I'm going with species native to the southern hemisphere, I'm this case using dinosaurs and prehistoric life from Australia, New Zealand, and Antarctica. Try and use species that may have been able to get there. I'd suggest looking at maps of different eras. ;)

EDIT- Here's an idea: in Lark Quarry, there was [url=http://www.dinosaurtrackways.com.au/en_US/stampede-story]a trackway[/url] disvovered. The footprints belong to as-of-yet unknown dinosaurs. Any of the three could be used for a speculative evolution project.