Little-known exposures of the Upper Jurassic Morrison Formation from the Purgatoire Valley of southeastern Colorado have yielded the world's largest continuously mapped assemblage of dinosaur trackways. Body fossils include plant, invertebrate, and vertebrate remains indicative of predominantly fresh-water conditions. The tracks occur in a lacustrine sequence characterized by (1) shallow-water shales; (2) micritic shoreface limestones with ooids, intraclasts, ripple marks, mud cracks; and (3) minor quartzose sandstone with salt-crystal casts. Analysis of these sedimentary facies suggests that in southeastern Colorado, lakes were larger and longer lived than in other Morrison paleoenvironments.
Trackway orientations and footprint-depth contours pinpoint the location of the paleoshoreline at successive levels in the section. Detailed mapping of 1,300 footprints in bed 2 has revealed more than 100 trackways which testify to the activity of both quadrupedal and bipedal dinosaurs. The respective ratio based on trackway counts is ∼40/60. The quadrupedal tracks are attributed to sauropods, and represent the first ever discovered in North America. They also exhibit the first known manus claw impressions and are in need of formal description. More than 90% of the bipedal tridactyl prints lack claw impressions and are tentatively therefore referred to the Ornithopoda, cf. Gypsichnites possibly a Camptosaurus. The small proportion with distinct claw impressions may represent Allosaurus. Resulting estimates of species diversity (∼5) and predator-prey ratio (1:30) are in general agreement with the estimates of other authors based on skeletal remains, and they suggest that the Purgatoire River tracks may accurately reflect the composition of the dinosaur fauna. Distinctive groupings of parallel, non-overlapping trackways suggest gregarious behavior among sauropods and tridactylous forms.
- Geological Society of America