Taphonomy and Paleoecology of the Dinosaur Beds of the Jurassic Morrison Formation

Abstract

The Upper Jurassic Morrison Formation has yielded one of the richest dinosaur faunas of the world. Morrison sediments are distributed over more than a million square kilometers in the western United States and represent a mosaic of riverine, lacustrine and floodplain environments developed on a vast alluvial plain nourished by debris from the ancestral Rocky Mountains. Plant productivity must have been reasonably high to support abundant large-bodied herbivores, but the absence of coals, scarcity of small aquatic vertebrates, the abundance of oxidized sediments, and presence of calcretes lead us to believe that water was periodically in short supply. A strongly seasonal climate may have necessitated annual large-scale movements of large herbivores, accounting in part for their remarkably broad and uniform geographic distribution. Dinosaur diversity is lower in the Morrison than in the Late Cretaceous, and taphonomic alteration is higher. Massed accumulations of thousands of bones are characteristic of the Morrison. Morrison dinosaurs were not confined to specific depositional environments but were distributed across the complete spectrum of available habitats, from lakes to dry floodplains; this type of distribution is similar to that of large terrestrial mammals such as elephants and rhinos and is different from that of hippos and crocodiles. Common Morrison taxa wereCamarasaurus, Apatosaurus, Diplodocus, AllosaurusandStegosaurus;these genera probably constituted a true dinosaur community.Stegosaurusmay have been partially segregated from the other genera, andCamptosaurusmore strongly so.CamarasaurusandDiplodocuswere gregarious, with juveniles and subadults of the former particularly common;Apatosauruswas less abundant and more solitary in its habits. Juveniles and subadults are known for a number of dinosaurs.

The reciprocal interaction of angiosperm evolution and tetrapod herbivory

“…Large bodies and long necks would presumably have permitted them to browse 10-12m above the ground (Bakker, 1978). Many sauropods appear to have been terrestrial (Dodson et al, 1980), and their feeding behavior is thought to have involved removing shoots or branches with their rake-like dentitions (Coombs, 1975). Sauropods probably lacked chewing capabilities, the teeth being widely separated and showing no signs of tooth-to-tooth occlusion.…”

Giants on the landscape: modelling the abundance of megaherbivorous dinosaurs of the Morrison Formation (Late Jurassic, western USA)

“…The giantism of sauropods and other Morrison herbivores, by affecting their abundance on the landscape, would thus presumably have influenced the extent to which they affected the physical structure of the ecosystem (Dodson et al 1980), their cropping impact on the vegetation and the amount of meat and other organic matter they provided to carnivorous dinosaurs, scavengers and (ultimately) decomposer organisms. So, if we could constrain how common the herbivorous giants were, we could better reconstruct the nature of interactions between the dinosaurs and their environment.…”

“…A number of authors have attempted to characterise the physical and floristic environment in which Morrison dinosaurs lived (e.g., Dodson et al 1980;Russell 1989Russell , 2009Bakker 1996;Ayer 1999;Dunagan 2000;Engelmann et al 2004;Turner and Peterson 2004;Foster and Lucas 2006;Foster 2007;Hotton and Baghai-Riding 2010). Evidence from ancient soils (Retallack 1997), plant fossils ) and invertebrate trace fossils (Hasiotis 2004) indicates that, for the most part, the climate of the Western Interior of the USA during deposition of the Morrison Formation was rather dry, at least seasonally.…”

“…Even so, it is clear that a half dozen or more species of really big plant eaters coexisted at any given place and time (Dodson et al 1980).…”

“…Although this scenario might be consistent with the hypothesis that Morrison megaherbivores had low metabolic rates, but also showed strong preferences for the habitats in which trackways were likely to be preserved, it seems very unlikely that all species of Morrison megaherbivores would have preferred the same habitats (cf. Dodson et al 1980;Bakker 1996). This scenario would instead, like that described in the preceding paragraph, seem more consistent with skeletal outcome 1, and with the hypothesis that Morrison megaherbivores had high metabolic rates, but with the proviso that Morrison megaherbivores lived in herds (cf.…”