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"Massive galaxies that stopped forming stars (known as massive quiescent galaxies) are plausible progenitors of giant elliptical galaxies."

Could someone explain a bit more what this means? Is it that those galaxies were "born" out of the quiet galaxies, or shaped into ellipses because of them?

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Not an expert, but here is my understanding: Elliptical galaxies are typically quiescent (and spiral galaxies aren’t). The “massive” quiescent galaxies observed in the early universe are however smaller than the “giant” elliptical galaxies observed in the more recent universe. It is assumed that the latter evolved from the former, most likely by merging with other galaxies. This question of how exactly they evolve(d) is the subject of continued research.

More generally, see also https://en.wikipedia.org/wiki/Galaxy_formation_and_evolution.

A "progenitor" is an ancestor; it means that the quiescent galaxy "plausibly" turns into an elliptical galaxy.

In this case precursor might be more precise than ancestor.

Agreed; I knew there was something wrong with "ancestor" when I typed it.

[Edit] A progenitor is literally an ancestor; I answered GP's question. It's "progenitor" that should instead be "precursor", in the featured article.

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