Found on South coast of Wales, I think it may be a vertebrae of an aquatic mammal? But not certain. Would be grateful for an identification of the bone and which animal it may have belonged to. It is 5.5cm in length and 4cm width.
This is a thoracic vertebra from fairly close to the neck end of the thorax. The spine is too long for it to have come from a seal and the vertebra is the wrong shape to be from a dolphin or porpoise. There are some seal vertebra here
I think the most likely is a sheep
I don't have access to sheep vertebra for a direct size comparison. However, here is a link to roe deer vertebra
The first thoracic vertebrae is in the second column from the left, second vertebrae down and measures 4.2cm wide and 9cm long, so I wouldn't see your vertebrae as being from a particularly large sheep.