Did you see the siphonophore?

https://boingboing.net/2020/04/16/giant-string-like-creature-com.html

Hate to take out the red pen on you hank, but as a teacher with no class: it's the Cambrian Explosion.

Stromatolites were more microbial mats (mostly cyanobacteria) than anything as organized as a sponge. But both filter feeders, sooo...

Been reading some new research on Snowball Earth (cnn -long story here*, I know this outcrop and one of the authors) and I personally think that this is what kept multicellular life from exploding far earlier in earth's history. But it could have been the terraforming of the atmosphere done by early lifeforms that could have caused the freezing feedback loop.

But yes, I do think there is this slow but sure way of creating multicellular life and higher life forms by increasing the cooperation of individuals. (competition too, but it's a balance.) I was fascinated by an idea I heard years ago about how the first cells formed. The question was how did these proteins and amino acids manage to create a sphere to contain themselves. The hypothesis was they started out in two dimensions, not three, organizing between the cleavages (crystallographic layers) of clay minerals. They didn't need to create a whole sphere, just a fence. Eventually, these fences shrunk in area until the surface tension led to creating spheres.

Finally, I'm not real convinced by the tiny leaf fossil shown at the link. I'd have to see the cited paper or others describing these rocks.

The final sentence of the abstract is always the big take-away. Even a billion years a hell of a long time ago on a tectonically active planet.

Thanks for the link.

* I wrote a big long post about this article, how it doesn't really get into what the scientific paper is about, but lost it to a bad keystroke. ARRRRgggg