Middle Jurassic (Bajocian and Bathonian) Dinosaur Megatracksites, Bighorn Basin, Wyoming, U.S.A.

Abstract

Two previously unknown rare Middle Jurassic dinosaur megatracksites are reported from the Bighorn Basin of northern Wyoming in the Western Interior of the United States. These trace fossils occur in carbonate units once thought to be totally marine in origin, and constitute the two most extensive Middle Jurassic dinosaur tracksites currently known in North America. The youngest of these occurs primarily along a single horizon at or near the top of the ''basal member'' of the ''lower'' Sundance Formation, is mid-Bathonian in age, and dates to ϳ167 ma. This discovery necessitates a major change in the paleogeographic reconstructions for Wyoming for this period. The older tracksites occur at multiple horizons within a 1 m interval in the middle part of the Gypsum Spring Formation. This interval is uppermost Bajocian in age and dates to ϳ170 ma.Terrestrial tracks found, to date, have been all bipedal tridactyl dinosaur prints. At least some of these prints can be attributed to theropods. Possible swim tracks of bipedal dinosaurs are also present in the Gypsum Spring Formation. Digitigrade prints dominate the Sundance trackways, with both plantigrade and digitigrade prints being preserved in the Gypsum Spring trackways. The Sundance track-bearing surface locally covers 7.5 square kilometers in the vicinity of Shell, Wyoming. Other tracks occur apparently on the same horizon approximately 25 kilometers to the west, north of the town of Greybull. The Gypsum Spring megatracksite is locally preserved across the same 25 kilometer east-west expanse, with the Gypsum Spring megatracksite more extensive in a north-south direction with tracks occurring locally across a 100 kilometer extent. Conservative estimates for the trackway density based on regional mapping in the Sundance tracksite discovery area near Shell suggests that over 150,000 in situ tracks may be preserved per square kilometer in the Sundance Formation in this area. Comparable estimates have not been made for other areas.Similarities between the two megatracksites include their formation and preservation in upper intertidal to supratidal sediments deposited under at least seasonally arid conditions. Microbial mat growth on the ancient tidal flats apparently initiated the preservation of these prints.

A new partial skeleton of a cryptocleidoid plesiosaur from the Upper Jurassic Sundance Formation of Wyoming

“…This set of beds represents the last transgressive-regressive cycle of the Sundance Seaway (Brenner and Peterson, 1994). Molluscan biostratigraphy places this member in the Oxfordian (Kvale et al, 2001). It is highly fossiliferous, with numerous invertebrate remains and occasional, usually isolated, vertebrate remains.…”

“…The Sundance Seaway was a shallow epicontinental seaway that covered much of Wyoming and parts of Montana, South Dakota, and Colorado in the late Jurassic (Kvale et al, 2001). Marine sediments deposited by this seaway in southeastern Wyoming were named the "Shirley Stage" by Knight (1900), and later the Sundance Formation by Reeside (1919).…”

“…In the Laramie Basin, Imlay (1947) divided the Sundance Formation into seven members. In the Bighorn Basin, where the specimens described here were recovered, Kvale et al (2001) divided the Sundance into Lower and Upper Members. This more general division is the one in current use given the lateral heterogeneity of the Sundance Formation and the fact that Imlay's (1947) detailed stratigraphy is highly localized.…”

Articulated bone sets of manus and pedes of Camarasaurus (Sauropoda, Dinosauria)

“…It is stratigraphically slightly higher than the Howe Quarry, but like all the excavation sites on Howe Ranch, it is located beneath the so-called clay change (see Schwarz et al, 2007a, figure 3), which was used for long-distance correlation by Turner and Peterson (1999) and Ikejiri (2004Ikejiri ( , 2005. The site was dated to 147 Ma by Kvale et al (2001, figure 5), based on its position somewhat above a bentonite layer dated to 151.5 +/-4.0 Ma (Kvale et al, 2001, table 1). FIGURE 1.…”

Sequence stratigraphic controls on synsedimentary cementation and preservation of dinosaur tracks: Example from the lower Cretaceous, (Upper Albian) Dakota Formation, Southeastern Nebraska, U.S.A.

“…During the past decade, numerous studies have suggested that the preservation of dinosaur tracks may be, in part, controlled by relative sea level rise (e.g. Lockley et al, 1992Lockley et al, , 2006Lockley and Hunt, 1995;Matsukawa et al, 1999;Kurtz et al, 2001;Kvale et al, 2001). Lockley and Hunt (1995) noted similarities in stratigraphic positioning of track beds/intervals (megatracksites) within the Upper Triassic Chinle Group (Utah, New Mexico, Texas, Oklahoma, Colorado, Arizona, and Wyoming) and the Lower Cretaceous Dakota Group (Colorado and New Mexico).…”