Coelacanth

  The coelacanth is the oldest living fish to date. Until it's recent discovery, it was believed to have been extinct over 65 million years ago. The coelacanth is believed to be the missing link between fish and the tetrapods, vertebrate animals with four limbs. The coelacanth was believed to have gone extinct at around the end of the Cretaceous period.
  The coelacanth is unique from other fish as it has a brain that occupies only 1.5% of the braincase, the other 98.5% is filled with fat. Its kidneys are fused into a single organ, sitting on the floor of its abdomen instead of under the backbone. Furthermore, its minuscule heart is more or less a straight tube.
  There are only two living species of the coelacanth.