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

Abstract

<jats:p>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.</jats:p>

Decomposition of dinosaurian remains inferred by invertebrate traces on vertebrate bone reveal new insights into Late Jurassic ecology, decay, and climate in western Colorado

“…Taxonomic tabulations of these traces showed the vast majority (78.261%) of all invertebrate traces occur on sauropod fossil material (342 individual traces). This could potentially reflect the large size of these animals and their proportionately longer exposure times in an attritional overbank depositional environment, but the frequency at which they are marked aligns with the overarching taxonomic composition of the site ( Foster, 2014 ; Foster et al, 2018 ). Theropod material, the second largest taxonomic group found at MMQ, contained 13.956% of the observed insect traces.…”

“…Other terrestrial and freshwater invertebrates known to modify vertebrate bone are carnivorous gastropods and burrowing bivalves ( Janssen, Van Baal & Schulp, 2013 ; Serrano-Brañas, Espinosa-Chávez & Maccracken, 2018 ; Tapanila et al, 2004 ) ( Table 3 ). We can exclude any putative marine actors, as the Brushy Basin of western Colorado is well-supported as a freshwater fluvial system ( Foster et al, 2018 , Maidment & Muxworthy, 2019 ; Turner & Peterson, 2004 ). Our working hypotheses for putative actors for each identified trace type are summarized in Table 4 .…”

“…Although this trace is known to be generated by vertebrate taxa, such as rodents, their traces are substantially longer and wider, form arcs, tend to overlap, and are often positioned on bone margins ( Mikuláš et al, 2006 ; Milan, Lindow & Lauridsen, 2011 ), whereas the traces observed here are not. Additionally, Mesozoic mammals are absent from the MMQ fossil assemblage ( Foster et al, 2018 ), although they are found at quarry sites in the surrounding region ( Davis, Cifelli & Rougier, 2018 ; Engelmann & Callison, 1999 ; Hughes et al, 2015 ; Luo & Wible, 2005 ; Rasmussen & Callison, 1981 ). Mesozoic mammals include many groups unrelated to modern rodents (e.g., docodonts, multituberculates) and tend to have paired incisors with fluted enamel, which have been shown to more deeply penetrate the periosteum, leaving subscores within the traces that can often form overlapping sets ( Kielan-Jaworowska, Cifelli & Luo, 2005 ; Klippel & Synstelien, 2007 ; Longrich & Ryan, 2010 ).…”

“…The main horizon at the MMQ is a 1–2 m-thick unit that varies from laminated to medium bedded gray silty mudstones that lies atop a layer of micritic pebbles and is stratigraphically located 64 m above the base of the Brushy Basin Member ( Foster, 2003 , 2014 ; Foster et al, 2018 ; Foster & Hunt-Foster, 2011 ; Kirkland & Carpenter, 1994 ; Mygatt, 1991 ) ( Fig. 1 ).…”

“…The MMQ is interpreted to preserve a riparian ecosystem, with abundant vegetation and a high water table, but without perennial standing water, similar to modern-day watering holes, bogs, and mires ( Foster, 2003 , 2014 ; Foster et al, 2018 ; McHugh, 2018 ; McHugh et al, 2018 ; Mygatt, 1991 ; Trujillo et al, 2014 ). Carbonized plant material is prolific within the quarry, however aquatic vertebrates such as crocodylomorphs, turtles, and fish are rare in the main horizon, supporting the interpretation that there was no continual standing water in the ecosystem, though in the limestone horizon (“fish layer”) of the quarry (stratigraphically above the dinosaur-bearing main horizon) this changes ( Foster, 2014 ; Foster et al, 2018 ). Previous taphonomic work established this as an autochthonous assemblage within an attritional overbank environment with seasonal deposition, resulting in few articulated specimens and no preferred orientation of skeletal elements ( Foster, 2014 ; Foster et al, 2018 ).…”