Paleontology, taphonomy, and sedimentology of the Mygatt-Moore Quarry, a large dinosaur bonebed in the Morrison Formation, western Colorado—Implications for Upper Jurassic dinosaur preservation modes

The Mygatt-Moore Quarry is a deposit of several thousand dinosaur bones in the Brushy Basin Member of the Morrison Formation in western Colorado. The site has been worked for more than 30 years and nearly 2400 mapped specimens have been collected. This study gathered data about the quarry from many sources to investigate the origin of the deposit. The Mygatt-Moore Quarry appears to be an attritional deposit of a relatively restricted diversity of dinosaurs, with few other non-dinosaurian taxa, that accumulated in a vernal pool deposit in an overbank setting. Bone modification was mostly by corrosion and breakage by trampling; scavenging was abundant. The paleofauna is dominated by Allosaurus and Apatosaurus (MNI and NIS), with the polacanthid ankylosaur Mymoorapelta less common. The matrix of the main quarry layer includes abundant carbonized fragments of plant material, and the mud during the time of deposition may have been often at least damp and occasionally acidic and dysoxic. The Cleveland-Lloyd Dinosaur Quarry is a close correlate of the Mygatt-Moore Quarry in terms of lithology and taphonomy, but demonstrates significant differences upon close inspection of matrix details and bone modification. Large quarries of fine-grained facies in the Morrison Formation possess a very different preservation mode as well as different taxon and relative abundance profiles from those in coarser sediments, which suggests that more may be learned in the future from taphofacies study of large quarries in mudstone beds.

First unambiguous dinosaur specimen from the Upper Triassic Chinle Formation in Utah

Triassic dinosaurs represent relatively rare but important components of terrestrial faunas across Pangea. Whereas this record has been well studied at various locales across the American West, there has been no previous systematic review of Triassic material assigned to Dinosauria from Utah. Here, we critically examine the published body fossil and footprint record of Triassic dinosaurs from Utah and revise their record from the state. In addition, we describe a sacrum from a locality within the Upper Triassic Chinle Formation of southeastern Utah. _is specimen represents the only unambiguous Triassic dinosaur body fossil from Utah. MWC 5627 falls within the range of variation known for sacrum morphology from Coelophysis bauri. Based on a literature review and examination of specimens available to us, we restrict the Triassic Utah dinosaurian record to _eropoda from the Chinle Formation. Preliminary reports of Triassic dinosaurs from other clades and formations in Utah are unsubstantiated.

Source within the seal—Distribution and implications of organic shale-bearing stringers within the Onion Creek diapir, northern Paradox Basin, Utah

The Onion Creek diapir is one of the best exposures of a dissected salt diapir in the world, offering a unique opportunity to better understand the internal character of heterolithic diapirs that are common in sedimentary basins worldwide. Large amounts of interbedded shale, carbonate, and evaporites are incorporated into the diapir as stringers or boudins, and excellent three-dimensional exposure allows us to document the nature, size, deformation, and distribution of these stringers. Blocks range in size from single, disaggregated layers of dolomite to several meters of coherent stringers that contain multiple cycles of dolomite- shale-evaporite and are upwards of 20 m thick and more than 100 m in observed length. The largest blocks are most commonly located along the margins of the exposed diapir, though stringers are common throughout the exposed caprock. In areas devoid of large stringers, there is more extensive deformation of the gypsum caprock, suggesting that the presence of stringers leads to a more heterolithic distribution of stress within the salt as it diapirically rises. These observations can help to better characterize similar diapirs elsewhere that are not well exposed at the surface. Black shale is present in all observed large stringers of the Onion Creek diapir. These shale beds are interpreted to have been deposited in a shallow, restricted marginal marine environment along with the interbedded carbonate and evaporite strata. Pyrolysis analysis of 13 samples from within the stringers shows a range of 2.56 to 60.22% total organic carbon (TOC), with an average value of 16.93%. These strata contain Type I/Type II hydrocarbon source facies, consistent with a restricted shallow marine environment. Tmax data suggest that these source rock facies have been exposed to sufficient thermal energy to generate hydrocarbons (average = 437o C), as evidenced by common hydrocarbon staining of intra-stringer carbonate strata and evaporite beds surrounding the stringers. Twelve additional samples were collected from these stained strata and pyrolysis analysis shows that all are enriched in free oil, as shown by elevated S1 peaks, high production index ratios, and TOC values of 0.64 to 1.66%. This hydrocarbon staining is found around stringers near the center of the exposed caprock, as well as stringers along the margins. Near the margins in particular, extensive alteration can be seen across tens of meters of evaporitic strata, showing that hydrocarbons are effectively generating within and migrating away from stringers fully encased in the anhydrite caprock of the Onion Creek diapir. This has important implications for potential seal integrity of diapiric caprocks, as well as providing a potential mechanism for caprock carbonate formation suggested by other researchers.

Shorelines and vertebrate fauna of Pleistocene Lake Bonneville, Utah, Idaho, and Nevada

Pleistocene Lake Bonneville created many classic examples of lacustrine shoreline landforms, which preserve a wide variety of vertebrate fossils. _is _eld guide provides a review of the published literature for a sampling of the lake’s world-class localities. _is guide also provides a brief overview of modern Great Salt Lake and its microbialites recently exposed by near-record low lake levels. Stops include G.K. Gilbert Geologic View Park, Draper spit, Steep Mountain beach, Point of the Mountain spit, American Fork delta, Stockton Bar, and Great Salt Lake State Park.

Upper Triassic lithostratigraphy, depositional systems, and vertebrate paleontology across southern Utah

The Chinle Formation and the lower part of the overlying Wingate Sandstone and Moenave Formation were deposited in fluvial, lacustrine, paludal, and eolian environments during the Norian and Rhaetian stages of the Late Triassic (~230 to 201.3 Ma), during which time the climate shifted from subtropical to increasingly arid. In southern Utah, the Shinarump Member was largely confined to pre-Chinle paleovalleys and usually overprinted by mottled strata. From southeastern to southwestern Utah, the lower members of the Chinle Formation (Cameron Member and correlative Monitor Butte Member) thicken dramatically whereas the upper members of the Chinle Formation (the Moss Back, Petrified Forest, Owl Rock, and Church Rock Members) become erosionally truncated; south of Moab, the Kane Springs beds are laterally correlative with the Owl Rock Member and uppermost Petrified Forest Member. Prior to the erosional truncation of the upper members, the Chinle Formation was probably thickest in a southeast to northwest trend between Petrified Forest National Park and the Zion National Park, and thinned to the northeast due to the lower Chinle Formation lensing out against the flanks of the Ancestral Rocky Mountains, where the thickness of the Chinle is largely controlled by syndepositional salt tectonism. The Gartra and Stanaker Members of the Ankareh Formation are poorly understood Chinle Formation correlatives north of the San Rafael Swell. Osteichthyan fish, metoposaurid temnospondyls, phytosaurids, and crocodylomorphs are known throughout the Chinle Formation, although most remains are fragmentary. In the Cameron and Monitor Butte Members, metoposaurids are abundant and non-pseudopalatine phytosaurs are known, as is excellent material of the paracrocodylomorph Poposaurus; fragmentary specimens of the aetosaurs Calyptosuchus, Desmatosuchus, and indeterminate paratypothoracisins were probably also recovered from these beds. Osteichthyans, pseudopalatine phytosaurs, and the aetosaur Typothorax are especially abundant in the Kane Springs beds and Church Rock Member of Lisbon Valley, and Typothorax is also known from the Petrified Forest Member in Capitol Reef National Park. Procolophonids, doswelliids, and dinosaurs are known but extremely rare in the Chinle Formation of Utah. Body fossils and tracks of osteichthyans, therapsids, crocodylomorphs, and theropods are well known from the lowermost Wingate Sandstone and Moenave Formation, especially from the St. George Dinosaur Discovery Site at Johnson Farm.

Vertebrate paleontology, stratigraphy, and paleohydrology of Tule Springs Fossil Beds National Monument, Nevada (USA)

Tule Springs Fossil Beds National Monument (TUSK) preserves 22,650 acres of the upper Las Vegas Wash in the northern Las Vegas Valley (Nevada, USA). TUSK is home to extensive and stratigraphically complex groundwater discharge (GWD) deposits, called the Las Vegas Formation, which represent springs and desert wetlands that covered much of the valley during the late Quaternary. The GWD deposits record hydrologic changes that occurred here in a dynamic and temporally congruent response to abrupt climatic oscillations over the last ~300 ka (thousands of years). The deposits also entomb the Tule Springs Local Fauna (TSLF), one of the most significant late Pleistocene (Rancholabrean) vertebrate assemblages in the American Southwest. The TSLF is both prolific and diverse, and includes a large mammal assemblage dominated by Mammuthus columbi and Camelops hesternus. Two (and possibly three) distinct species of Equus, two species of Bison, Panthera atrox, Smilodon fatalis, Canis dirus, Megalonyx jeffersonii, and Nothrotheriops shastensis are also present, and newly recognized faunal components include micromammals, amphibians, snakes, and birds. Invertebrates, plant macrofossils, and pollen also occur in the deposits and provide important and complementary paleoenvironmental information. This field compendium highlights the faunal assemblage in the classic stratigraphic sequences of the Las Vegas Formation within TUSK, emphasizes the significant hydrologic changes that occurred in the area during the recent geologic past, and examines the subsequent and repeated effect of rapid climate change on the local desert wetland ecosystem.

Paleontology and stratigraphy of Middle Eocene rock units in the southern Green River and Uinta Basins, Wyoming and Utah

The Bridger Formation is restricted to the Green River Basin in southwest Wyoming, and the Uinta and Duchesne River Formations are located in the Uinta Basin in Utah. These three rock units and their diverse fossil assemblages are of great scientific importance and historic interest to vertebrate paleontologists. Notably, they are also the stratotypes from oldest to youngest for the three middle Eocene North American Land Mammal Ages—the Bridgerian, Uintan, and Duchesnean. The fossils and sediments of these formations provide a critically important record of biotic, environmental, and climatic history spanning approximately 10 million years (49 to 39 Ma). This article provides a detailed field excursion through portions of the Green River and Uinta Basins that focuses on locations of geologic, paleontologic, and historical interest. In support of the field excursion, we also provide a review of current knowledge of these formations with emphasis on lithostratigraphy, biochronology, depositional, and paleoenvironmental history, and the history of scientific exploration.

Late Cretaceous stratigraphy and vertebrate faunas of the Markagunt, Paunsaugunt, and Kaiparowits plateaus, southern Utah

The Late Cretaceous succession of southern Utah was deposited in an active foreland basin circa 100 to 70 million years ago. Thick siliciclastic units represent a variety of marine, coastal, and alluvial plain environments, but are dominantly terrestrial, and also highly fossiliferous. Conditions for vertebrate fossil preservation appear to have optimized in alluvial plain settings more distant from the coast, and so in general the locus of good preservation of diverse assemblages shifts eastward through the Late Cretaceous. The Middle and Late Campanian record of the Paunsaugunt and Kaiparowits Plateau regions is especially good, exhibiting common soft tissue preservation, and comparable with that of the contemporaneous Judith River and Belly River Groups to the north. Collectively the Cenomanian through Campanian strata of southern Utah hold one of the most complete single region terrestrial vertebrate fossil records in the world.

Major bonebeds in mudrocks of the Morrison Formation (Upper Jurassic), northern Colorado Plateau of Utah and Colorado

The Morrison Formation contains a number of large quarries that have yielded dinosaurs and other vertebrates, and many of these occur in sandstone beds representing ancient river channels. However, a number of very productive sites occur in mudstone beds representing other environments such as ephemeral ponds, and some of these yield both large dinosaurs and microvertebrates; these localities in mudstone beds represent different taphonomic modes of preservation and often preserve vertebrate taxa in different relative abundances from the channel sandstone sites. Among these important and very productive mudstone localities are the Cleveland-Lloyd Quarry, the Mygatt-Moore Quarry, and the microvertebrate sites of the Fruita Paleontological Area, and each of these preserves distinct vertebrate paleofaunas, different from sandstone sites and from each other, suggesting that mudstone localities had a very different mode of sampling the local biotas than did sites in sandstone.

A preliminary report of the fossil mammals from a new microvertebrate locality in the Upper Jurassic Morrison Formation, Grand County, Utah

The first Mesozoic mammals in North America were discovered in the Morrison Formation during the closing decades of the 19th century, as by-products of dinosaurs quarried by teams led by O.C. Marsh. These tiny fossils served as foundational specimens for our understanding of Mesozoic mammal evolution. There are now nearly 25 mammal-bearing localities known from the Morrison Formation, distributed across the Western Interior from the Black Hills to southern Colorado and west into Utah; the most historically important of these are in Wyoming (e.g., Como Quarry 9). Most Morrison mammals are known by jaws or jaw fragments, and several important Mesozoic groups (e.g., docodonts, dryolestoids, and to a large extent triconodonts and symmetrodonts) were established based on Morrison material, shaping the perception of mammalian diversity on a global scale. Despite heavy sampling of coeval sites elsewhere, the Morrison remains the most systematically diverse (at high taxonomic levels) assemblage of Jurassic mammals in the world. Here, we describe two mammalian specimens and highlight other remains yet to be fully identified from a new microvertebrate locality in the Morrison Formation of eastern Grand County, Utah. The site is positioned low in the Brushy Basin Member and is similar in lithology and stratigraphic level to the famous small vertebrate localities of the Fruita Paleontological Area, located less than 50 km to the northeast. In addition to small archosaurs and squamates, limited excavation to date has yielded at least 20 mammalian specimens representing a minimum of six taxa, several of which are new and quite different from typical Morrison taxa. Preservation is generally excellent and includes partially articulated cranial and postcranial elements of small vertebrates. This new site has great potential to contribute new taxa and more complete morphological data than typical Morrison localities, underscoring the importance of continued field work in the Morrison.