Please can you help my son Thomas (aged 8) identify this skeleton of what looks like a fin found at our local beach, Poppit sands in West Wales. Shark, porpoise, or something else completely?

Length 10cm. Width 6cm (widest point). Height 4.5cm (base to top of fin)

Thomas is a keen forager and biologist in the making.

Thank you

S. Cartwright

Post's pictures

IMG_4172.jpg, 955.7 kb, 1936 x 1936

IMG_4176.jpg, 142.69 kb, 640 x 640

As correctly spotted by reader 'Paul', this is probably a sternum (breastbone) from a gull.