Benjamin Burger

The discovery of a Permian-Triassic boundary section in northeastern Utah provided an opportunity to study the chemistry and geology associated with this event that led to one of the greatest mass extinctions on the planet. From 83% to 97% of the species living on the planet went extinct during this relatively short interval of geological time that defines the major geological boundary between the Paleozoic and Mesozoic Eras. The cause and resulting sequence of events that led to this extinction have puzzled geologists for years. The new stratigraphic section in Utah provided a framework for reconstruction of the chemistry that was in the ocean during the mass extinction. Geochemical analysis of the section demonstrates, moving across the boundary from the Paleozoic to the Mesozoic, a significant drop in carbonate and total organic carbon, and a delayed occurrence of pyrite. Carbon isotope ratios (δ13C) show a negative excursion in carbonate with ratios dropping from 2.59‰ to −3.63‰. Elevated mercury is present at the boundary with a 4-fold increase from background levels. The boundary layer also has elevated zinc, lead, strontium, and nickel, but not at high enough levels to indicate a volcanic ash source. There is no evidence in concentrations of siderophile and chalcophile elements for an extraterrestrial impact. The stratigraphic section in Utah supports the theory of a massive release of carbon dioxide, resulting in the acidification of the oceans. High levels of mercury, as well as elevated levels of zinc and lead, implicate a coal source triggered by the contemporary Siberian Traps sill complex. The delay of enriched sulfur and barium content in the stratigraphic record indicates an anoxic ocean and upwelling of methane hydrates from depth. Further study of this stratigraphic section will aid our understanding of the global impact these catastrophic events had on life.

We report on the first occurrence of an early horned brontothere in the Brennan Basin Member of the Duchesne River Formation in northeastern Utah. This is the first record of a brontothere from the Brennan Basin Member. Previously, brontotheres have been reported from the higher stratigraphic La Point Member (Duchesneodus uintensis) and the lower stratigraphic Uinta Formation (Sphenocoelus uintensis, Fossendorhinus diploconus, Metarhinus fluviatilis, Metarhinus abbotti, Sthenodectes incisivum, Metatelmatherium ultimum, Protitanotherium emarginatum, Pollyosbornia altidens, Diplacodon elatus). The recovered specimen consists of an upper third molar, which is comparable to the species Diplacodon elatus. The specimen supports the continued presence of brontotheres throughout the deposition of the Duchesne River Formation across the late Uintan to early Duchesnean North American Land Mammal Age. The previous lack of brontotheres within the lower beds of the Duchesne River Formation is likely a result of poor sampling and the relative rarity of fossils from this unit.

An abrupt global warming event marks the Paleocene–Eocene boundary, known as the Paleocene–Eocene Thermal Maximum (PETM). The event is distinguished in the strata globally by a significant negative excursion of δ13C ratio values. The response of the terrestrial biota to the abrupt climatic change has been well studied in northern Wyoming in the Bighorn Basin, where it has been observed that the mammalian fauna during the global warming event is represented by smaller, but morphologically similar species to those found later in the Eocene. Various hypotheses have been proposed to explain the observation smaller body sizes during the global warming event. In this article, evidence is presented to support the hypothesis that the observed body size decrease during the PETM was influenced by the appearance of smaller southern species who extended their geographic range northward during the abnormal global warming event. Using disperse organic carbon isotopic ratios of bulk sediment, the negative excursion of δ13C was located in the Piceance Creek Basin of western Colorado, 400 km to the south of the Bighorn Basin. Below the stratigraphic level marking the negative carbon excursion in the Piceance Creek Basin are five specimens of the phenacodontid mammal (Ectocion parvus), a diminutive species of the genus Ectocion restricted to the basal Eocene (Wa-0 Biozone) in northern Wyoming. The five specimens of E. parvus are associated with a late Paleocene (Clarkforkian) mammalian fauna in Colorado, implying that the diminutive species extended its geographic range northward during the global warming event. This evidence supports biogeographic models that assume poleward biogeographic shifts during global warming events, and will have modern day implications for the conservation of species as global temperatures rise in the near future.

The description of a skull of the omomyid primate Omomys carteri from the middle Eocene Bridger Formation of southwestern Wyoming provides important new information relevant to our understanding of Eocene primate systematics and evolution. More specifically, Omomys carteri exhibits a long and wide nasal region and lacks the forward orientation of the orbits found in other omomyid primates. Furthermore, other features such as an unreduced cranial arterial system, primitive auditory bulla, expanded internal choanae, and an unreduced canine indicate that the cranium of Omomys carteri is primitive. Phylogenetic analysis of 52 cranial, 194 dental, and 56 postcranial characters, including the new characteristics of Omomys carteri discussed here, suggests that omomyiforms belong to a monophyletic tarsiiform clade exclusive of anthropoids. Anthropoids are still viewed as a sister group to tarsiiforms, forming a haplorhine clade, with an uncertain position for the problematic fossil primate Rooneyia. The reconstructed cranial anatomy of the haplorhine morphotype is more primitive than originally thought and likely exhibited many of the primitive traits featured in the skull of Omomys carteri.
Keywords: mammal, haplorhine, tarsiiform, omomyiform, anatomy, Bridger Formation

Despite recent drilling for hydrocarbon tight-gas resources in conglomerates and sandstones in the Piceance Creek Basin, controversy still exists over the age and lateral extent of the Ohio Creek Formation. This controversy originated from the proposition that these deposits belonged in the late Cretaceous Mesaverde Group. However, recent pollen analyses suggest a late Paleocene age. Here I report the first fossil vertebrate fauna from these deposits, which corroborates the late Paleocene age and further refines it to the early Tiffanian NALMA (Ti3 lineage zone). Identified fossils comprise several genera of mammals and reptiles. Reptilian fossils consist of a dorsal osteoderm of a eusuchian with affinity to Borealosuchus and a partial hyoplastron of the turtle Compsemys. Both genera range from the late Cretaceous through the Paleocene. Mammalian taxa represented are: Gelastops, Haplaletes, Promioclaenus, Colpoclaenus, and Mimotricentes, which range from the Torrejonian into the early part of the Tiffanian NALMA. Presence of the index fossil Nannodectes simpsoni further constrains the deposits to an early Tiffanian age  (Ti3 lineage zone). Lithologically the Ohio Creek Formation is characterized by the absence of coal and shale, and the presence of tan mudstones and siltstones spaced between thick and widespread units of clay pebble and chert conglomerates and sandstones. The revised age and unique lithic characteristics of the Ohio Creek Formation excludes it from the late Cretaceous Mesaverde Group. Revising the biostratigraphic zonation of the late Cretaceous and early Tertiary strata of the Piceance Creek Basin is important for understanding the complex tectonic history of the Rocky Mountain region.

The spatial and temporal record of the early Eocene mammal Hyopsodus exemplifies the complex and sinuous nature of evolution. I transform the dualism of punctuated equilibrium and phyletic gradualism patterns into a discussion of regional and local evolutionary patterns.  In this paper, I show that evolution follows a gradual pattern at the local level, while punctuated changes occur through migration, climate change and ecological shifts at the regional level. These conclusions are demonstrated by comparing two long fossil records of the small mammal Hyopsodus from the Bighorn Basin of northern Wyoming and the Piceance Creek Basin of western Colorado. Separated by a large geographic distance and the high Rocky Mountains, the two sedimentary basins preserve a long record of evolutionary change during the Wasatchian Land Mammal Age (55 to 50 million years ago). In both basins Hyopsodus diversifies into a number of different sized species during the late Wasatchian. Northward migration played an important role in the diversification of Hyopsodus. The large species H. powellianus appears earlier in the Piceance Creek Basin and expanded northward into the Bighorn Basin as global temperatures warmed during the early Eocene. Gradual in situ changes, such as the gradual transformation of H. simplex into H. minor in the Bighorn Basin, are not witnessed in the Piceance Creek Basin. Additional fossils may prove H. minor migrated into the Piceance Creek Basin only after originating from the Bighorn Basin. There currently is a stratigraphic gap between the last appearance of H. simplex and first appearance of H. minor in the Piceance Creek Basin. This paper draws attention to the dense fossil record of the Eocene and the value it serves as a window into understanding evolutionary patterns spanning millions of years.

Utilizing the occurrence of plesiadapid mammals, we recognize both Tiffanian and Clarkforkian ages in the Fort Union Formation of the Piceance Creek Basin of Colorado. Five species of plesiadapids are described from eighteen fossil localities. The presence of Nannodectes gazini, Plesiadapis fodinatus and a new species Chiromyoides gigas indicate that at most eleven  of the reported fossil localities are Tiffanian in age. The majority of the fossil specimens come from UCM locality 92177 (USGSD-2001) located 35.8 meters above the base of the Fort Union Formation, which we place in the Ti5 lineage zone. The occurrence of Plesiadapis dubius and Chiromyoides caesor in the Plateau Valley area suggests a younger Clarkforkian age for these stratigraphically higher localities. We are the first to recognize Tiffanian aged fossils in the Piceance Creek Basin, extending the mammalian record back to more than 58 million years ago.

Page 189. Chapter 6 Stratigraphy and Taphonomy of Grizzly Buttes, Bridger Formation, and the Middle Eocene of Wyoming JOHN P. ALEXANDER and BENJAMIN J. BURGER 1. Introduction 165 2. Geology 170 2.1. Palaeoclimate 171 2.2. Lithology 172 3. Taphonomy 176 3.1. ...

As scientists, we strive for a coherent understanding of what occurs during periods of rapid global warming, specifically the effects on biodiversity. The Paleocene-Eocene boundary and the corresponding Paleocene-Eocene Thermal Maximum (PETM) represent one such event in Earth’s history when global temperatures rose 5° to 10° C in fewer than 10,000 years and lasted ~170,000 years. Intense paleontological research has focused on the Paleocene-Eocene boundary in the Bighorn Basin of northern Wyoming, yet little is known from other geographic areas. Discovery of the Paleocene-Eocene boundary defining Carbon Isotope Excursion (CIE) in the Piceance Creek Basin, 788 kilometers to the south in Colorado, facilitates comparison of
contemporaneous mammalian faunas between the two basins. My research in the Piceance Creek Basin includes over 300 fossil vertebrate localities spanning late Paleocene (middle Tiffanian) through the early Eocene (late Wasatchian) time. Over 150 mammalian species occur in the Piceance Creek Basin during this six-million-year interval. A significant shift in lithology occurs at the Paleocene-Eocene boundary wherein sandstones replace coals and carbonaceous shales, and red beds with cyclicpaleosols predominate the Eocene section. The observed lithologic change is consistent with climate change associated with the PETM event. The Paleocene-Eocene boundary marks the extinction of many mammalian species. Only 4 of the 50 Paleocene mammal species survive the PETM event in the Piceance Creek Basin. The PETM
seemed most detrimental to arboreal mammal species; none of the Paleocene species survived, which is consistent with loss of forest habitat due to climate change.
Furthermore, most of the larger terrestrial mammals (6 species) went extinct, including Barylambda, Titanoides, and Probathyopsis. Northward migration from Colorado to
northern Wyoming of smaller Paleocene species of Ectocion and Haplomylus appears to have occurred during the PETM event. The fact that the global PETM event in Colorado coincides with major mammalian extinction and northward migration of terrestrial species suggests a expansion of the subtropical dry zone during the PETM event. This study gives historical evidence in support of recent climate models predicting the expansion of the subtropical dry zone as a result of current rising atmospheric CO2 levels.

Directly overlaying the fossiliferous yellow-gray siltstone dominated Uinta Formation, the Brennan Basin Member of the Duchesne River Formation consists of sandstone and conglomeratic lenses, inter-bedded by reddish brown siltstone. Fossil mammals from this unit are rare, but stratigraphically important in the understanding of mammalian evolution across a poorly sampled period in the fossil record of Utah. During this interval of time, the Carnivora diversified into felids and canids, while the Primates disappeared from the fossil record in Utah. Pig-like entelodonts and anthracotheriids first appear, while the rhino-like brontotheres came to dominance. Most fossil mammals from the Duchesne River Formation are associated with discoveries made in 1929 and 1930 from a large death assemblage of twenty individuals of the horn-less brontothere Duchesneodus. From this quarry came several other fossil mammals including the large carnivorous mesonychid Hessolestes, the advance creodont Hyaenodon, and early camel Poabromylus. Fairly well preserved specimens of the bizarre clawed agriochoerid artiodactyl Diplobunops are known elsewhere in the formation, in addition to the transitional early horse Duchesnehippus only known from a single lower jaw. More abundant fossilized remains were discovered near Randlett, Utah near the boundary with the lower Uinta Formation. This stratigraphic lower fauna includes the early rhino Uintaceras, the sheep-sized Protoreodon, the primitive tapiroid Dilophodon and amynodont Megalamynodon. The rodents Pareumys and Mytonomys as well as the early lagomorph Mytonolagus are also preserved from the Randlett Horizon. In 2009, fossils from a separate location in the lower Brennan Basin Member were discovered south of Vernal Utah. The new fauna includes the hyracodont Epitriplopus, the protoceratid Leptotragulus, the small carnivorian Miacis, and the cylindrodont rodent Pareumys, as well as an alligatoroid provisionally identified as Procaimanoidea. While not extensively diverse, the new fauna helps expand what little knowledge there is of the fossil mammals from the lower Brennan Basin Member of the Duchesne River Formation. Study of the sparse mammalian fauna from the late Eocene of Utah continues highlight the formation’s importance.