The earth was once a super continent conected together. It was formed millions of years ago holding many species. When the continental drift happened it broke appart the continent and then they shifted away acrost the sea. The animals were forced to adapt and change as their climates did. The species evolved and pretty soon they were widespread creatures.
This would explain why tropic plant fossils were found in the antarctic. They died from the climate change and were preserved in the snow. This caused all tropic creatures in the continent to die. The snow covered the remains and the fossils