Evolutionary biologists have theorized specifically that the creatures respiratory systems, fed by efficient gills, were present in the species since they first diverged on Earth more than 400 million years ago. A study of the 325 million year old shark-like creature, published in scientific journal Nature, suggests that ancient sharks might have developed their gills after bony fish did. The authors of the study say the fossil, which represents the earliest identified cartilaginous fish with a preserved respiratory system, has a gill structure that makes it look more like an average bony fish than a shark.
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