This is an excellent short manuscript that is in my opinion important for several reasons. First, it allows for the detection of a Southern Hemisphere signal for body size evolution in mysticetes that went completely overlooked until now, despite the study of quite a number of southern localities. Second, it stresses the fact that even very fragmentary specimens have the potential to contribute significantly to the advance of this topic. I am sure that the publication of this work will make us look even more carefully at fossils from our collections, and I applaud the efforts of the authors for extracting as much as possible information from such a poorly attractive fossil. Third, together with other recent works, this manuscript points to the huge potential of austral fossils to the study of the evolution of marine mammals. There is still a lot to be learnt from Southern Hemisphere localities, and this ms is an important contribution in that direction.

The text is perfectly written, finely organized, and well illustrated. I could not find any major issue in this work, and only list below some suggestions and comments that the authors should feel free to use or not:

• The body length estimates compiled in the highly informative figure 2a are obtained using different proxies, I guess. The method you use to estimate body length for the Australian specimen takes account of mandibular shape variation among extant mysticetes, and thus covers most (though probably not all) of the variation among crown Chaeomysticeti. What about stem Chaeomysticeti? Wouldn't it be useful to take the mandibular measurements proposed here to estimate body length in a few stem chaeomysticetes, and then compare the numbers to estimates obtained with other proxys (e.g. bizygomatic width)? This may allow you to test if the method based on the mandible tip dimensions yields results that are comparable to other methods, not only in extant mysticetes, but also in the stem group. Only a suggestion.

• You convincingly conclude that a collection bias against Southern Hemisphere localities led to a size evolution signal that is not representative of the genuine pattern. Don't you think that another bias could occur among Southern Hemisphere localities, namely the under-sampling of localities representing a more open oceanic assemblage? This has been evoked in previous works and may also be briefly mentioned in the discussion. Again, only a suggestion.

• Proposing that mysticetes grew larger in the Southern Hemisphere means that there were barriers to their dispersion towards northern waters at that time. You briefly evoke regional productivity differences as a potential driver in this scenario. Is there any other example of size-related distribution pattern (in the past or present) that could be mentioned here? And/or did you note a higher degree of endemism for certain groups of marine mammals in the past, maybe related to other oceanographic features? We all know that many extant mysticetes (especially giant species) migrate between high productivity areas and breeding grounds, meaning that higher regional productivity may not necessarily be the sole driver for geographic distribution. Just some thoughts!

• You very rightly discuss the role of baleen whales as ecosystem engineers, especially for nutrient transfer from feeding regions to the surface, and the giant size of part of them is a fine justification for the focus on mysticetes (and not cetaceans as a whole). As a perspective, one may find interesting to cite the potential impact of large odontocetes as ecosystem engineers. Indeed, the fact that they use different feeding strategies probably does not prevent them for being considered as major actors. Just think about deep diving sperm whales, beaked whales and large delphinids, which reach body lengths not too far from the specimen described here. I would not be surprised if a Southern Hemisphere signal remained similarly hidden due to the lack of data.

• I made a series of additional comments in the main text and supplementary information documents, but this is only for very minor issues. I suspect that an inversion between Southern and Northern Hemispheres occurred in the caption of figure S4. Probably to be checked.

O Lambert