Maybe saurians were like hippos, and spent most of their lives in the water where fossils tend to break up faster. Occasionally, they might have to travel to new sources of water, especially during droughts, and they would be more vulnerable to predators and stress and more likely to leave fossils. This is a lot like hippo behavior, and I suspect their fossil distribution in the future might show similarity to saurian bone patterns and sedimentology.

There is some physiological evidence the saurians were not warm blooded like the raptors, and an aquatic existence would offer more opportunities for temperature regulation than a terrestrial existence.

I must rely on the experts, of course, but they've been wrong before by their own admission on saurian habitat and ecology. My instinct tells me evolution would not result in a saurian body design for a completely terrestrial animal. Today, elephants are clumsy because of their size, the big thunder lizards would only be much more so.