Bighorn Basin Research Articles

Sandstone body character and river planform styles of the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA

AbstractThe lower Eocene Willwood Formation of the Bighorn Basin, Wyoming, USA, is an alluvial succession with a sand content varying around 25%. It has been studied intensively for palaeontology, palaeoenvironments and palaeoclimates, as well as sedimentological and stratigraphic analysis. Channel dynamics were studied at a relatively low resolution throughout the basin over the geological time from late Palaeocene to early Eocene. Here, a high‐resolution study is reported to complement previous research at the basin scale. Efforts are made to document the characteristics and river planform styles of most sandstone bodies encountered through ca 300 m of alluvial stratigraphy in a 10 km2 area of the Deer Creek part of the McCullough Peaks area situated in the basin axis of northern Bighorn Basin. Four channel facies associations are recognized and ascribed to four river planform styles: crevasse channel, trunk channel, braided‐like channel and sinuous‐like channel, with the latter two types dominant. Braided‐like and sinuous‐like channel sandstone bodies differ significantly in thicknesses, being on average 6.1 m versus 9.0 m, but they have similar palaeoflow–perpendicular widths of on average 231 m and palaeoflow directions of on average N 003°. Braided‐like and sinuous‐like river planform styles show no spatial dependency in the 10 km2 study area. Results of this study are in line with existing basin‐scale depositional models that are composed of a single axial system fed by several transverse systems dominantly from the west. The feeding of these systems could be influenced by palaeoclimate changes possibly controlling their contribution over time, thereby impacting river planform styles. At the same time, changing water discharge hydrograph, sediment load and overbank cohesiveness may have equally driven the observed river planform style changes within the basin without a major role of catchments.

Local differences in paleohydrology have stronger influence on plant biomarkers than regional climate change across two Paleogene Laramide Basins, Wyoming, USA

Plant biomarkers in the geologic record are commonly used to reconstruct paleovegetation and paleoclimate. Yet the representativeness of these biomarker-based reconstructions is not entirely straightforward because both local and regional climatic influences are convolved in the recovered biomarker composition. Furthermore, geomorphic processes including sedimentation, deposition, burial rates, and surface and groundwater levels can also influence biomarker preservation and taphonomy. The combination of leaf compression fossils with the biomarker record provides a robust framework in which ecological reconstructions can be made. Herein we present a case study examining the biomarker responses to a major regional climatic signal, the Paleocene Eocene Thermal Maximum (PETM), from local effects in two basins with different paleohydrology: the Hanna Basin (HB) and Bighorn Basin (BHB), in Wyoming (USA). Data collected include (1) leaf wax n-alkanes and plant-derived non-steroidal di- and triterpenoids, (2) pristane/phytane vs. terrestrial to aquatic ratio (Pr/Ph:TAR), plant material source proxy (Paq), hopanes, and bacteria which provide constraints on organic matter sourcing, redox conditions, and bacterial inputs, and (3) carbon (δ13Cn-alk) and hydrogen (δ2Hn-alk) isotopic compositions of plant wax n-alkanes. We find the two basins differed significantly in weight % total organic carbon (TOC), alkane and terpenoid concentrations, and δ13Cn-alk and δ2Hn-alk values. We examined the effects of δ13Catm on δ13Cn-alk values by assuming comparable regional climate in the two basins. Differences in paleohydrology and paleovegetation were assessed using the above proxies and previously published paleobotanical data. Local paleohydrologic conditions, an abiotic factor, likely drove variability in plant biomarker preservation and paleovegetation structure. Notably, an abundance of water in the HB during the PETM created a conifer refugium while drying within the BHB caused a local extirpation. This study highlights the importance of considering the influence of local environment, specifically paleohydrology, on the local fossil record and the need to compare multiple sites to disentangle regional climate effects on terrestrial plant communities. Distinguishing local from regional differences is imperative if we are to understand how ecosystems respond to future climate change.

Terrestrial carbon isotope stratigraphy and mammal turnover during post-PETM hyperthermals in the Bighorn Basin, Wyoming, USA

Abstract. Paleogene hyperthermals, including the Paleocene–Eocene Thermal Maximum (PETM) and several other smaller events, represent global perturbations to Earth's climate system and are characterized by warmer temperatures, changes in floral and faunal communities, and hydrologic changes. These events are identified in the geologic record globally by negative carbon isotope excursions (CIEs), resulting from the input of isotopically light carbon into Earth's atmosphere. Much about the causes and effects of hyperthermals remains uncertain, including whether all hyperthermals were caused by the same underlying processes, how biotic effects scale with the magnitude of hyperthermals, and why CIEs are larger in paleosol carbonates relative to marine records. Resolving these questions is crucial for a full understanding of the causes of hyperthermals and their application to future climate scenarios. The primary purpose of this study was to identify early Eocene hyperthermals in the Fifteenmile Creek area of the south-central Bighorn Basin, Wyoming, USA. This area preserves a sequence of fluvial floodplain sedimentary rocks containing paleosol carbonates and an extensive record of fossil mammals. Previous analysis of faunal assemblages in this area revealed two pulses of mammal turnover and changes in diversity interpreted to correlate with the ETM2 and H2 hyperthermals that follow the PETM. This was, however, based on long-distance correlation of the fossil record in this area with chemostratigraphic records from elsewhere in the basin. We present new carbon isotope stratigraphies using micrite δ13C values from paleosol carbonate nodules preserved in and between richly fossiliferous mammal localities at Fifteenmile Creek to identify the stratigraphic positions of ETM2 and H2. Carbon isotope results show that the ETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, are recorded at Fifteenmile Creek. ETM2 and H2 overlap with the two previously recognized pulses of mammal turnover. The CIEs for these hyperthermals are also somewhat smaller in magnitude than in more northerly Bighorn Basin records. We suggest that basin-wide differences in soil moisture and/or vegetation could contribute to variable CIE amplitudes in this and other terrestrial records.

Swift Weathering Response on Floodplains During the Paleocene‐Eocene Thermal Maximum

AbstractSilicate weathering is thought to increase and offset the rapid, massive input of CO2 into the atmosphere and ocean during the Paleocene‐Eocene Thermal Maximum (PETM), but few nonmarine records have been used to quantify this. We probe changes in silicate weathering intensity by measuring Li isotope ratios of bedrock and ancient floodplain deposits spanning the PETM in the Bighorn Basin, Wyoming (USA). Our results reveal a rapid increase in silicate weathering intensity during the PETM that remained high during at least the initial stage of climate recovery. Additionally, we determine that soils that formed farthest from ancient river channels underwent larger weathering changes than near‐channel soils. Alongside increased temperatures and pCO2, the simplest explanation for this response relates to increased seasonal fluctuations in water table height in the floodplain that promote dissolution and precipitation reactions. These findings newly demonstrate that weathering on floodplains, like mountain hillslopes, responds to climate change.

Bug-bitten leaves from the early Miocene of Ethiopia elucidate the impacts of plant nutrient concentrations and climate on insect herbivore communities

High-resolution paleoecological studies elucidate the effects of biotic and abiotic factors on the diversity and distribution of insect herbivory in deep time. Previous analyses of fossil leaf assemblages from intervals of significant environmental change have shown that insect damage frequency and diversity are strongly influenced by temperature and carbon dioxide levels. Similar studies that span intervals of environmental quiescence are a necessary complement and can help disentangle the relative influences of biological versus abiotic factors. The 21.73 ± 0.03 million year old (early Miocene) Mush Valley lagerstätten, Ethiopia, provides one such example. Insect damage censuses were conducted at six stratigraphic levels that span ~50,000 years and whose plant ecology has been studied in detail. A total of 2200 leaves (47 morphotypes) were examined and 35 damage types (DTs) observed. We documented variations in insect damage frequency, diversity, and composition among plant host taxa and across stratigraphic levels and tested for correlations with abiotic and biotic factors. Interspecific variations in damage frequency and diversity among host plants at Mush with >20 leaves in the damage census could not be explained solely by leaf mass per area, presence/absence of trichomes, symbiosis with nitrogen-fixing bacteria, or host plant abundance. The strongest predictor of insect herbivore damage on the bulk floras (all leaves) was the prevalence of legumes, whose symbiotic relationships with nitrogen-fixing bacteria likely made them more nutrient-rich. This pattern was also observed across the climatically-variable Paleocene-Eocene boundary in the Bighorn Basin, USA, suggesting that the soil microbiome, plant nutrient composition, and climate work synergistically in regulating insect herbivore distribution, population sizes, and feeding. • A new Miocene record of fossil insect herbivory from the African tropics. • Insect damage frequency and diversity strongly correlate to abundance of plants with N 2 -fixing symbionts. • Predicting variability in herbivore damage among fossil plant hosts is challenging.

Paleo-vegetation and environmental history of Natural Trap Cave based on pollen and carbon isotope analyses

Pollen and carbon isotope data from Natural Trap Cave provides insights into the environmental conditions of the Bighorn Basin, in north-central Wyoming, USA during the time intervals 151.1 to 132.2 cal ka BP, 51.8 to 17.4 cal ka BP, and 10.4 cal ka BP to present. Similarities between the buildup of the penultimate and last glacial maxima include abundant Artemisia, Amaranthaceae, Asteraceae, and Poaceae pollen, which, when combined with high δ13C values (>−25.0‰) and low plant discrimination values (18.0–19.0‰) from sedimentary organic matter, indicate steppe-like, cold-dry conditions. Between 151 and 142.5 cal ka BP, abundant Poaceae associated with much lower δ13C values (−28.0‰ to −26.0‰) and higher plant discrimination values (19.0‰–21.0‰) could be indicative of relatively wet summers. Similarly, after 25 cal ka BP increases in Poaceae may indicate wet conditions during the growing season. Holocene conditions differ from the earlier sections of the record, with greater arboreal pollen abundance (i.e., Pinus), consistent with woodland expansion in the region. Carbon isotope data from sedimentary organic matter show the Holocene was more humid than the glacial conditions of the late Pleistocene. This is especially clear at the start of the Holocene when a large negative carbon isotope excursion (CIE, < −28.0‰) occurred, providing a distinct chemostratigraphic marker defining the Pleistocene-Holocene boundary within these cave sediments. The association of this CIE with a pronounced spike in Pinus pollen supports an interpretation of a short-lived interval (<1000 years) of extremely warm, wet, forested conditions at this site.

Evidence for Stress Localization Caused by Lithospheric Heterogeneity From Seismic Attenuation

AbstractThe Wyoming Craton underwent tectonic modifications during the Laramide Orogeny, which resulted in a series of basement‐cored uplifts that built the modern‐day Rockies. The easternmost surface expression of this orogeny ‐ the Black Hills in South Dakota ‐ is separated from the main trend of the Rocky Mountains by the southern half of the Powder River Basin, which we refer to as the Thunder Basin. Seismic tomography studies reveal a high‐velocity anomaly which extends to a depth of ∼300 km below the basin and may represent a lithospheric keel. We constrain seismic attenuation to investigate the hypothesis that variations in lithospheric thickness resulted in the localization of stress and therefore deformation. We utilize data from the CIELO seismic array, a linear array that extends from east of the Black Hills across the Thunder Basin and westward into the Owl Creek Mountains, the BASE FlexArray deployment centered on the Bighorn Mountains, and the EarthScope Transportable Array. We analyze seismograms from deep teleseismic events and compare waveforms in the time‐domain to characterize lateral variations in attenuation. Bayesian inversion results reveal high attenuation in the Black Hills and Bighorn Mountains and low attenuation in the Thunder and Bighorn Basins. Scattering is rejected as a confounding factor because of a strong anticorrelation between attenuation and the amplitude of P wave codas. The results support the hypothesis that lateral variations in lithospheric strength, as evidenced by our seismic attenuation measurements, played an important role in the localization of deformation and orogenesis during the Laramide Orogeny.

Tracking 40 Million Years of Migrating Magmatism across the Idaho Batholith Using Zircon U-Pb Ages and Hf Isotopes from Cretaceous Bentonites

Cretaceous strata preserved in Wyoming contain numerous large bentonite deposits formed from the felsic ash of volcanic eruptions, mainly derived from Idaho batholith magmatism. These bentonites preserve a near-continuous 40 m.y. chronology of volcanism and their whole-rock and mineral chemistry has been used to document igneous processes and reconstruct the history of Idaho magmatism as emplacement migrated across the Laurentian margin. Using LA-ICP-MS, we analyzed the U-Pb ages and Hf isotopic compositions of nearly 700 zircon grains from 44 bentonite beds from the Bighorn Basin, Wyoming. Zircon populations contain magmatic autocrysts and antecrysts which can be linked to the main pulses of the Idaho batholith and xenocrysts ranging from approx. 250 Ma to 1.84 Ga from country rocks and basement source terranes. Initial εHf compositions of Phanerozoic zircons are diverse, with compositions ranging from −26 to nearly +12. Based on temporal trends in zircon ages and geochemistry, four distinct periods of plutonic emplacement are recognized during the Mid- to Late Cretaceous that follow plutonic emplacement across the Laurentian suture zone in western Idaho and into western Montana with the onset of Farallon slab shallowing. Our data demonstrate the utility of using zircons in preserved tephra to track the regional-scale evolution of convergent margins related to terrane accretion and the spatial migration of magmatism related to changes in subduction dynamics.

The largest and earliest known sample of dental caries in an extinct mammal (Mammalia, Euarchonta, Microsyops latidens) and its ecological implications

Dental cavities or caries is a common disease among modern humans, affecting almost every adult. Caries frequency has been used to study dietary change in humans over time, based on an inferred tie between the incidence of caries and a carbohydrate-rich diet. However, the disease is not unique to our species. Among non-human primates, there is also variation in caries frequency associated with diet, suggesting that this metric may provide a mechanism for studying diet in broader contexts, and across geological time. To date, very few studies have examined caries among fossil mammals, and none have done so among Eocene mammals. Here, we present our analysis of the largest sample to date of fossil caries in a single extinct mammal species, Microsyops latidens, a stem primate from the early Eocene, which is known from over a thousand specimens from the Southern Bighorn Basin of Wyoming (n = 1030). Our results show that Microsyops latidens is characterized by a high prevalence of dental caries (7.48% of individuals), with notable variation through time, reaching 17.24% of individuals from a particular interval. This interval is also associated with a change in overall dental form, as quantified by dental topographic analysis, which measures functional aspects of the chewing surface of teeth. These observations suggest that this species experienced a shift in their diet to include more fruit or other sugar rich-foods for a short period. Our analysis, therefore, suggests that the diet of M. latidens fluctuated over time, as well as providing a framework for assessing caries in other fossil taxa.

Phosphorus fertilization and enhanced efficiency products effects on sugarbeet

A major cash crop in the Bighorn Basin of Wyoming (WY) and the Yellowstone River valley of eastern Montana (MT) is Sugarbeet (Beta vulgaris L.). This growing area is characterized by alkaline/calcareous soil conditions that inhibit phosphorus (P) availability. Phosphorus, however, is reported to increase root growth in sugarbeet. This study evaluated the effects of two P fertilizer sources and rates on sugarbeet productivity in Sidney, MT and Powell, WY. The P fertilizer sources were liquid ammonium polyphosphate (APP) and dry monoammonium phosphate (MAP) applied with or without a P availability enhancer (Avail®). A fifth fertilizer treatment consisting of APP + Avail® + starter fertilizer was tested. The APP was banded at 5 cm beside and below the seeds, the starter fertilizer was seed-placed and the MAP was broadcast-incorporated to 5-cm depth. Five P application rates (0, 34, 67, 134, and 201 kg P2O5 ha−1) were tested in Montana, and seven rates (0, 34, 67, 134, 201, 269, and 336 kg P2O5 ha−1) in Wyoming. Results showed significant effects of P fertilizer products and application rates on sugarbeet productivity and quality. The starter fertilizer increased plant stands in Wyoming, which was due to lower temperature (3–6 °C) condition at planting. Avail® increased root yield when APP was applied in Powell, WY, but had no effects when MAP was applied. There was no benefit from treatment with Avail® in Sidney, MT, regardless of fertilizer source. Phosphorus application rates had significant effects on root and sugar yields at both locations. The relationship between P application rate and root and sugar yield was linearly positive and failed to reach a plateau, despite the high P rates tested in this study. Such high P rates however, are not recommended in Wyoming and Montana because of the high economic and environmental cost associated with such rates.

Dietary shifts in a group of early Eocene euarchontans (Microsyopidae) in association with climatic change

AbstractMicrosyopidae is a family of plesiadapiforms known from over 1500 stratigraphically controlled specimens from the southern Bighorn Basin of Wyoming spanning the first three million years of the early Eocene. The early Eocene is characterized by rapid fluctuations in climate during the period represented by this collection of microsyopids, making this an ideal sample to examine how climate may have influenced early stem primate biology, particularly diet. An evolving lineage of microsyopids is known from before, during, and after Biohorizon A, a faunal turnover event associated with a period of localized cooling. Dental topographic analysis (DTA) metrics quantify functional aspects of molars including curvature, complexity and relief, and covary with diet in extant taxa. Here, we use DTA to examine microsyopid dietary change over time, particularly in association with this cooling event. Our results suggest that microsyopids had molars that are functionally like extant insectivorous/omnivorous euarchontans. The earliest occurring species, Arctodontomys wilsoni, is characterized by molars that became more like modern insectivores over time. During Biohorizon A, A. wilsoni is replaced by A. nuptus, which has molars that are more like those of extant omnivores with a mixed diet including fruit. After Biohorizon A, A. nuptus appears more insectivorous, as is the later occurring Microsyops angustidens, which evolves from A. nuptus. Overall, we provide potential evidence for a causal scenario in which local climate change coincided with a dietary transition among microsyopids. Our results have implications for understanding how diet was a prime mover for the evolution of Primates.

Comparison of Satellite Driven Surface Energy Balance Models in Estimating Crop Evapotranspiration in Semi-Arid to Arid Inter-Mountain Region

The regional-scale estimation of crop evapotranspiration (ETc) over a heterogeneous surface is an important tool for the decision-makers in managing and allocating water resources. This is especially critical in the arid to semi-arid regions that require supplemental water due to insufficient precipitation, soil moisture, or groundwater. Over the years, various remote sensing-based surface energy balance (SEB) models have been developed to accurately estimate ETc over a regional scale. However, it is important to carry out the SEB model assessment for a particular geographical setting to ensure the suitability of a model. Thus, in this study, four commonly used and contrasting remote sensing models viz. METRIC (mapping evapotranspiration at high resolution with internalized calibration), SEBAL (surface energy balance algorithm for land), S-SEBI (simplified surface energy balance index), and SEBS (surface energy balance system) were compared and used to quantify and map the spatio-temporal variation of ETc in the semi-arid to arid inter-mountain region of Big Horn Basin, Wyoming (Landsat Path/Row: 37/29). Model estimates from 19 cloud-free Landsat 7 and 8 images were compared with the Bowen ratio energy balance system (BREBS) flux stationed in a center pivot irrigated field during 2017 (sugar beet), 2018 (dry bean), and 2019 (barley) growing seasons. The results indicated that all SEB models are effective in capturing the variation of ETc with R2 ranging in between 0.06 to 0.95 and RMSD between 0.07 to 0.15 mm h−1. Pooled data over three vegetative surfaces for three years under irrigated conditions revealed that METRIC (NSE = 0.9) performed better across all land cover types, followed by SEBS (NSE = 0.76), S-SEBI (NSE = 0.73), and SEBAL (NSE = 0.65). In general, all SEB models substantially overestimated ETc and underestimated sensible heat (H) fluxes under dry conditions when only crop residue was available at the surface. A mid-season density plot and absolute difference maps at image scale between the models showed that models involving METRIC, SEBAL, and S-SEBI are close in their estimates of daily crop evapotranspiration (ET24) with pixel-wise RMSD ranged from 0.54 to 0.76 mm d−1 and an average absolute difference across the study area ranged from 0.47 to 0.56 mm d−1. Likewise, all the SEB models underestimated the seasonal ETc, except SEBS.

Decreased soil carbon in a warming world: Degraded pyrogenic carbon during the Paleocene-Eocene Thermal Maximum, Bighorn Basin, Wyoming

Global warming will likely perturb carbon storage and cycling throughout many components of the exogenic carbon cycle, but its net impact on the long-term fate of organic carbon stabilized in soils is unclear. Abrupt warming during the Paleocene-Eocene Thermal Maximum (PETM) profoundly altered vegetation and hydrologic patterns globally. To assess the consequences for soil carbon in a mid-latitude region we measured total organic carbon (%TOC), polycyclic aromatic hydrocarbons (PAHs), charcoal, and sporomorphs (pollen and spores) at two paleo-floodplain depositional sites in the Bighorn Basin, Wyoming, USA. At both sites %TOC, PAHs, charcoal, and sporomorphs declined during the PETM. The decline in pyrogenic carbon, which is more severe than the decline in %TOC, is consistent with isotopic and fossil evidence for degradation of labile organic compounds and preservation of highly refractory allochthonous organic carbon. The severe loss of less-labile contemporaneous PETM (autochthonous) soil carbon, illustrated by the fate of pyrogenic carbon, indicates intensified rates of organic matter decay during the PETM. Because of the highly degraded signature of organic matter in these PETM sections, it is difficult to discern if less pyrogenic carbon is in part a consequence of less fire occurrence during the PETM. We propose that in this mid-latitude region of the western USA increased soil carbon oxidation hindered soil carbon sequestration during this period of hotter climate with more seasonal precipitation.

Evidence of integumentary scale diversity in the late Jurassic Sauropod Diplodocus sp. from the Mother’s Day Quarry, Montana

The life appearance of dinosaurs is a hotly debated topic in the world of paleontology, especially when it comes to dinosaur integument. In the case of sauropods, however, the topic is harder to properly discuss due to the limited amount of fossilized skin impressions that have been discovered. Thus far, the fossil record of sauropod integument fossils include titanosaur embryos from Patagonia, possible keratinous diplodocid dorsal spines, track ways with foot impressions, and other isolated skin impressions found in association with sauropod body fossils. Several prominent integument fossils have been found at the Mother’s Day Quarry, located in the Bighorn Basin, Montana. These discoveries may bring new important information about diplodocids, specifically Diplodocus sp. Here we describe newly uncovered fossilized skin that gives evidence of scale diversity in the genus Diplodocus. The scales themselves represent tubercles, and exhibit various shapes including rectangular, ovoid, polygonal, and globular scales. The tubercles are small in size, the biggest of which only reach about 10mm in length. Considering how diverse the scale shapes are in such a small area of skin, it is possible that these distinct scale shapes may represent a transition on the body from one region to another: perhaps from the abdomen to dorsal side, or abdomen to shoulder. Based on analysis of extant integument and scale orientation of crocodilians, it is possible to hypothesize on the location of the integument relative to the body as well as the size and relative maturational status of the individual.

New specimens of the mesonychid Dissacus praenuntius from the early Eocene of Wyoming and evaluation of body size through the PETM in North America

The Mesonychia is a group of archaic carnivorous mammals of uncertain phylogenetic affinities with a Holarctic distribution during the Paleogene. Intensive fossil collecting efforts in the Bighorn Basin, Wyoming, have resulted in recovery of the largest sample and most complete specimens yet known of the mesonychid Dissacus praenuntius from the second biozone of the Wasatchian North American Land Mammal Age (Wa-0). The Wa-0 biozone corresponds to the body of the Paleocene-Eocene Thermal Maximum (PETM), a brief but intense global warming event that occurred ~56myr ago that significantly impacted terrestrial mammal faunas, including dwarfing in many mammal lineages. To evaluate the potential response of this lineage to climate change, we compared the PETM sample of D. praenuntius with those recovered from just before the PETM in the last biozone of the Clarkforkian North American Land Mammal Age (Cf-3) and just after the PETM in the Wa-1 biozone. While the sample size is still too small to say with certainty, tooth size (as a proxy for body weight) of D. praenuntius appears to be smaller during the late PETM than during either the pre-PETM Cf-3, or post-PETM Wa-1 biozones, suggesting the possibility of a muted dwarfing response to the PETM. However, the pattern observed for D. praenuntius differs from that of many other PETM mammals, as the shift to smaller body size is less pronounced and may have only occurred in late Wa-0.

Evaluating the responses of three closely related small mammal lineages to climate change across the Paleocene–Eocene thermal maximum

AbstractInterpreting the impact of climate change on vertebrates in the fossil record can be complicated by the effects of potential biotic drivers on morphological patterns observed in taxa. One promising area where this impact can be assessed is a high-resolution terrestrial record from the Bighorn Basin, Wyoming, that corresponds to the Paleocene–Eocene thermal maximum (PETM), a geologically rapid (~170 kyr) interval of sustained temperature and aridity shifts about 56 Ma. The PETM has been extensively studied, but different lines of research have not yet been brought together to compare the timing of shifts in abiotic drivers that include temperature and aridity proxies and those of biotic drivers, measured through changes in floral and faunal assemblages, to the timing of morphological change within mammalian species lineages. We used a suite of morphometric tools to document morphological changes in molar crown morphology of three lineages of stem erinaceid eulipotyphlans. We then compared the timing of morphological change to that of both abiotic and other biotic records through the PETM. In all three species lineages, we failed to recover any significant changes in tooth crown shape or size within the PETM. These results contrast with those documented previously for lineages of medium-sized mammals, which show significant dwarfing within the PETM. Our results suggest that biotic drivers such as shifts in community composition may have also played an important role in shaping species-level patterns during this dynamic interval in Earth history.

Jurassic dinosaurs on the move: Gastrolith provenance and long‐distance migration

AbstractHere, we present the detrital zircon age spectra for five (n = 36, 68, 66, 41, 29) red quartzite gastroliths collected from the top of the Upper Jurassic Morrison Formation in the eastern Bighorn Basin, Wyoming, USA. The detrital zircon age spectra reveal geon 17 maximum depositional ages, and age peaks that are Yavapai (geon 17), Penokean (geon 18) and Archean (&gt;geon 25). The colour, texture, composition and zircon age spectra of these exoliths are indistinguishable from those of geon 16 (i.e. Baraboo interval) quartzites present in the Laurentian midcontinent more than 1,000 km to the east. We interpret that these gastroliths were ingested by dinosaurs, most likely sauropods, in the Laurentian midcontinent and then transported in their digestive tracts to the site of deposition. These data support the hypothesis of long‐distance dinosaur migration, perhaps following low energy, continental‐scale drainage systems that flowed from the Appalachian highlands to the Morrison Formation depositional basin.

Cladogenesis and replacement in the fossil record of Microsyopidae (?Primates) from the southern Bighorn Basin, Wyoming.

The early Eocene of the southern Bighorn Basin, Wyoming, is notable for its nearly continuous record of mammalian fossils. Microsyopinae (?Primates) is one of several lineages that shows evidence of evolutionary change associated with an interval referred to as Biohorizon A. Arctodontomys wilsoni is replaced by a larger species, Arctodontomys nuptus, during the biohorizon interval in what is likely an immigration/emigration or immigration/local extinction event. The latter is then superseded by Microsyops angustidens after the end of the Biohorizon A interval. Although this pattern has been understood for some time, denser sampling has led to the identification of a specimen intermediate in morphology between A. nuptus and M. angustidens, located stratigraphically as the latter is appearing. Because specimens of A. nuptus have been recovered approximately 60 m above the appearance of M. angustidens, it is clear that A. nuptus did not suffer pseudoextinction. Instead, evidence suggests that M. angustidens branched off from a population of A. nuptus, but the latter species persisted. This represents possible evidence of cladogenesis, which has rarely been directly documented in the fossil record. The improved understanding of both evolutionary transitions with better sampling highlights the problem of interpreting gaps in the fossil record as punctuations.

A dense sample of fossil primates (Adapiformes, Notharctidae, Notharctinae) from the Early Eocene Willwood Formation, Wyoming: Documentation of gradual change in tooth area and shape through time.

The Willwood Formation of the southern Bighorn Basin, Wyoming is a fluvial rock sequence that spans approximately 3 million years of early Eocene time. It has yielded one the largest collections of fossil mammals in the world including thousands of dentitions of extinct lemur-like primates known as notharctines. In the southern Bighorn Basin, specimens of these primates have been collected on numerous paleontological expeditions and the stratigraphic levels yielding the dentitions have been carefully recorded. Notharctine dentitions represent a rare opportunity to study morphological variation in a single anatomical system through time among closely related individuals. Prior studies of Bighorn Basin notharctines through time produced measurements of hundreds of specimens but I report here results from measurement and comparison of the dentitions and dentaries of more than 3,000 specimens, all stratigraphically mapped. Variation in premolar and molar area and variation in dentary depth are apparent throughout the section. Specimens with relatively small teeth and dentaries are known from the older part of the section. In younger rocks, variation in tooth area among specimens increases. Variation in tooth area is continuous and overlaps extensively both within and between stratigraphic levels. Other dental variables examined by inspection change in a mosaic and continuous fashion through the section. These features include variation in the presence and number of paraconids on the lower fourth premolar (p4), the size and shape of the entoconid notch on the lower first and second molars, and the relative development of the pseudohypocone, mesostyle, and cingula on the upper molars. These broad patterns can be identified despite notharctine alpha taxonomy being in need of extensive revision and, importantly, simplification. Such revision is beyond the scope of this article but is essential if we are to develop a taxonomy that is both free of stratigraphic influence and useful for rapid, repeatable species assignment. Boundaries among the patterns of tokogenesis, anagenesis, and cladogenesis are blurred in this dense sample of extinct primates. While pattern of evolution, a population-level phenomenon, may be difficult to falsify in the fossil record, this notharctine sample suggests that in the rare instance such as this, when the fossil record is densely sampled, change through time is continuous and more consistent with gradual evolution.

Soil carbon degradation during the Paleocene-Eocene Thermal Maximum in the Piceance Basin, USA

The Paleocene-Eocene Thermal Maximum (PETM) warming event is marked by a negative carbon isotope excursion (CIE). The magnitude and shape of the CIE differs among carbon archives, but bulk organic carbon isotopes (δ13Corg) in terrestrial sections are often particularly variable and attenuated. Multiple lines of evidence from the Bighorn Basin, Wyoming, USA have revealed that extensive organic carbon degradation and increased refractory or allochthonous carbon inputs diminished the δ13Corg excursion magnitude. To test if this was a spatially widespread phenomenon and to understand possible underlying mechanisms, we examined the abundance of total organic carbon (%TOC) and polycyclic aromatic hydrocarbons (PAHs) as metrics of soil degradation at a neighboring terrestrial basin, the Piceance Basin in Colorado. Outcrop samples reveal both diminished %TOC and PAH concentrations across the Paleocene-Eocene boundary. Using PAHs as a proxy for intermediate refractory carbon, such as found in mineral soils, evidence from the Piceance Basin supports increased organic carbon degradation and greater proportions of refractory allochthonous carbon remaining. Correlations between %TOC and elemental oxides (e.g., Al2O3 and TiO2) suggest soil organic carbon was stabilized by its association with clay minerals. We hypothesize that decreased clay content in the soils (which reduced soil capacity to stabilize fresh carbon) and increased fluctuations in soil moisture (which destabilized older, refractory carbon), in conjunction with increased temperatures (which increased microbial decomposition rates), contributed to reduced soil organic matter preservation during the PETM. These mechanisms destabilized carbon on millennial timescales and, with sustained higher temperatures across the PETM (~150,000 years), increased soil carbon degradation persisted for tens of thousands of years.