W.H. Flower's Ungulate -> Cetacean hypothesis

I do not have much time to write today, so rather than type something from scratch I have decided to share an excerpt from my book (still in-progress).

In my recent post "Ancient Armored Whales" I briefly drew attention to a quote from Richard Lydekker deriding William Flower's hypothesis that whales may have evolved from ungulates. Presented below in the passage on this subject as it presently appears in the chapter "As Monstrous as a Whale";

A lack of other transitional forms had stirred debate about the place of Basilosaurus among mammals (the biologist D'Arcy Thompson, for example, thought it was more closely related to seals than to whales), but the anatomist William Henry Flower was certain it was relevant to whale evolution. In an 1883 lecture reviewing what was known about whales at the time Flower made it clear that whales had evolved from terrestrial ancestors. Even among living meat-eating mammals there was a continuum of increasing adaptation to the water, and it was not unreasonable to suppose that land-dwelling carnivores had gone through a seal-like stage before evolving into something like Basilosaurus.

Yet this hypothetical sequence was not without its problems. Flower noted that seals and sea lions used their limbs to propel themselves through the water while whales lost their hind limbs and swam by oscillations of their tail. He could not imagine that early cetaceans used their limbs to swim and then switched to just using their tails at some later point. Semi-aquatic mammals like otters and beavers provided a better alternative model for the earliest terrestrial ancestors of whales, and Flower suggested that the large, broad tails of such hypothetical creatures caused them to evolve a mode of swimming different from the pinnipeds. This ran counter to the popular idea that whales had evolved from creatures resembling modern mammalian carnivores, but Flower thought he had found a more appropriate stock from which to derive cetaceans. Ungulates, or hoofed mammals, closely resembled whales in parts of their skeletal anatomy, and to Flower the skull of Basilosaurus had more in common with those of ancient "pig-like Ungulates" than seals. On this connection he mused;

Though there is, perhaps, generally more error than truth in popular ideas on natural history, I cannot help thinking that some insight has been shown in the common names attached to one of the most familiar of Cetaceans by those whose opportunities of knowing its nature have been greatest - "Sea-Hog," "Sea-Pig," or "Herring-Hog" of our fishermen, Meerschwein of the Germans, corrupted into the French "Marsouin," and also "Porcpoisson," shortened into "Porpoise."

If ancient omnivorous ungulates could eventually be found, Flower reasoned, there could be little objection to whales being derived from them. Flower hastened to add that his ideas were still speculative, but he ended his lecture by envisioning the unknown cetacean ancestor treading into the shallows;

We may conclude by picturing to ourselves some primitive generalized, marsh-haunting animals with scanty covering of hair like the modern hippopotamus, but with broad, swimming tails and short limbs, omnivorous in their mode of feeding, probably combining water plants with mussels, worms, and freshwater crustaceans, gradually becoming more and more adapted to fill the void place ready for them on the aquatic side of the borderland on which they dwelt, and so by degree being modified into dolphin-like creatures inhabiting lakes and rivers, and ultimately finding their way into the ocean.

The bones of such a creature remained elusive. By the turn of the 20th century the oldest fossil whales were still represented by Basilosaurus, Dorudon, and Protocetus, all of which were fully aquatic. As E.D. Cope admitted in an 1890 review of whales, "The order Cetacea is one of those of whose origin we have no definite knowledge." This state of affairs continued for decades, and by the 1960's the yawning gaps in the cetacean fossil record were almost too much to bear. Were there no animals that showed even the slightest similarity to the oldest whales?

[I do not wish to give creationists false hope. We presently have a wonderful collection of fossils detailing the evolution of whales from terrestrial ancestors and their adaptive radiation during that transition. There is still more to learn, but the gaps that puzzled Flower, Cope, and other naturalists have been filled in by an abundance of strange creatures.]