I think the problem here is that we are not looking at it with ancient eyes. The engraver will likely have never seen a dolphin but would have seen a dolphin in a mosaic etc. When we look at how a dolphin is depicted in a mosaic does this help?
This reminds me of a discussion I saw some year back about Roman elephants on coins and the cross hatching that is depicted on them. There was debate as to whether the cross hatching represented armour or not... For example But then an image of the Piazza Armerina mosaic was used to illustrate that the cross hatching was used to represent the wrinkled skin. Until we know how they thought thinks looked then it is sometimes difficult to guess what they were depicting on coins.