Oldest known* not first. If this was the first then you're saying there was a major jump from single cellular to billions of cellular animals. Oh accuracy of headlines...
See https://www.smithsonianmag.com/smart-news/one-oldest-known-a... for a fossil of an arguable animal that at 600 million years predates this one. In fact we have evidence that the sponge/other animals split was one of the early taxonomic splits to happen, so it seems likely to me that Dickinsonia could be an ancestor of every animal EXCEPT sponges. Which may well have naturally developed from colony organisms like Stromatolites. (Which in turn date back over 3 billion years.)
As far as I know, Animalia currently only includes multicellular organisms by definition, but maybe different definitions are used for the distant past.
I don't know about the definition you're using, but taxonomically speaking there is at least one single-celled organism that's indisputably part of (as in descended from) Animalia: Canis lupus incertae sedis[1][2], a infectious disease that evolved from dogs[3]. (On further wiki-walking, there also appear to be similar diseases evolved from devils, hamsters, and clams[4].)
There are also choanoflagellates[5], which might have reverted to single-celled life after a stint as multicellular, or might not, but are in any case more closely related than the opisthokont[6] common ancestor of animals and fungi. (These are rather more relevant to the context of "were there single-celled animal ancestors?". If opisthokonts already formed differentiated clonal colonies, then there was no point in the main [modern normal animals] lineage that was defensibly already animal but not yet multicellular. Apoikozoa[7] allegedly formed colonies, but it's not clear to what extent they were differentiated and/or obligate.)
I wonder if one day we will have technology that does a comprehensive scan for fossils without having to dig them up. Right now it's surprising that they find anything considering how manual and hit and miss the process is. i can't even imagine how much good stuff we will never find with current methods.
Wouldn't the world's first animal be prehistoric by default? I mean, I assume the priority of any first animal on any planet is not going to be scribbling things down for posterity.
If we allow that humans and other extant creatures are not prehistoric, then it is possible (though not at all likely) that the first animal is one that is still with us and therefore not prehistoric.
I had the same sensation, but the adjective prehistoric is not applied to the animal here. If I'm not wrong could be rephrased as "a pancake-shaped inhabitant of a prehistoric ocean".
