Researchers found evidence that the joints evolved as the species became active predators with jaws and mobile skeletons.

Zebrafish Embryo - Dr. Philipp Keller and Raghav K. Chhetrii Janelia Research Campus, Howard Hughes Medical Institute / Ashburn, Virginia, USA / Spatially isotropic whole-animal functional imaging ...

“It is a viviparous fish, meaning the male fish fertilizes the egg inside the female fish’s body. The embryo develops directly in the body, and the young fish can swim independently after birth,” he ...