hello my name is not jeff Known colloquially as "The ironing-board shark" due to the large- ironing-boards shaped fin on its back, Stethacanthus was an early shark that behaved much like modern reef sharks. It was alive 370-345 million years ago, and ranged from 70 centimetres to 2 metres in length. No-one knows what the ironing-board shaped fin was used for, though some speculate it may have been a way of attracting females, or scaring larger predators.