Sort of. It's the first real mass extinction with animals -- that is the end Ediacaran extinction. The Ediacaran actually lost a lot of its weirder life before the end of that period and then more animal-like fossils started dominating. From what I remember(and I can't remember the name of that boundary) it's correlated with evidence of animal predation(which used to be believed to have started in the Cambrian but it likely predates both for at least microscopic animals). Namely they found evidence of bore holes and in biomineralized remnants(shells) and also the very presence of biomineralization suggests predation.
The Ediacaran was believed to have had its mass extinction because of the ancestors to the bilaterian lineages that would dominate the Cambrian though. The idea being that the very worm-like bilaterians were consuming and altering the dominant habitat at the time, microbial mat biomes. These biomes were great for anchoring to for all of the sessile organisms(like many
Rangeomorphs, which had "fractal symmetry".) and it was also a major food source for other organisms like
Dickinsonia which has trace fossils showing that it was feeding on the mats through some method with its ventral(belly) side.
You can see in this fossil the outlines on the fossilized microbial mat and then the actual fossilized specimen who left them. The mechanism for this theoretical extinction event though is called the
Cambrian Substrate Revolution and beyond just destroying the habitat of the more simplistic organisms of the Ediacaran it also drastically altered the dynamics of the oceans to allow for more nutrients into the water column which made more active animal life possible.
Oxygen levels went way down way way down. I think that was before trees right?
Yeah trees first showed up, to what's found at least, around the Devonian(before the Carboniferous which is where you're thinking of with the giant insects and sky-high oxygen levels, the Devonian is also called "The Age of Fish" because it saw the dominance of fishes in the food chain and their rise in diversity) and there's not too much reason to doubt that. There were precursors to trees on land before that that are a mystery to paleontologists and have confounded them for over a century now in
Prototaxites(means "first yew") and until more recently it was widely accepted that they were giant mushrooms, more or less(there were debates on if they were giant lichen as well but isotopic analyses poked holes into the idea that they were properly photosynthesizing like lichen can do with their symbiotic algae). Prototaxites actually got really big for its time and I have my own theory for it, but I'll save that for another time and preferably some place its better put threadwise.
Funnily enough though the colonization of land by the first plants is related to oxygen levels dropping in the oceans. The theory for this is that the mosses and lichens that were colonizing land at the time rapidly sped up erosion of various minerals and because the land was largely just rocky, sandy and barren all of those minerals just continuously poured into the oceans and led to massive algal blooms like
red tide today. I think this was for the Ordovician? I can't properly remember, but the Ordovician is after the Cambrian. For a modern example to compare it to, look at the Sahara desert and how sands blown from it
fertilize the Amazon and the southern Atlantic ocean. Just imagine that on a larger scale.
The major extinction event that saw oxygen levels plummet both in the oceans and in the air was the Permian, or "The Great Dying". That was after trees though and it was the prelude to the rise of the dinosaurs and their relatives. The low oxygen conditions actually helped to select for adaptations in their larger group that helped give dinosaurs the edge(like better hearts and respiratory systems to cope with the low oxygen levels.). Generally though a lot of the mass extinction events for the oceans see ocean oxygen levels drop from what I remember. Not all of them though, but quite a lot.
Isn't oxygen technically bad for the earth too? Like I know we need some to breathe but it destroys so much just by being into contact with it.
Yeah, high oxygen levels lead to extreme wildfires(which you find in the fossil record in the form of large charcoal deposits. This is evidence of extremely hot forest fires.) and also it's directly harmful to animal life. One of the reasons it was believed that insects got so large was actually to cope with oxygen levels, as from what I remember insect young raised in high oxygen environments have a lower survival rate for the smaller young. The high oxygen levels lead to oxidative stresses that are harder to deal with for tiny insects due to the square-cube law. Less volume to a higher ratio of surface area. The same thing applies to us, fungi, plants and microbes too.
. The leading idea for why this is is actually to alert predatory insects to eat whatever is eating them. You see something similar in the deep sea with bioluminescence being used also as an alarm to attract predators. Here's a wonderful video showing some of that off and talking about it too(the jellyfish at the start of the video are doing those alarm lights).
