Southern Bighorn Basin Research Articles

New material of Lophiparamys debequensis from the Willwood Formation (early Eocene) of Wyoming, including the first postcrania of the genus

Abstract We report new material of the rare early Eocene rodent Lophiparamys debequensis Wood, 1962 from the Willwood Formation of the southern Bighorn Basin, north-central Wyoming. The new material constitutes the first record of L. debequensis from the Bighorn Basin and documents aspects of the anatomy of Lophiparamys that were previously unknown, including a portion of the maxilla and a portion of the tarsus. The maxillary fragment demonstrates that Lophiparamys has a small P3 and a relatively large infraorbital canal. The tarsus of Lophiparamys is similar to that of other early rodents but differs in a few features that suggest an arboreal locomotor repertoire, including an asymmetric astragalar trochlea, long astragalar neck, transverse astragalar sustentacular facet, short calcaneal tuber, elongate calcaneal ectal facet, and circular calcaneal cuboid facet. The presence of arboreally adapted features in the tarsus of Lophiparamys is consistent with a hypothesized relationship between small-bodied Eocene microparamyine rodents and extant Gliridae. Phylogenetic analysis fails to consistently support this relationship or monophyly of Microparamyinae, but both remain plausible. Comparison of L. debequensis with other species of the genus emphasizes the distinctiveness of L. debequensis and suggests the presence of multiple lineages of Lophiparamys.

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.

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.

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.

New fossils of the oldest North American euprimate Teilhardina brandti (Omomyidae) from the paleocene–eocene thermal maximum

More than 25 new specimens of Teilhardina brandti, one of the oldest known euprimates, are reported from earliest Eocene strata of the southern Bighorn Basin, Wyoming. The new fossils include the first upper dentitions, a dentary showing the lower dental formula for the first time, and the first postcrania ascribed to T. brandti (tarsals and terminal phalanges). The elongated navicular and long talar neck suggest that T. brandti was an active arboreal quadruped, and the terminal phalanges constitute the oldest evidence for nails in Euprimates. Phylogenetic analysis incorporating the new data indicates that T. brandti is more derived than T. belgica but less so than T. americana. The hypothesis that Teilhardina originated in Asia (T. asiatica) and dispersed westward to Europe (T. belgica) and then to North America (T. brandti and T. magnoliana) during the earliest Eocene Paleocene-Eocene Thermal Maximum is most consistent with available evidence, including the relative age of fossil samples and their stage of evolution.

Early Eocene Paromomyidae (Mammalia, Primates) from the southern Bighorn Basin, Wyoming: Systematics and evolution

We present a critical review of the alpha taxonomy and evolution of Eocene North American paromomyid primates, based on analysis of more than 570 stratigraphically controlled dental and gnathic specimens from the early Eocene of the southern Bighorn Basin, Wyoming (Wasatchian, Willwood Formation). In addition to documenting numerous previously unpublished specimens of known taxa (including deciduous teeth), we also describe a new species, Phenacolemur willwoodensis n. sp., from the upper part of the Willwood Formation (Wa 5 and 6). The new species is intermediate in size between Phenacolemur simonsi and Phenacolemur citatus and has both primitive features (e.g., retention of m2–3 paraconids, relatively long molar trigonids) and derived traits (e.g., relatively reduced paraconid on m1 and no p4 paracristid, unlike Paromomys). Overall patterns of dental evolution in southern Bighorn Basin paromomyids provide some support for previously hypothesized periods of faunal change (Biohorizons). In particular, Phenacolemur praecox evolves into the similarly sized but morphologically distinct Phenacolemur fortior at Biohorizon A, and P. fortior is replaced by P. citatus just below Biohorizon B. Two taxa previously believed to have become extinct at Biohorizon A (Ignacius graybullianus, P. simonsi) are shown to have persisted about a million years longer than previously thought. The Bighorn Basin paromomyids are of general interest in comprising a very dense sample that allows for the study of patterns of evolution against the backdrop of well-understood patterns of change in other mammalian lineages, and in climatic variables.

Early Eocene Paromomyidae (Mammalia, Primates) from the southern Bighorn Basin, Wyoming: Systematics and evolution

We present a critical review of the alpha taxonomy and evolution of Eocene North American paromomyid primates, based on analysis of more than 570 stratigraphically controlled dental and gnathic specimens from the early Eocene of the southern Bighorn Basin, Wyoming (Wasatchian, Willwood Formation). In addition to documenting numerous previously unpublished specimens of known taxa (including deciduous teeth), we also describe a new species,Phenacolemur willwoodensisn. sp., from the upper part of the Willwood Formation (Wa 5 and 6). The new species is intermediate in size betweenPhenacolemur simonsiandPhenacolemur citatusand has both primitive features (e.g., retention of m2–3 paraconids, relatively long molar trigonids) and derived traits (e.g., relatively reduced paraconid on m1 and no p4 paracristid, unlikeParomomys). Overall patterns of dental evolution in southern Bighorn Basin paromomyids provide some support for previously hypothesized periods of faunal change (Biohorizons). In particular,Phenacolemur praecoxevolves into the similarly sized but morphologically distinctPhenacolemur fortiorat Biohorizon A, andP. fortioris replaced byP. citatusjust below Biohorizon B. Two taxa previously believed to have become extinct at Biohorizon A(Ignacius graybullianus, P. simonsi)are shown to have persisted about a million years longer than previously thought. The Bighorn Basin paromomyids are of general interest in comprising a very dense sample that allows for the study of patterns of evolution against the backdrop of well-understood patterns of change in other mammalian lineages, and in climatic variables.

High-resolution carbon isotope stratigraphy and mammalian faunal change at the Paleocene-Eocene boundary in the Honeycombs area of the southern Bighorn Basin, Wyoming

The Paleocene-Eocene boundary is defined chemostratigraphically by the onset of a 100 kyr global carbon isotope excursion (CIE) that is recognized in marine and continental strata in both carbonate and dispersed organic carbon. The CIE is important in continental sections as a proxy for environmental change during the Paleocene-Eocene thermal maximum (PETM), which clearly affected faunal succession in Paleocene-Eocene mammals. At Polecat Bench in the northern Bighorn Basin, Wyoming, the CIE is in the Willwood Formation, where it spans four mammalian biozones from part of latest Clarkforkian Cf-3, through early Wasatchian Wa-M (<b><i>Meniscotherium</i></b>), Wa-0, and part of Wa-1. Here we report a new high-resolution study of the CIE recorded in dispersed organic carbon of the basal Willwood Formation in the Honeycombs area of the southern Bighorn Basin. In the Honeycombs area, 58 isotope sample sites span a vertical section of 71 meters, including the Wa-M locality ‘Halfway Hill North’ with both <b><i>Meniscotherium</i></b> and a <b><i>Heptodon</i></b>-like perissodactyl, and the richly fossiliferous Wa-0 wash site ‘Castle Gardens’ yielding a diverse mammalian fauna. Total organic carbon in the Honeycombs section is in the range of 0.1 to 0.4 percent by weight, and CaCO₃ is notably lacking except in the Castle Gardens interval where it reaches ca. 3 percent. Pre- and post-excursion values of δ¹³C_org (PDB) range from −26.2 to −24.1 permil. The Honeycombs CIE is about 42 m thick, and CIE excursion values range from −28.8 to −26.2 permil. The Honeycombs area is important because the Halfway Hill North locality is the first to yield a perissodactyl from the Wa-M <b><i>Meniscotherium</i></b> zone, and the Castle Gardens wash site is the first to yield abundant evidence of Wa-0 zone microvertebrates. As at Polecat Bench, the <b><i>Meniscotherium</i></b> zone in the Honeycombs area is in the lower part of the CIE, and Castle Gardens and other Wa-0 sites are in the longer middle and upper parts, starting near or after the maximum negative excursion. The CIE at Polecat Bench includes a series of 5 to 6 negative-excursion landmarks that are spaced, on average, about 8.5 m apart. These are tentatively interpreted as 21-kyr orbital precession cycles like those inferred for paleosol color. Negative-excursion landmarks are present in the Honeycombs CIE as well, but these are fewer and part of the Honeycombs section may be condensed or missing. Full understanding of the Paleocene-Eocene biotic transition will require investigation at even finer scales of temporal resolution, and results reported here suggest that finer-scale studies should be possible.

Carved Steatite and Sandstone Tubes: Pipes for Smoking or Shaman’S Paraphernalia

Six broken and refitted carved stone pipes or tubes were recovered at the Coal Draw site (48H0469) in the southern Bighorn Basin in north-central Wyoming. In addition there were an unbroken tube and pieces offive others. Several different types of carvable stone are represented, many of which are not found locally. These items may or may not be associated with a large Dinwoody style petroglyph panel nearby. They may also have been burial offerings, since a human skull fragment and a probable human molar were found in the immediate area. Ritual breakage of most of the items is suggested. It is also suggested that this occurred while the items were in containers such as pipe bags, and that they were not removed afterward.

Floodplains and paleosols in the Wyoming Eocene sequence: implications for the taphonomy and paleoecology of faunas

The Willwood Fm. of the southern Bighorn Basin of Wyoming, U.S.A., comprises 700 m of lower Eocene alluvial molasse, nearly all of which contains relict pedogenic features. These rocks are grouped into pedofacies–alluvial sediment prisms, thick with immature paleosols proximal to streams and thinner with mature paleosols distally. Pedofacies are bounded by either trunk-stream channel or crevasse-splay deposits, which represent time-stratigraphic markers. The floodplain widths of the Willwood rivers varied from 15 to 20+ km. Paleosols occur throughout the Willwood Formation and the most mature paleosols required about 60 Ka to form whereas the least mature, required 0.5 to 1.0 Ka. Paleosol thicknesses vary from about 0.3–8.0 m and are directly related to net sediment accumulation rate (NSAR) and profile maturity. Pedofacies also reflect NSAR controls; pedofacies are continuously superposed, 15–35-m-thick, and represent time intervals of 30–60 Ka.In the earliest Eocene, paleosol maturity rose sharply, and NSAR plummeted (Fort Union Fm./Willwood Fm. contact), after which maturity gradually declined (and NSAR rose) throughout the early Eocene. This decline was punctuated by two episodes of severe decline, each corresponding with major increases in NSAR, increased tectonism, and episodes of faunal turnover (“Biohorizons” A and B). Above the biohorizons, species earlier tied to particular paleosol maturities were replaced by closely related though more generalized species with no marked paleosol preferences. Time-stratigraphic reconstruction of the Willwood Fm. shows that “Biohorizons” B and C record the same faunal event; B the extinctions, and C the immigrations.The 1,300 Willwood fossil vertebrate localities, which are distributed throughout the entire formation, occur in the surface horizons of cumulative alluvial paleosols. All fossil accumulations in paleosols are attritional and formed during pedogenesis. The most complete remains occur in immature paleosols, whereas the most abundant remains are found in mature paleosols. Within the large-scale Willwood ecologic setting, studies of discrete (m's to tens of m's thick) stratigraphic intervals suggest that the paleontology and sedimentology of these intervals can be significantly influenced by lateral differences in paleosol hydromorphy (soil wetness) and maturity (lateral position of a fossil-bearing paleosol with respect to an ancient river channel). These smaller-scale controls on fossil occurrences are important for distinguishing between real and apparent changes in faunal compositions over time and emphasize the value of three-dimensional stratigraphic analysis for interpreting paleontologic events.Supported by National Geographic Society grant 3985-89.

Spectral Reflectance Characterization (0.4 To 2.5 and 8.0 To 12.0 &amp;micro;m) of Phanerozoic Strata, Wind River Basin and Southern Bighorn Basin Areas, Wyoming

ABSTRACT Reflectance spectroscopy, a useful analytical tool available to geologists for over 100 years, has not been widely used in petrologic or stratigraphic studies of sedimentary rocks. We are routinely using laboratory spectral reflectance data covering the 0.4 to 12.0 µm wavelength interval in an investigation of the stratigraphic and structural evolution of the Wind River and Bighorn basin areas, Wyoming. Two types of laboratory spectral reflectance measurements were made on 192 weathered outcrop, bedrock, and core samples: 1) hemispherical reflectance, from 0.4 to 2.5 µm (229 spectra) and 2) biconical reflectance, from 8.0 to 12.0 µm (84 spectra). Results show stratigraphic variations in the spectral properties of sedimentary rocks as encountered in the field. Sample mineralogy was determined using X-ray diffraction and interpretation of spectra. The mineralogy, lithology and stratigraphic distribution of samples are diverse and representative of Phanerozoic siliciclastic and carbonate strata encountered in foreland sedimentary basins. Spectra were sorted into 36 distinct groups, according to the presence or absence of 68 visible and infrared reflectance features. Many spectral features can be assigned to specific molecular phenomena and allow rapid determination of specific minerals. Minerals detectable spectrally include: gypsum, kaolinite, smectite, analcime, goethite, jarosite, calcite, dolomite, and feldspar. Results are contributing to our understanding of the depositional history of the Wind River and Bighorn basin areas; more importantly, results suggest that laboratory, field and remote reflectance spectroscopy have more general geologic applicability in studies of sedimentary rocks. Reflectance spectroscopy can augment conventional laboratory and field methods for petrologic analysis, correlation, interpretation of depositional environments, and construction of facies models.

Basin Analysis Aided by Remote-Sensing Data: Geologic Results in Wind River/Bighorn Basin Area, Wyoming: ABSTRACT

Aircraft and satellite remote-sensing data combined with surface and subsurface geologic and geophysical data have been used to investigate the formation and evolution of Laramide foreland basins in central Wyoming. Quaternary through Holocene geomorphology reveals post-Laramide basin excavation history. Six terrace/pediment surfaces have been recognized in the northern Wind River basin and have been correlated with equivalent surfaces in the Bull Lake and central Wind River basin areas, respectively. Permian through Eocene stratigraphic relationships distinguish late pre-Laramide through Laramide paleogeography. New type sections established in the eastern wind River basin/northern Casper arch area refine lithostratigraphic correlations with: (1) surface sections measured in the southern Bighorn basin and (2) subsurface sections on the southern Casper arch. Structural mapping in the southern Bighorn basin indicates predominantly compressional Laramide deformation. Numerous northeastward-dipping thrust faults that do not appear on published geologic maps have been recognized at the surface and have been confirmed with seismic data. Similar strategies of data acquisition and geologic analysis developed should be applicable in studies of similar foreland basins.

Cross Folding in Southern Bighorn Basin: ABSTRACT

Cross Folding in Southern Bighorn Basin: ABSTRACT

Discovery Paper--Ordovician Oil in Southern Big Horn Basin, Wyoming: ABSTRACT

Discovery Paper--Ordovician Oil in Southern Big Horn Basin, Wyoming: ABSTRACT