Pre-whales, like Maiacetus (Fig. 1), started swimming by converting terrestrial bipedal hopping, as in Leptictidum, (Figs. 1,2) to simultaneous hind limb paddling which promoted lumbar flexion and extension with the large tail snaking up and down in waves as in living whales. Here’s how it happened in a bit more detail:
- Leptictidium was a bipedal tenrec that nests as a sister to land (stem) whales and aquatic whales (Fig.3).
- On land, Leptictidium might have hopped, like a kangaroo. Or maybe not.
- In water, Leptictidium let its legs extend backward as it undulated its supple spine and greatly elongated robust tail (Fig 1) in the manner of sea otter (see video). Of course all four legs contributed to propulsion and steering.
- Over time, the longer tail and lumbar region became more efficient, reducing the need for drag-inducing hind legs. The pelvic connection was already loose at this stage. So were the leg joints.
- Leptictidium was preserved with a complete pelage of thick fur. That disappears, of course, in whales.
Current and traditional thinking from Wikipedia:
“Leptictidium (Early Eocene, 50 mya) is on a short list of bipedal mammals. Combination of primitive eutherian traits (prepubic bones) with derived adaptations such as the powerful hind limbs and long tail. Adapted to a forest ecosystem, Leptictidium is considered omnivorous and its lineage became extinct 35 mya.
“The ankles and the sacroiliac joint were quite loosely fixed, while the pelvis had a flexible joint with only one coccygeal vertebra. The anteorbital muscle fenestrae in their crania suggest they probably had a long and mobile snout, similar to that of elephant shrews. Its dentition was quite small in comparison to the size of the mandible and the animal as a whole.
“Studies of the bone structure of Leptictidium have yielded contradicting information: its leg articulations appear too weak to have supported the shock of repeated jumps, but its long feet were obviously adapted for jumping rather than running.”
That extinction statement is not exactly true
as both tenrecs and whales continue this lineage, though neither are bipedal nor do they have such proportions anymore. But this is news that the Wiki writer was not aware of.
It’s interesting and supportive of the present hypothesis
that the leg, ankle and hip joints of Leptictidium are considered “loose”. On whales, of course, the sacral connection to the pelvis is long gone. The Messel pit, from which several fossils were found, was a former lake. Maybe Leptictidium was not a hopper, but a swimmer. The long flexible snout hypothesized for Leptictidium could have been used as a snorkel to keep it underwater while breathing. And so, this is likely the origin of the cetacean blow hole.
Traditional renderings of whale swimming origins
usually employ a wolf-sized predator at the water’s edge. Now we can reduce that to a rabbit-size or smaller predator/insectivore.
If you still like hippos for whale ancestors
as DNA, prior morphological studies and several helpful paleontologists suggest, with hippos (or the earlier ungulate Elomeryx) you don’t get a long supple lumbar area, a long tail, long flat feet with long toes, a long narrow snout with small triangular teeth or really anything else whales are famous for, except, perhaps, blubber.
This heretical discovery
should be seen as one more nail in the coffin of DNA studies that try to draw cladograms from their unsupportable results. We’ve seen similar untenable cladograms nesting turtles as archosaurs and glyptodonts as armadillos. It’s time to put our thinking caps on and see hard evidence for what it is. Only morphological studies, and good ones, will recover gradual accumulations of derived taxa. Put all such studies under the magnifying glass to see if they make sense. Let’s put DNA studies on the shelf until they produce results that confirm morphological studies.
This hypothesis could not have been recovered
without the data and skeletons provided by Phillip Gingerich, Hans Thewissen and other stem whale discoverers over the last few decades. Phylogenetic analysis based on morphological data permitted seeing Leptictdium in a new light, as a swimmer, not a hopper with loose joints.
Storch G and Lister AM 1985. Leptictidium nasutum, ein Pseudorhyncocyonide aus dem Eozän der “Grube Messel” bei Darmstadt (Mammalia, Proteutheria). Senckenbergiana lethaea 56:1-37.