We (collectively) have made many assumptions concerning certain "design" characteristics of animals and humans. Words like "vestigial," "junk," and other connotations, such as "meat eater," etc., have often proven wrong once the issues at hand are more fully understood.
I am not saying that such is true for T-rex, but I am saying that we probably don't know God's original design parameter before Genesis 3, but it was likely not a meat eater. Other large reptiles (and we do know that reptiles continue to grow -- seemingly without limits -- as they age and with long life spans of early creation, many critters that we have specially called dinosaurs may just have been largely grown reptiles of one sort or another) are demonstrably plant eaters and the roundish peg-type teeth of many of the reptiles purportedly meat eaters work equally well to strip leaves from trees. In fact, the lack of forward appendages (stunted) may be a greater call to tree stripping behavior than meat harvesting, which would have been cumbersome at the very least.
Oh, and how do we know the coloration and more so, the noise these critters made?
But we do know what every creature ate prior to the flood.
Genesis 1: 29 And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, in the which is the fruit of a tree yielding seed; to you it shall be for meat.
30 And to every beast of the earth, and to every fowl of the air, and to every thing that creepeth upon the earth, wherein there is life, I have given every green herb for meat: and it was so.
31 And God saw every thing that he had made, and, behold, it was very good. And the evening and the morning were the sixth day.
Then after the flood God commanded Noah in
Genesis 9:1 And God blessed Noah and his sons, and said unto them, Be fruitful, and multiply, and replenish the earth.
2 And the fear of you and the dread of you shall be upon every beast of the earth, and upon every fowl of the air, upon all that moveth upon the earth, and upon all the fishes of the sea; into your hand are they delivered.
3 Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things.
This verse explains a lot to us pre-flood the animals had no reason to fear mankind and so they readily came with Noah into the Ark. Pre-flood man had no fear of the animals for they ate the herbs and fruit from trees and plants. After the judgement of sin at the flood this changed. The only animals that died pre-flood were those offered in sacrifice to God as we see in
Genesis 4:3And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
4And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering: