Did stem tetrapods have tadpoles?

 So I saw some sensational headlines declaring that it turns out, stem tetrapods didn’t have tadpoles, despite that long being considered the case, so of course I read the article, because I know popular science news is often overstated, and then also did a little supplementary wiki-diving. I did not, however, actually read the original paper, which would be the proper thing to do in this situation to get an accurate read on how serious this result really is. I think it could be be fun to be a science communicator, but I’m not one, I’m not trying to be one, these blog posts are just for writing practice, so I don’t feel compelled to raise the level of my research to truly reliable levels just because I want to write about it to an audience of zero for a bit. All of that is to say, this is just my opinion, man, of a layperson on a subject I have read a lot about but have no formal education in, based on some popular science coverage of the research but without actually delving into the research itself, so you shouldn’t trust anything I say to be reliable, but like… this feels like a nothing burger.

I mean, not really, it actually sounds like super interesting. Basically, they found some baby early tetrapods called embolomeres, and they were just miniature versions of adults, not tadpoles, and previously scientists had assumed that early tetrapods had tadpoles, like amphibians. That sounds super important when phrased like that, and even moreso when phrased in breathless “This changes everything we thought we knew about early tetrapod development!” language being used in the news, but I think some context is necessary why this is probably not (if my read on it is right, see my qualifications above for why it might not be) that big a deal but why its still super cool.

First, why did scientists assume that early tetrapods had tadpoles? There’s actually a lot of reasons for it, some scientifically rigorous, others less so, but I’ll just sum up the most relevant and scientific reasons to think they might have: modern amphibians have tadpoles, and are have thought to evolved from fish ancestors, and most fish also have a stage known as larva where they have significantly different bodyplans, essentially a tadpole stage, and thus its been assumed that amphibian tadpole stages are the same as fish larval stages, developmentally. If modern fish and modern amphibians both have “tadpole” stages, then we would expect their most recent common ancestor to have had one as well. It is also thought that reptiles and mammals evolved from amphibian-like early tetrapods, and who would likely have the same most recent common ancestor with modern fish that modern amphibians do, which means that at least SOME ancestor of reptiles and mammals probably had a tadpole stage.

That was the logic, however these early tetrapods, the embolomeres, appear to lack one, since we found small babies that just look like small adults, not completely different like a tadpole would. So theory disproven, no tadpoles, right? Well, there are some issues to jumping to that conclusion. One, although classification of early tetrapods is no easy task, its generally thought that embolomeres are more closely related to reptiles (and mammals) than they are to modern amphibians. Which means they likely evolved after the split between the last common ancestor of amphibians and reptiles. So what this would actually tell is is that the ancient tetrapods who would eventually evolve into reptiles had already lost their tadpole stage at this point, whereas the ancient tetrapods who eventually would become modern amphibians retained it, and our most recent common ancestor with amphibians would still have had tadpoles, right? That is closer to what this means, but even that is probably an overstatement. Embolomeres are generally thought to have been an evolutionary dead end, with no living descendants, which means that just because they had lost a tadpole stage at this point, does not necessarily mean our ancestors did - although it does make it significantly more likely. Thus, this research probably doesn’t mean our ancestors never had tadpoles, nor does it necessarily mean they stopped having tadpoles earlier than we thought, but it does mean that is far more likely (and will probably become the scientific consensus, especially if we find more corroborating evidence).

The headlines, though, all seem to be making a stronger claim, that this means that all early tetrapods did not have tadpoles (and by extension, none of our tetrapod ancestors did). This would only be true if amphibians evolved their tadpoles stage after they split off from other tetrapods, which would also require that the amphibians tadpole stage is likely developmentally different in some way than the very similar fish larval stage. This is where my lack of scientific expertise really shows, because I bet if I was a herpetologist or ichthyologist I could tell you off the top of my head whether that could be possible, but honestly I have no idea. Its very possible we understand the two stages well enough that we can be certain they are the same and that amphibians could not have evolved it independently, which would mean our tetrapod ancestors must have had a tadpole stage unless everything we know about tetrapod evolution is wrong. But maybe the similarity is not that certain and fish larval stages and amphibian tadpole stages could be convergent or coincidentally similar, amphibians evolved them secondarily, and tetrapods really did all start with direct development. I think thats highly unlikely and I doubt it will become the scientific consensus unless I massively am misunderstanding something (see my qualifications above for why thats entirely possible) but I certainly cannot rule it out, and that is what the headlines are implying.

Its also important to note that technically, it could also be this species just transformed from its tadpoles at a small size and then continued growing, and that this discovery actually means nothing. We probably could only rule that out if the fossils were clearly in eggs, and I think embolomeres are not amniotes (reptiles and mammals) and thus did not have shelled eggs.

Still, even simply the idea that early tetrapods on the way to becoming reptiles and mammals lost their tadpole stage very early after splitting from amphibians is an interesting implication, so I think this is really cool. Plus, it’s rare to find fossils this small anyway. Basically, still cool research, but the popular coverage also seems quite overblown to me. But maybe I’m wrong, I’ll wait and see what the science communicators I trust have to say about it.