confusion over which test to use...

#1
Hello! I was just wondering about the appropriate statistical test to see if there is a significant difference in the data for plants and animals. I have attached a graph that has data points for plants and animals and each data point is basically a date range (i.e 3 to 5 million years ago - MYA) to depict the time when plant or animal lineages diverged after crossing a geographical barrier.
Here's a bit of background. South America split from Africa 100 MYA and collided with North America 3 MYA with the formation of the Panama Isthmus. This provided a terrestrial route for animals from south america to move to North america and vice versa and led to a number of speciation events. I hypothesis that the ranges for the timing of speciation for animals should coincide with the closure of the Panama Isthmus (i.e. 3 MYA or later - as in more recently) whereas plants because of their dispersal abilities will have crossed from S to N america or vice versa much earlier because they can disperse over the ocean barrier.


Firstly i would like to see if there is a statistical difference in the ranges of speciation between plants and animals. And secondly, is there any statistical way of knowing whether the data point of plants altogether cover a broader date range (eg from 1 MYA to 70 MYA) than animals ( which have data points that are more clustered around the 3 Million mark)

Any help at all would be greatly appreciated. thank you thank you thank you. Ask me to clarify anything if it is still unclear. On the graph P = Plants and A = animals. Thanks!

Sarah