Sponges
Heard the talk, from a colleague of mine. He's as dumb-funded as anyone. He's not sure they're sponges, but they're "sponge-like" in some things (body shape, mostly) but they are also hard and rigid and sup[port 1 km-high near-vertical subsea cliffs at "reef' edges. They need to be hard and strong to do that.
Nobody knows what they are, what modern analogues they have (if any - there's an argument based on carbon isotope data that they are weird early lifeforms with 'different" metabolism that is long-extinct)
The world is more strange than you, or I, can imagine, Horatio...