Box Butte Formation (Ogallala Group). The two techniques, general vs. specific, produced results that differ markedly. Data from the general approach indicate the presence of relatively closed habitats in late Eocene to early Oligocene, typified by the presence of phytoliths from bambusoid grasses, woody and herbaceous dicotyledons, and palms. The introduction and spread of modern, open-habitat grasses, such as pooids, arundinoids, and panicoids, occurred between the late Oligocene and early Miocene. This resulted in a savanna or woodland type habitat by the early Miocene. In contrast, analysis using the specific method suggested the presence of open habitats in northwestern Nebraska from the late Eocene onward. The difference in interpretation based on the two approaches is principally due to inclusion of phytoliths >50 mm and the use of an expanded reference collection in the current study (general analytic approach). These results point to the importance of methodological choices in phytolith analysis and emphasize the usefulness of phytoliths as paleoecological indicators in the fossil record.

The auditory region and nasal cavity of Oligocene Nimravidae (Mammalia: Carnivora)

ABSTRACT Highly detailed microscopic preparation of both newly discovered and previously undescribed nimravine crania from the White River Group of North America reveals hitherto unreported morphological features of the basi-cranium and auditory region, particularly the composition of the auditory bulla and form of the petrosal. The monophyly of Dinictis, Hoplophoneus, and Nimravus is supported by a remarkably uniform configuration of the auditory bulla: (1) the form and placement of the entotympanics; (2) a near-vertical ectotympanic with elaborate styliform process partially enclosing the Eustachian tube; and (3) an anterior bulla wall formed by a unique overlap of the anterior entotympanic lamina by the elongate styliform process. Although an intact auditory bulla is unknown in an Eurasian nimravine, the auditory regions of the species represented by crania (Eusmilus bidentatus and Nimravus cf. intermedius from Quercy) demonstrate a definite affinity with New World genera, affirming the Holarctic distribution of the Nimravinae. In addition to identifying basicranial similarities among nimravines, we describe the first largely intact turbinate bones within the nasal cavity of a nimravid. CT (computed tomography) scanning of nimravid rostra reveals differing configurations of the maxilloturbinate in Dinictis and Hoplophoneus, the former more derived than the latter. Compared to other Carnivora, nimravid maxilloturbinates are most similar to those of living aeluroids, which are confined to the ventral part of the anterior nasal cavity. Nimravids lack the elaborately branching (dendrolamellar) maxilloturbinates of arctoid and cynoid carnivorans.

Eocene and Oligocene Paleosols of Central Oregon

Eocene and Oligocene Paleosols of Central Oregon, Gregory J. Retallack, Erick A. Bestland, and Theodore J. Fremd (Editors), 2000, Geological Society of America Special Paper 344, Boulder, 196 p. (Paperback $58.00) ISBN: 0-8137-2344-2. In the past 20C years, paleopedology (the study of paleosols, or ancient soil horizons) has become one of the hottest areas of research in sedimentary geology. A number of geologists have added paleopedology to their repertoire of skills, and many important discoveries have emerged from the study of paleosols. One of the scientists who sped up this process in the United States was Greg Retallack of the University of Oregon. Beginning with his pioneering studies of Eocene-Oligocene paleosols of the White River Group in the Big Badlands of South Dakota (published in 1983 as GSA Special Paper 193, and summarized in his 1990 book Soils of the Past ), Retallack has made many important discoveries using ancient soils. GSA Special Paper 344 presents the results of many years of research by Retallack, his former student Erick Bestland, and John Day Fossil Beds park paleontologist Ted Fremd. Focusing on the spectacular color-banded buttes and cliffs of the Eocene-Oligocene Clarno and John Day Formations of central …

Paleopedology of the Chadron Formation of Northwestern Nebraska: implications for paleoclimatic change in the North American midcontinent across the Eocene–Oligocene boundary

The Big Cottonwood Creek Member is a newly recognized lithostratigraphic unit of the Chadron Formation of northwestern Nebraska. I measured 23 superimposed paleosol profiles within 29 m of strata exposed 0.4 km (0.25 mile) east of the type section of the Big Cottonwood Creek Member at Toadstool Park in northwestern Nebraska and recognized four main types of paleosols. The paleosols are all well drained, moderately alkaline to alkaline, oxidized, and range from weakly developed azonal Ustifluvents, to moderately and well-developed Paleustalfs and Argiustolls with argillic and petrocalcic horizons. Based on changes in soil structure, horizons, root traces, and geochemical trends, these paleosols record a gradual change from humid, forested conditions of the late Eocene to more seasonal, semi arid, open range conditions leading up to the global climatic deterioration event of the early Oligocene at 33.2 Ma. This interpretation is supported by associated changes in sedimentology, fluvial architecture, floras, and faunas over this period of time. The section at Toadstool Park is significant in that it establishes a link between the Big Badlands of South Dakota and correlative deposits of Wyoming. The contact of the Chadron and Brule Formations in the Big Badlands of South Dakota is marked by an unconformity that is in part correlative to the earliest Oligocene sediments that elsewhere record the global climatic deterioration event at 33.2 Ma. The paleosols of the Big Cottonwood Creek Member of northwestern Nebraska fill in a significant part of this gap and record paleoclimatic conditions of the late Eocene leading up to the global climatic deterioration event of the early Oligocene. Using the latest lithostratigraphic, biostratigraphic, magnetostratigraphic, and tephrostratigraphic data for the White River Group, this change to more arid conditions in the midcontinent appears to have progressed from west to east over time.

Recognition and implications of Eocene tufas and travertines in the Chadron Formation, White River Group, Badlands of South Dakota

There are few descriptions of pre‐Quaternary tufa or travertine, yet these deposits have important climatic, hydrological and tectonic implications. In the Badlands of South Dakota, the Late Eocene Chadron Formation contains a variety of tufa (fluvial, lacustrine, paludal and perched springline types), travertine and calcrete (pedogenic and non‐pedogenic types). In fluvial channels, tufas and tufa barrages (oncolites, tufa lithoclast conglomerates and phytoherms) are transitional to travertines (pseudostromatolitic boundstone gravels and small pinnacle resurgences with rimstone edges and pools). Lacustrine tufas formed in small ponds adjacent to other types of tufas and are similar to the ‘low‐energy, bench‐margin facies’ types observed in modern, groundwater‐fed, temperate‐zone hardwater lakes. Paludal tufas are laterally discontinuous sheets of pisoid‐intraclast wackestone that fill palaeotopography. Distal perched springline tufas are mounds of numerous sloping tufa sheets. Concretions and rhizoliths associated with palaeosols are recognized as pedogenic calcretes, whereas non‐pedogenic calcretes are variably cemented, vertically oriented conduits for palaeogroundwater discharge that are located in fault zones. These deposits resulted from palaeogroundwater discharge along faults during the Late Eocene. Coeval unroofing of the Black Hills uplift created a regional groundwater flow system through karsted Palaeozoic limestones exposed in the Black Hills. The Chadron Formation is an avulsion‐dominated, siliciclastic unit, yet important bodies of carbonate accumulated in it as a result of groundwater processes.

Late Quaternary loess in northeastern Colorado: Part II—Pb isotopic evidence for the variability of loess sources

A new application of the Pb isotopic tracer technique has been used to determine the relative importance of different silt sources for late Wisconsin loess in the central Great Plains of eastern Colorado. Samples of the Peoria Loess collected throughout the study area contain K-feldspar derived from two isotopically and genetically distinct sources: (1) glaciogenic material from Early and Middle Proterozoic crystalline rocks of the Colorado province, and (2) volcaniclastic material from the Tertiary White River Group exposed on the northern Great Plains. Pb isotopic compositions of K-feldspar in loess from two dated vertical sections (at Beecher Island and Last Chance, Colorado) vary systematically, implying climatic control of source availability. We propose a model whereby relatively cold conditions promoted the advance of Front Range valley glaciers discharging relatively little glaciogenic silt, but strong winds caused eolian erosion of White River Group silt due to a decrease in vegetation cover. During warmer periods, valley glaciers receded and discharged abundant glaciogenic silt, while surfaces underlain by the White River Group were stabilized by vegetation. Isotopic data from eastern Colorado loess sections record two warm-cold-warm cycles during late Wisconsin time between about 21 000 and 11 000 radiocarbon yr B.P., similar to results from other studies in the United States and Greenland.

Late Quaternary loess in northeastern Colorado: Part I—Age and paleoclimatic significance

Loess in eastern Colorado covers an estimated 14 000 km 2 , and is the westernmost part of the North American midcontinent loess province. Stratigraphic studies indicate there were two periods of loess deposition in eastern Colorado during late Quaternary time. The first period spanned ca. 20 000 to 12 000 14 C yr B.P. (ca. 20‐14 ka) and correlates reasonably well with the culmination and retreat of Pinedale glaciers in the Colorado Front Range during the last glacial maximum. The second period of loess deposition occurred between ca. 11000 and 9000 14 C yr B.P. This interval may be Holocene or may correlate with a hypothesized Younger Dryas glacial advance in the Colorado Front Range. Sedimentologic, mineralogic, and geochemical data indicate that as many as three sources could have supplied loess in eastern Colorado. These sources include glaciogenic silt (derived from the Colorado Front Range) and two bedrock sources, volcaniclastic silt from the White River Group, and clays from the Pierre Shale. The sediment sources imply a generally westerly paleowind during the last glacial maximum. New carbon isotope data, combined with published faunal data, indicate that the loess was probably deposited on a cool steppe, implying a last glacial maximum July temperature depression, relative to the present, of at least 5‐6 °C. Overall, loess deposition in eastern Colorado occurred mostly toward the end of the last glacial maximum, under cooler and drier conditions, with generally westerly winds from more than one source.

Faunal stability during the Early Oligocene climatic crash

Abstract Traditional Neo-Darwinism views species as highly flexible entities which adapt to climatic change by gradually evolving new morphologies. Of the 177 species of mammals from the upper Eocene-lower Oligocene (37-30 Ma) White River Group in the High Plains, most species are static for 2.4 million years on average, and some persist much longer. Only three examples of gradualism can be documented in the entire fauna, and these are mostly size changes. Contrary to expectations, most mammalian species show no change during the earliest Oligocene climatic crash (33.2 Ma), in spite of the fact that the vegetation changed from dense forests to open forested grassland, mean annual temperatures dropped 13°C, and conditions got much drier and more seasonal. Only a few mammalian lineages speciated, a few more went extinct, and the vast majority (62 out of 70) persisted through this climatic event with no observable response whatsoever. This evidence shows that organisms are much less responsive to the environment than short-term neontological studies suggest.

Megacerops kuwagatarhinus n. sp., an unusual brontothere (Mammalia, Perissodactyla) with distally forked horns

A new species of Megacerops, M. kuwagatarhinus, is described from deposits belonging to the White River Group in southeastern Montana. Megacerops kuwagatarhinus is distinguished from all other species of Megacerops by the peculiar morphology of the horns, which are bifurcated distally into two well-developed branches. Two specimens from the Hunter Quarry of Saskatchewan, Canada, also share this horn morphology and are referred to the species.

Pedogenesis and paleoclimatic implications of the Chamberlain Pass Formation, Basal White River Group, Badlands of South Dakota

The Chamberlain Pass Formation is a new lithostratigraphic unit representing the first phase of deposition of the Paleogene White River Group in western South Dakota. Analysis of pedogenic features within the channel and overbank deposits of the Chamberlain Pass Formation allows for a refinement of Middle to Late Eocene paleoclimates of this area. Pedogenic features within distal overbank deposits of the Chamberlain Pass Formation have long been recognized and classified as the Interior Paleosol Series ( Retallack, 1983). Previous interpretations held that distal overbank sediments and soils of this time were deposited and formed under humid, acidic environments. This research demonstrates that, while this interpretation is correct, soils of the Chamberlain Pass Formation were also subjected to periods of dryness. Pedogenic features preserved within the channel sandstone and proximal overbank deposits (which include root traces, peds, cutans, glaebules, Bt, Btc and Bc horizons, silcretes and altered detrial sandstone mineralogy) suggest the formation of Aqualfs, Aquults and Durixerults. Soils appear to have been acidic to slightly alkaline, oxidizing and moderately-to well-drained, depending upon the physiographic position of the soil and the prevailing climate at the time of formation. Soils formed within the channel and proximal overbank deposits represent a new soil series, herein designated as the Weta Paleosol Series. The Weta and Interior Paleosol Series have a catenary soil relationship, based on differences in parent material and soil moisture conditions. Middle to Late Eocene paleoclimate appears to have fluctuated between humid and dry conditions. Within the Weta Paleosol Series, soils formed within one physiographic and climatic regime were successively overprinted by pedogenic features of soils formed during later periods under different physiographic and climatic regimes. This has resulted in the presence of humid climate soil pedorelicts in dry climate soils, dry climate pedorelicts within humid climate soils and the overlapping of various physiogrpahically controlled pedogenic processes as baselevels changed. Sediments of similar pedostratigraphic and lithostratigraphic arrangement can be found in Nebraska, northern South Dakota and North Dakota. The recognition of these similar sediments in different basins across the northern Great Plains may allow for an increased understanding of regional tectonic, depositional, climatic and geomorphic events during the Paleogene.

The significance of incision and fluvial sedimentation in the Basal White River Group (Eocene-Oligocene), Badlands of South Dakota, U.S.A.

A newly defined lithostratigraphic unit, the Chamberlain Pass Formation (CPF), records the initial episode of incision, fluvial sedimentation, and pedogenesis in SW South Dakota following the retreat of the Cretaceous Interior Seaway. The CPF is Middle(?) to Late Eocene in age, and consists of fluvial sandstone and mudstone. Pedogenic modification of the unit has created a distinctive pedostratigraphic unit, the Interior Paleosol Series. The CPF thickens from west to east, achieving a maximum channel-belt thickness of ⩾ 11 m. Paleoflow data indicate that deposition of the CPF was restricted to a fault-controlled basin southeast of the Black Hills uplift. Sandstones in the CPF were derived from a recycled sedimentary rock source area to the west. In contrast, sandstones in the overlying Chadron Formation (Late Eocene) had a variety of sources including the Precambrian core rocks of the Black Hills uplift. Deposition of the CPF brackets four significant Paleogene changes in baselevel that occurred in this region. These events were: (1) Late Cretaceous to Middle(?) Eocene relative baselevel fall, weathering and erosion of the Cretaceous Pierre Shale to form the Yellow Mounds Paleosol, and fluvial incision; (2) Middle(?) to Late Eocene relative baselevel rise and deposition of the CPF; (3) Late Eocene relative baselevel fall, weathering and erosion of the CPF to form the Interior Paleosol, and fluvial incision; and (4) Late Eocene to Oligocene relative baselevel rise and deposition of the Chadron Formation. The first event was probably eustatic, the second was controlled primarily by local subsidence in a fault-bounded basin, the third records the tectonic uplift and unroofing of the Black Hills, and the fourth event was probably primarily controlled by eustasy, but other factors may have been important.