an ecological interaction, just as some animals today congregate for mutual beneficial reasons, such as augmented resource acquisition or lesser predation pressure. It is also possible that a herd of hadrosaurids and therizinosaurids walked across this terrain at different times and did not encounter each another. Regardless of these explanations, the therizinosaurid trackway reflects a gregarious behaviour in these theropods. Distribution Although the vast majority of all therizinosaurids have come out of Asia (especially from China and Mongolia), the relatively complete remains of Nothronychus have been found outside Asia in North America. Though the fossil record of therizinosaurids indicates that the family flourished near the Turonian stage of the Late Cretaceous, by the Barremian stage there was already a pan-Laurasian distribution of therizinosauroids. Since Laurasia was beginning to break apart during the Cretaceous, two theories have arisen to explain how therizinosaurs could have spanned across Laurasia. One theory is that primitive members were already present in both Asia and North America before it began to drift apart, suggesting an emergence for therizinosaurs of at least the Late Triassic. A second theory is that the group dispersed between Asia and North America via a "Cretaceous" Beringian land bridge during the Berriasian-Valanginian stages. This last proposal is supported by the presence of therizinosaurids in North America during the early Late Cretaceous such as Nothronychus. Nevertheless, therizinosaurid-grade footprints are known from remote locations such as Europe or Africa. Several four-toed tracks were found in Poland, and a series of footprints dating back to the Late Cretaceous were found in Morocco. Both findings may indicate that therizinosaurids were far more disperse than previously thought. The idea of a land bridge is even more supported by the multiple co-occurrence of hadrosaurid and therizinosaurid footprints at the lower Cantwell Formation of the Denali National Park, which reflects an important faunal exchange between landmasses. These tracks show the dominant presence of hadrosaurids and a reduced concentration of therizinosaurids in the same location at potentially the same time. As indicated by the discovery of a waterlily-like impression representing a single fossil leaf from the same site, the trackway was made by the dinosaurs as they crossed a shallow body of water away from the main river channels, which is curious given that during the Late Cretaceous this part of North America was a semi-arid habitat. This occurrence at this single locality within the lower Cantwell Formation has not