My comment does not indicate that sharks were once terrestrial (as with cetaceans), but rather states they are adapted to an aquatic life...I apologize for any confusion.
it is an interesting question though... at any time in those 420 million years did a shark species of some sort evolve to a terrestrial existence and then go extinct at a later date, it is possible...
fossils are very very rare, so I can't dismiss it... sharks are well adapted to their environment so there doesn't appear to be any need to evolve to a terrestrial existence but that can be said about every terrestrial life forms previous aquatic existence .... all terrestrial life came from the sea because there was a niche to be filled, some returned to the sea and many probably went extinct without leaving any fossil evidence of their terrestrial existence...I agree until such definitive fossil evidence is discovered it just didn't happen but it's very plausible...
That's exactly what I meant. Of course, I still question our total understanding of how all of this fits together.
I see. When I read the OP, I took it as though you meant the sharks adapted from life on land.... Which of course would totally contradict the theory of evolution..... Well, parts of it anyway... Nonetheless, interesting topic from a science standpoint.
It's like trying to piece together an enormous 3D puzzle with millions of pieces missing...you can see it taking shape and know where it's going but there huge gaps...
