After many false starts, breaks, and dead ends, I'm finally nearing the completion of at least one chapter of the book. I still am not entirely sure how I'm going to end it and I still have some details to fill out, but there appears to be a good chance that I'm going to finish the chapter on whale evolution on time.
Reading The Emergence of Whales has provided me with more interesting examples of the evolution of systems and adaptations, allowing me to do more than just retell the same old story. There is much more detail than I can fit in but I think it's important to relate the particulars of whale evolution rather than just point to the skeletons, and I'm doing what I can to include ichthyosaurs, swimming giant sloths, and other animals for a broader perspective on tetrapods that returned to the sea.
I've been particularly interested in the reversal of bone density in whale evolution. Some of the earliest known archaeocetes like Pakicetus already had dense, brittle bones that better suited them to life in the shallows than running after prey on land. This trend was continued in younger relatives but by about 10 million years after Pakicetus the fully aquatic whales like Zygorhiza had bones that were more osteoporotic. If you're a deep diver having a heavy skeleton is not advantageous if you need to return to the surface to breathe (especially after expending a lot of energy in a dive) and it's relatively clear how natural selection would have favored a reversal in the trend towards bones as ballast. Ontogeny and the regulation of the cells that lay down and resorb bone are probably the systems by which such changes were affected, but even if such mechanisms still await research the overall trend is still stunning.
Out of all the evolutionary transitions I document in the book there are perhaps none so spectacular as the evolution of whales, and I certainly don't want to shortchange my readers by skipping over interesting details. I still have plenty of work to do (I need to spend some more time on Ambulocetus, Rodhocetus, etc.) but I do feel like there's a good chance I'll be wrapping this chapter up soon. Here's a Wordle derived from what I've written so far;
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Congratulations, and whatever you do, don't stop now! :-)
"If you're a deep diver having a heavy skeleton is not advantageous if you need to return to the surface to breathe (especially after expending a lot of energy in a dive)"
Diving down (speed sinking) takes little energy (some phocids apparently snooze on the way down, so it's possible that odontocetes employ unihemispheric sleep as well on long dives), it's the searching-chasing and rising that requires energy.
Dense bone offsets the (buoyant) thermoinsulative blubber, deep divers are proportionately larger than kin and have proportionately thinner blubber, so need less dense bones.