I was so trilled with this find!
I completely forgot to photograph it while it was still laying on the beach (smile)
In the past we found some small (fossil) vertebras,
but never such a big one like this (10,5 x 6,5 x 1,2 cm)
with an intervertebral disk on top.
It probably belonged to a fish, dolphin or seal...
Update: I found some similar examples of harbour porpoise
(in Dutch: bruinvis) vertebrae on the internet.