Hello, I have encountered a lot of these unusual structures while looking for Middle Triassic conodonts. The sample was dissolved in formic acid and has yielded conodonts, agglutinated foraminifera and fish teeth.

Could these perhaps be pieces of a sponge skeleton, or are they more likely pieces of plastic foam or something similar? Fused sponge skeletons seem to have more rounded edges, an these seem to have pronounced angles. Sponges also seem to rarely form pentagonal meshes...

I washed the sample pieces before dissolving them, but I wouldn't be surprised something got stuck in the cracks and nooks (e.g., there was a very small insect leg part in one of the fractions). I tested if these structures sink in water; they do, but very slowly. I am aware that some plastic does, however, sink in water.

Could anyone more experienced with micropaleontology help?