Transitional fossils are fossilized remains that have traits common to both an ancestral group and its descendant groups. Now if we talk about them as a species instead of as individual fish, Matt Friedman from the University of Chicago (now at the University of Michigan) noted that there are transitional fossils from the Eocene epoch, 56 to 33.9 million years ago. They also stop swimming freely in the water and settle down to live as a bottom-dweller for the rest of its life. The now eye-less side turns pale, while the double-eyed side becomes colored like the ocean floor. They start metamorphosing at around one to two weeks old, their skulls twisting and moving around so that one eye (and their inner ear) migrates from one side of their head to the other. When they hatch as young fish, called larvae, they look like normal (transparent) fish, with their eyes on each side of their body as they swim around. In fact, the word sole comes from the Latin word for sandal, solea, referring to how flat they are! However, even though we usually see them as these flat things with eyes on one side of their head, sole fish don't start that way. "Sole" refers to many different flatfish. Let's start with the second question: "When did the sole fish start having eyes on the same side of its head"? There's two ways to go about this question: "When do baby sole fish start migrating one of its eyes to the other side of the head?" and Why is the reason a sole fish goes flat on the sand instead of moving? When the sole fish did start having the eyes on the same side of its head?
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