First, I’ll note that, at this rate, there is absolutely no way I’ll be able to keep up with all these posts! Too much time spent in class, lab, meetings with students, commuting, etc. But I’ll do my best to answer questions as I have time.
Second, I’ll note that the Maiacetus paper is open access and available here. Many of the specific questions you might have about this particular fossil can probably be answered there.
Now to @swamidass’s questions first…
Before you answer, it might be worth explaining to everyone why this is consider an important step in the evolution of mammals. This has to do with fully aquatic births right? Breech birth means that the newborn’s nostrils and lungs are not exposed to the sea till the very last second, when the mother can help the little one to the air. Note, of course, that whales are not mouth breathers, because their esophagus is not connected to their windpipe. Do we even know this is the case for Maiacetus too? I digress. Did I get that explanation right?
There are multiple possible explanations for why tail-first presentation is ubiquitous in cetaceans, but reducing the possibility of drowning during birth is among the most plausible reasons. (Gingerich et al. discuss some of these possibilities briefly on p. 15 of the paper.) Head-first presentation is pretty much universal in land mammals, so this implies that at some point, early cetaceans made the switch to tail-first delivery.
My first question concerns this. Can you help me understand why this is a valid inference? It seems that the chances of seeing a pregnant Maiacetus are very, very low. I wonder if the only reason we see this fossil is because there was something wrong with the pregnancy, say, that the little one was oriented in correctly. This would explain why we even see this fossil. I am filling some details here, assuming that pregnant Maiacetus fossils are rare. I also do not know nearly enough to speak confidently here. This is really an opportunity for us to probe how paleontologists think through questions like. How do you know that this is really the normal orientation of delivery for Maiacetus ?
The inference that this is a near-term fetus positioned for headfirst delivery is certainly something that one could contest. In fact, Hans Thewissen (another whale paleontologist) posted a comment on the PLoS ONE article shortly after publication, arguing that this was in fact the whale’s last meal rather than its fetus. I know that Gingerich et al. considered this, but as they describe in the article (p. 10), they think it is more likely to be a fetus based on the preservation of the skull. I think you’re fine in assuming that finding pregnant fossils is not very common, but it’s not super rare either. I could cite many examples of fossils that are clearly pregnant (including live-bearing marine reptiles from the time of dinosaurs and also fossil horses). Given that there are only two fossil specimens known for Maiacetus, only one of which was possibly pregnant, it is difficult to infer whether or not this would have been typical or atypical presentation. But given its overall anatomy, its presumed terrestrial competence, and the universality cephalic presentation in land mammals, headfirst delivery seems like a reasonable interpretation.
Ok, I’ve gotta hit the road for my commute. I’ll resume writing when I can!