Why did the same plant species exist on multiple continents even without a supercontinent?
The existence of the same plant species on multiple continents, even without a supercontinent, can be attributed to a phenomenon called "biogeographic dispersal." This occurs when plant species are capable of dispersing across geographic barriers such as oceans or mountain ranges.
To understand how this happens, we can look at a few key mechanisms:
1. Geographical Continuity: Even if continents are separated, certain land formations can connect different regions. For example, during the Cretaceous period, when flowering plants (angiosperms) were diversifying, land bridges like the Bering Land Bridge between Asia and North America provided a pathway for plant migration.
2. Long-distance Dispersal: Some plant species have mechanisms that allow them to disperse over long distances. For instance, wind-dispersed seeds or fruits can be carried by air currents across vast distances, enabling plants to colonize new territories. Animal dispersal is another means, where animals consume fruit or seeds and transport them to different areas before excreting them.
3. Continental Drift: Over millions of years, tectonic plate movement causes continents to drift apart or come together. During periods of fragmentation, plant species on a particular landmass may become isolated, leading to independent evolution. However, when continents later reunite, either partially or completely, these evolved species can encounter each other again and potentially mix through migration.
4. Vicariance: Vicariance refers to the splitting of a population by the formation of geographic barriers. This process can occur due to geological events such as the uplifting of mountain ranges or the formation of new bodies of water. Over time, distinct populations may evolve in isolation on each side of the barrier. If the barrier eventually disappears, these populations can come into contact again, often resulting in the mixing of plant species.
It's important to note that plant dispersal and continental movement are complex processes that have occurred over millions of years. The presence of the same plant species on multiple continents without a supercontinent is the result of various interconnected factors, including geographical continuity, long-distance dispersal, continental drift, and vicariance.