What would the orthocone rayonnoceras have eaten?

[andrewsaurus rex]

Trilobites would seem a likely food source as they were plentiful.  If not trilobites, what would have been on the menu?

[Halichoeres]

I don't know if it's possible to know that. But here, for examples, is a list of the fauna of one formation it's found in:
https://en.wikipedia.org/wiki/Fayetteville_Shale#Fauna
Snails, brachiopods, bivalves, crustaceans, trilobites, other cephalopods, even some fishes. Many of these were small enough to be prey to a good-sized nautiloid.

[andrewsaurus rex]

I've read that trilobites have been found, preserved inside orthocone shells, but I took that to mean that they were living in empty shells when the died, not that they had been consumed   I wonder about the ability of the beak of rayonnoceras to break through the shell of a trilobite.  Perhaps the other prey you've listed would have been easier pickins.

[Newt]

It's right there in the name: Rayonnoceras lived entirely on the semi-synthetic fiber Rayon. Of course, Rayon, AKA Viscose, was not invented until 1894, but Rayonnoceras, like other orthocones, had the ability to project itself through time and space - not permanently, due to the constraints of paradox, but long enough to forage for its favorite fibers before being snapped back to its own proper position in space-time. This unique temporal travel process is called orthoconic projection.

The mass production of Rayon in the early twentieth century caused a spike in Rayonnoceras populations; these orthocone plagues ravaged the Rayon industry and led to the near cessation of Rayon production and the extinction of Rayonnoceras itself - victim to its own success.

Also, octopodes prey heavily on crabs - I don't see why nautiloids shouldn't be able to feed just as easily on trilobites. 

[Halichoeres]

I read a phylogenetic analysis in which Rayonnoceras was synonymized with Viscoceras.