biology help :)?

That would depend on the phylum. The extinction of phylum Cycliophora, for example, might mean only that some other organisms would be able to attach themselves to certain lobsters.

https://simple.wikipedia.org/wiki/Cycliophora

The extinction of phylum Chordata might mean that there would be no "large terrestrial animals" for a couple of hundred million years.

Similarly events have happened. It would enable other species to occupy the ecological niches previously occupied by the now extinct animals. They probably did not occupy them previously because the animals just gone extinct out competed them in those niches. I may enable animal taxa not represented by large numbers of species to radiate and become both a more common and dominant group. At the time of the dinosaur extinction about 65 mya mammals did exactly that.

Other phyla of animals will evolve to exploit the empty niches left by the extinct phyla. Not all niches will be filled but scientists call this sort of event an adaptive radiation, which often happens after a mass extinction. For example, mammals really evolved into quite a wide variety after the dinosaurs were wiped out. Dinosaurs in turn evolved to fill the niches left open by the extinction of most of the pelycosaurs (our ancestor was a pelycosaur), the dominant reptiles at the time, at the end of the Permian.