Originally answered by me on Quora.
Here is an interesting thought experiment. Before the answer, here is the detailed question:
It seems that rotifers are hardly ever found as fossils: the only finds are more recent than the death of the dinosaurs.
But there is little doubt they are a very ancient group and probably around in the Cambrian, since their genetic and evolutionary links seem to be to phyla already found there.
The common assumption is that rotifers were once the normal size for multicellular animal and then shrunk. But if they are a rare leftover from an era where all multicellular animas were that size, this would explain why they are not found before the Cambrian.
Is this plausible?
It’s certainly plausible that there is a hidden diversity of Precambrian multicellular animals, the fossil record can be fickle like that. There is a model of the Cambrian Radiation that postulates just such a “long fuse” of cryptic diversity before the appearance of the larger animals known from Chengjiang, the Burgess Shale, and other Cambrian localities.
The Cambrian is very unique taphonomically – no other time period shows such a variety of exceptional preservation at all size ranges. The picture above comes from a recent paper, and it shows preserved crustacean mouthparts from 510 Ma. The scale bar is: 60 µm (A, B), 15 µm (A’), 100 µm (C, D), 25 µm (C’), 35 µm (D’). In other words, super-tiny, certainly in the range of fossilisable rotifer trophi (“jaws”, let’s say they’re 20 µm on average).
The Ediacaran Doushantuo Formation preserves acritarchs at a cellular level – and these aren’t foraminiferal-like organisms with a calcified shell, these are completely organic. The above (scale bar 10 µm) are probably algal cysts, but other larger ones like Megasphaera have been convincingly (and controversially) interpreted as animal embryos.
In other words, organisms that normally would never fossilise under any circumstance get fossilised in Ediacaran and Cambrian localities. That’s both good news and bad news for your hypothesis. The bad news is that nothing rotifer-like has been found. The good news is that now, there is a lot of interest in this microdiversity, thanks to better research methods and findings like Megasphaera and the crustacean mouthparts, which means that at any time, a rotifer, or at least a stem-group rotifer, could be found in the Cambrian. It’s certainly not a possibility that can be dismissed out of hand.
However, there are some other lines of evidence that suggest it’s not so likely that rotifers back then were tiny, namely evidence from other taxa.
Recent entoprocts (relatives of bryozoans) have a maximal size of 7 mm, most are around sub-1mm in my experience. The earliest suggested stem-entoproct is Cotyledion tylodes from Chengjiang (520 Ma), pictured above (source). It’s comparatively enormous.
But okay, many entoprocts are visible to the naked eye, not fair. So I’ll go to the Loricifera, also pretty much unfossilisable animals. Above is a typical loriciferan, around 100 µm, so in the size range of a rotifer (source).
And below is a 3D reconstruction of the proposed Cambrian stem-group counterpart, Sirilorica from the Sirius Passett, Greenland (518 Ma), last redescribed here.
The lorica alone of this animal was on average 5 mm, with fragmentary fossils suggesting the animal could grow up to 70 mm.
Of course, the evolutionary history of animals is riddled with exceptions, and it could well be that rotifers are one of them (or that the two examples I used are the exceptions!). That’s just a matter of more research – I’ve seen deacdes-old unopened crates of Cambrian material just waiting to be examined, and I bet each of those crates has at least one unwritten Nature paper inside it. Maybe the elusive stem-rotifer is hiding in there.
There is also the possibility that we shouldn’t expect rotifers in the Cambrian. That section of animal phylogeny is an unresolved mess – rotifers aren’t even an accepted group anymore (Syndermata is the right name), the precise relationship between “rotifers”, Acanthocephala, and Gnathostomulida still needs to be cleared up, and where these guys belong also has little consensus (Platyzoa? Gnathifera?).
Without any clarity here, inferring their ancestry is impossible, and this is made significantly harder than impossible (!) by none of the groups around the rotifers having a noteworthy fossil record, Cambrian or otherwise.
So, to sum up: it’s a plausible hypothesis, but for now, it’s not something that can be studied with any reliability. We need either a decently-resolved phylogeny (non-existent) or an early fossil record of any rotifer relative (doesn’t even have to be a rotifer at this point). If rotifers were around back then, we’ll get a fossil eventually. Evidence from other taxa says it’s unlikely that the hypothesis is true, but that’s certainly not a reason to forget about it.