Cartilaginous fishes divide into two branches. The elasmobranchs, sharks and rays, are one; the holocephalans, chimaeras, are the other. Fossil and molecular evidence place the split in the Devonian, roughly 400 million years ago, since when the chimaera line has continued alone.
The differences follow. Chimaeras have a single gill opening on each side covered by a fleshy operculum, rather than exposed slits. The upper jaw is fused to the braincase, which is what holocephalan means. Instead of endlessly replaced rows of teeth, they carry six permanently growing tooth plates. They lack a true stomach, the oesophagus running more or less directly into the intestine, and they lack the rough denticle covering that makes shark skin abrasive.
Rabbit fish live on soft slope bottoms, feeding largely on invertebrates, and lay elongate horny egg capsules. Deepwater trawling takes them in numbers as bycatch, and slow growth leaves little room to absorb it.