During the Permian, these two regions were separated by a narrow sea-way, but it is not clear why one geographically isolated group should evolve a sail, but the other group not.
在二疊紀期間，這兩個地區被狹窄的海道分開，但目前還不清楚為什麼被地理上分離的群體，會一群演化出背帆，而另一群沒有。