A dinosaur adventure (Cretaceous period)

Pretty self explanatory this is a scenario in which you accidentally enter a portal and end up in the Cretaceous period. (Ik it realistically wouldn't be possible because of the different atmospheres but just imagine its possible) Short beginning summary. You are a paleontologist who is currently working on a newly found site. You go explore the cave further and fall through the floor as it collapses and fall through a portal. You awaken during the Late Cretaceous.