Please tell us a bit more:
- where (country,county.town, etc.) ?
- details of particular locality (roadside cutting, quarry, beach, etc) ?
- was it taken from an exctual exposure of rock or from soil or somewhere loose?
The Aust Cliff area is "famous for its Rhaetic bone bed, and is also the most productive locality in Britain for Triassic insects."
There are some good references on the geology, eg:
But unfortunately I do not recognize your fascinating specimen.
Let's hope somebody else here does.
(I'm tempted by the idea of dragonfly abdomen, but that is just a guess.)