Distance: The distance between the Americas and Europe is vast, especially when considering that the organisms of the time had to cross the ocean. This presented a significant barrier to dispersal.
Habitat Differences: The habitats in Europe were significantly different from those in the Americas. Many species were adapted to live in particular environments, and could not survive in the different conditions found on the other continent.
Competition and Predation: The species that existed in Europe had their own ecological roles and niches, and any new species would have faced strong competition and predation from the existing species. This would have made it difficult for new species to establish and survive.
Environmental and Climatic Factors: The climate and environmental conditions in Europe during the Quaternary period were often harsh, including glacial periods and periods of aridity. These factors would have made it challenging for species from the Americas to adapt and thrive in these conditions.
Overall, the combination of these factors, and others, limited the dispersal of animals from the Americas to Europe during past geologic time periods.
