Re: why only now?
Many thanks for providing a link to your 2005 post: I've missed that one. I do hope that 'lubech' will do what he promised to do and put all of your posts together.

I am not sure about this 'steppe'/ceral grasses argument; it does not seem complete. The 'steppes' were the consequence of cooling/drying at the end of the Eocene; hence the open spaces. The late Triassic was also dry, with herds of large herbivorous reptiles roaming the open spaces. The first flies appeared at the same time. I would think that the dung was as abundant back then as now and it was also available on the open spaces (rather than hidden in forests). It must be something else. Perhaps your idea that it is not dung per se which is important but a certain kind of dung: the one that contains semidigested seeds of angiosperms packing the calories -- is more to the point. My own guess is that it could've been the evolution of complex stomachs that made the difference. There is no credible evidence that the reptiles ever had one, or practiced bacterial fermentation (although there is one bird, Hoatzin, that does it), so what came out was probably not too good. Perhaps it is the revolution in digestion rather than cereal grasses on steppes as such that was the driving force for the sudden popularity of coprophagy. Does this make sense?