North America’s largest assemblage of Late Jurassic dinosaur trackways, a new look at the Purgatoire Valley dinosaur tracksite (Dinosaur Lake), Morrison Formation, southeastern Colorado, United States

Abstract

ABSTRACT The Purgatoire Valley dinosaur tracksite (‘Dinosaur Lake’) is North America’s largest assemblage of Late Jurassic dinosaur tracks. Roughly 1,300 tracks representing ~100 trackways were reported from a 5,600 m2 area in 1986. Extensive new excavations have revealed more than 800 additional tracks in a 2,600 m2 area. Less abraded tracks farther from the river channel, excavated below bed rock not alluvium, provide high-quality-of-preservation morphometric data. Sauropod (Parabrontopodus) trackways up to 88 m long, some with over 100 successive pes prints, comprise nearly 500 manus and pes tracks. Parallel sauropod trackways confirm previous interpretations of gregarious behaviour, now including a mixed herd of subadult and adult individuals. A preferred westward trackway progression trend likely suggests a single or short-lived episode of gregarious movement along a lake shoreline. Nearly 300 newly recorded tridactyl (theropod) tracks range from 25 to 40 cm in length, with several trackways exceeding 25 or more successive steps. Theropod tracks are assigned to ichnogenus Megalosauripus, also known from the Late Jurassic of Utah, Europe, Central Asia and North Africa. Other theropodan ichnogenera (Hispanosauropus and Therangospodus) from this epoch and region have not been identified. Cross-cutting (over-printing) trackway relationships help constrain the sequence of track registration events.