Or did they they ate herbivors of a different order/phylum? (earlier)
Pretty much.
As the article indicates the immediate pre-dinosaurs which gave rise to the "ruling reptiles" were small agile bipeds.
However once some dinos started to rely on vegwtation, the advantage is less to agility (as your food can'y run) and more to size both from a food requirement/body mass ratio, and more imnportantly, it held a browasing animal to be at least as tall as it's food. But once sixe increases, bipedalism becomes less tenable./
The quick solution is the quadraped Jurassic brontasaurs. the longer term Creataceous solution is modification of the hips to allow bipedalism in larger sizes.
And to complicate things the small agile terrestrial Triassic bipeds also were ancestral to the crocidilians - aquatic environment requires swimming which encourages return to the quadripedal semi-sprawling danve as a by-product.
Evolution is opportunistic rather than planned.
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