[cabezon]

I agree that it is a stonefly molt. As stoneflies crawl out onto the shore to molt, their air-filled molts could be carried off when rising water (and we've had that recently....) inundated the stream bank. A dry, air-filled molt could easily be carried from the banks of one of our streams into the estuary where you found it. And you are correct that there are no marine stoneflies.
As a side-note, because stoneflies are very sensitive to water quality, the fact that you found this molt would indicates that the adjacent stream (if that was the source) flows with unpolluted water.
Steve

 

[cabezon]

I take it then that we're in agreement? If @Taxon thinks it's a stonefly, it's a stonefly. I'd expect them to be hatching later in the year, would this be a molt?

Big sucker, eh?

Hi P,
It looks to me like the terminal molt from the aquatic stage to a flying adult. There are a number of stoneflies (for example, Skwalas and winter stones) that emerge in the winter and early spring. The giant stoneflies tend to be later and golden stoneflies in early summer and little yellow (yellow Sallie) later in summer.
Steve