This is for a book I’m writing where mermaids and sirens are more of a natural plausible predators instead of magic, so i more want to know the movement of a animal having a certain tail aswell as the possibility of a mammal having it.
In: Biology
Ok, so I’m going to give it a go, fish have evolved a laterally/horizontally flexible spine, that later translated to amphibians and later still to reptiles, it makes sense fish have vertical tail fins because that way it’s perpendicular to the direction of the motion of the tail, of the spine, if you look at modern legged reptiles, they all have legs splayed out to the side of their bodies for the same reason, their spines are especially flexible laterally, if you look at a lizard running you can see how it moves it’s body side to side,
Now mammals on the other hand have evolved vertically flexible spines, mammals generally have better range of motion and stronger movements of the spine on the vertical axis, you can also see that mammals have legs underneath their bodies and subsequently (since all marine mammals have evolved from land mammals) marine mammals retain that vertical flexibility hence evolved horizontal tail fins also perpendicular to the direction of the motion of the spine.
P.S.: there have been exceptions: dinosaurs, genetically far removed from modern reptiles, yes, but still technically reptiles, all(or vast majority) had legs under the body and vertically flexible.
Crocodilians, non alive today but there have been many crocodilians that have evolved to have more vertical spine flexibility, land crocs really, see: Kaprosuchus.
Cheers!
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