"An anachronism is something that is chronologically out of place: a typewriter or floppy disc in a modern office. Leather helmets at the Super Bowl. Or, hopefully, the internal combustion engine in the near future. An ecological anachronism is an adaptation that is chronologically out of place, making its purpose more or less obsolete. A tree with big fruits to attract huge mammals as dispersers of its seeds is anachronistic in a world of relatively small mammals."
This is definitely a fascinating and well-written read.
Also, just for fun, I want to show you what some of these extinct megafauna looked like.
Diagrams (in Portuguese, but they have some nice perspective) of North American megafauna
Glymptodont (an oversized relative of the armadillo)