It’s a tricky business and ScienceAlert offers some famous examples, including Dickinsonia:
Dickinsonia has had one long identity crisis since it was first described in 1947. It hails from the Ediacaran era, prior to the Cambrian, before the emergence of the major phyla we know today. The Ediacaran biota were mostly soft-bodied organisms, and very few of them resemble any living or extinct organism. So they’ve been very hard to contextualize.
Dickinsonia looks a lot like a strangely ribbed oval, and it could be anything, really. On its discovery, it was classified as a type of jellyfish. Scientists have also thought it could be a worm, a polyp, or a mushroom or lichen. It’s even been proposed that Dickinsonia belonged to some unknown, extinct kingdom that was neither animal, plant, nor fungus.
A study a few years ago into the way the organism grew seems to have solved it. According to the scientists’ analysis, Dickinsonia is an animal, belonging either to Placazoa, which are among the simplest of animal organisms, or Eumetazoa, which are a step up from sponges.Michelle Starr, “The Famous Fossils Scientists Got Incredibly Wrong” at ScienceAlert (December 28, 2021)
Gunter Bechly would take issue with the contention that Dickinsonia is an animal. See: Gunter Bechly: Dickinsonia Is NOT Likely An Animal Dickinsonia does not seem to have the bilateral symmetry of an animal. Also, some life forms other than animals produce cholesterol, Bechly says.
Well, if Dickinsonia doesn’t have bilateral symmetry, maybe “animal” is a flag of convenience? Call it an animal and close the file? These stories remind us why dogmatism is not an asset in science.