ABSTRACT The Morrison-Golden Fossil Areas National Natural Landmark, Colorado, USA, including Dinosaur Ridge, is rich in geological and paleontological history, ranking historically as the premier location and type area for Late Jurassic dinosaurs like Stegosaurus and Diplodocus . As the type area for the Morrison Formation, it famously became central in the “Bone Wars” underway in the 1870s. After a brief historical introduction at Stop 1, the trip will explore the ‘mid’-Cretaceous Dakota Sandstone (Stop 2), which yields the top-ranked dinosaur tracksite in the United States, with two type ichnospecies and the most accessible “nest scrape display trace” evidence of dinosaurian sexual display found anywhere. It also an important location for the study of microbial mat in association with dinosaur tracks, now photogrammetrically surveyed in detail. Dinosaur Ridge serves as the type area for the “Dinosaur Freeway” and the transgression of the Western Interior Seaway. Its diverse invertebrate traces have also been described in detail. After lunch at the Dinosaur Ridge Visitor’s Center exhibit (Stop 3), the trip will move a short distance to Golden to view the type localities for the first bird and crocodilian tracks ever reported from the Mesozoic (Stop 4). We will also visit the younger (Late Cretaceous) Laramie Formation, exposed in the Golden clay pits, cut by the Golden fault, and the source of historically famous paleofloras, the first known Ceratopsian tracks, and other type traces now developed as Triceratops Trail (Stop 5), constituting part of the well- documented Fossil Trace and School of Mines Geological Trail complex. The field excursion will involve easy walks and no strenuous climbs.

ABSTRACT The Colorado Plateau in the southwestern United States is within the Paleozoic transcontinental arch, an area of thin, cratonic strata. The plateau was broken by latest Mississippian to early Permian Ancestral Rocky Mountain orogenesis, which produced bedrock uplifts that influenced lower Mesozoic sedimentation before Jurassic burial. Clastic sediments shed from uplifts interfinger with eolian Permian strata ultimately derived from eastern Laurentia. Triassic and Jurassic strata of the Colorado Plateau are here divided into five depositional systems, each representing a different sedimentary and tectonic setting and forming stratal associations referred to as “deposystems.” The five deposystems, which largely but not entirely correspond to formation or group names, were deposited during northward continental drift from tropical latitudes (fluvial, tidal, and nearshore marine Moenkopi and fluvial Chinle) through desert latitudes (the erg-dominated Glen Canyon and San Rafael) to temperate latitudes (fluvial Morrison). Paleomagnetically determined paleolatitudes, corrected for inclination shallowing due to postdepositional sediment compaction, place the Glen Canyon and San Rafael eolianites firmly within expected latitudes for desert environmental conditions. Lower Triassic strata of the Moenkopi deposystem form a westward-thickening wedge of fluvial and shallow marine strata and are overlain by entirely fluvial strata of the Chinle deposystem. Both contain 240–280 Ma detrital zircon populations derived from the east Mexico magmatic arc, but more northern Chinle fluvial deposits contain a higher fraction of zircons derived from Paleozoic, Neoproterozoic, and Grenville provinces in eastern Laurentia. Westward thickening of Moenkopi strata is attributed to subsidence in the proforeland basin of the east-vergent Sonoma orogeny in central Nevada, whereas accommodation space for Chinle sedimentation was provided by dynamic subsidence above the upper Triassic subduction zone behind the newly established Cordilleran magmatic arc to the southwest. Overlying, largely Jurassic Glen Canyon and San Rafael deposystems are dominantly eolian. Detrital-zircon geochronologic analysis indicates that eolian sands were derived largely from eastern Laurentia. Interbedded marginal marine, ­lacustrine-sabkha, and fluvial strata have been associated with regional unconformities, but evidence for such unconformities is here regarded as indicating facies transgressions without development of plateau-wide unconformities or disconformities. Upper Jurassic northward continental drift carried the plateau out of the desert belt and into the zone of prevailing westerly winds. This coincided with a flare up of magmatism in the Cordilleran magmatic arc, leading to transgression of Morrison fluvial sediments over erg deposits of the San Rafael deposystem. Eastward dispersal of Morrison sediments marked the initiation of the Cordilleran orogen as the dominant topographic feature of the plateau region.

Abstract: Pterosaur fossils are rare in the Morrison Formation, and most are poorly preserved. The Breakfast Bench Facies (BBF) at Como Bluff produces incomplete but uncrushed limbs. One proximal and two distal femora match a complete femur (BYU 17214) referred to Mesadactylus . Unexpectedly, both of the BBF distal femora possess a large intercondylar pneumatopore. BYU 17214 also possesses an intercondylar pneumatopore, but it is smaller than in the BBF femora. Distal femoral pnuematicity is previously recognized only in Cretaceous azhdarchoids and pteranodontids. A peculiar BBF jaw fragment shows strongly labiolingually compressed, incurved crowns with their upper half bent backwards; associated are anterior fangs. We suspect this specimen is a previously undiagnosed pterosaur. Additional BBF material documents a diverse pterosaur fauna including a humerus with a greatly expanded ectepicondyle possibly from a non-pterodactyloid monofenestratan.