"… the text of Genesis itself would suggest that, while man was meant to be immortal, animals were not. … giving “every green plant” to animals as food does not mean that some of them weren’t also carnivores. It’s not as if, before original sin, lions ate dandelions and toadstools and only afterward did they begin picking on poor old wildebeest. This is something Thomas Aquinas wrote about in his Summa Theologica:
In the opinion of some, those animals which now are fierce and kill others, would, in that state, have been tame, not only in regard to man, but also in regard to other animals. But this is quite unreasonable. For the nature of animals was not changed by man’s sin, as if those whose nature now it is to devour the flesh of others, would then have lived on herbs, as the lion and falcon.
(We should add, lest anyone be tempted to think that this is a forced retreat in the face of modern evolutionary theory, that Thomas wrote these words nearly 550 years prior to the birth of Charles Darwin.)"
From “Was There Death Before the Fall?”
And as far as human death:
“According to the well-known Augustinian consideration, at the beginning of human history, God gave men and women, despite their being creatures, the “capacity not to die” (posse non mori, cf. De Genesi ad litteram, VI, 36,25: CSEL 28,197): resurrection (or a state of union with God no longer reversible) would have represented the immediate crowning achievement of each historical life, without the interruption of death. This does not mean that physiological laws would have been invalidated, and that human beings would have continued their earthy life indefinitely: what is promised as achievement of the universal history in a redeeming history marked by sin, could have been the achievement of each individual life if there had been no sin (cf. Schmaus, 1953)…”