Seals have two joints where the rib attaches to the vertebra which gives the head end of the rib a distinctive shape, which is not like this rib. On the other hand the lower ribs of cetacea have one attachment and the overall shape of the rib is exactly like this one. The longest porpoise rib is about 25cm, White beaked dolphin 45cm and Minke whale 70cm. These are the only measurements I have scientific papers for.This rib is 40cm end to end so I would suggest a dolphin, a small white beaked or a smaller species. 28/06/2013 there was a dead white beacked dolphin washed up at holkam gap.