So what are the actual, observational actual details that make paleontologists decide that whales and other cetaceans like dolphins come from land and evolved from land artiodactyls?
But for that I would lke to take Dorudon rather than Indohyus and Ambulocetus as the video does.
Dorudon was a cetacean that lived in the Eocene. Beyond any doubt it was a marine animal,
you can tell this from its streamlined body shape.
First of all, let's examine why paleontologist find that early cetaceans like
Dorudon (or Ambulocetus for that matter) are mammals. You can tell from anatomy, DNA, physiology and behavior, like:
- they breath through lungs, not gills
- females produce milk to feed their newborns
- specifically they belong to the placental animals (and not to the marsupials or monotremes)
- they give live birth
- the inner ear anatomy (mammalian middle ears contain three tiny bones known as the ossicles: malleus, incus, and stapes and this threefold structure is unique in the animal kingdom, so paleontologist may only find a fossil part of the skull and they can tell it belongs to a mammal)
- they are warm-blooded
- they propel through the water by up/down instead like fish left/right movement of the spine (unlike, for instance, reptiles, these still move around the way fish do by a left/right undulation)
- and some hundreds of other traits that link them to mammals.
Note: the traits above are mostly unique to mammals but there are some individual exceptions, for instance, live birth is also observed in other non-mammal species like some sharks but it is the total picture of hundreds of traits that make the point (and of course especially telling concerning traits are certainly unique for mammals).
But the most telling trait that binds cetaceans to the mammals is their genetic make-up: of all animals living on earth, the one that resembles cetaceans genetically most by DNA comparison, is hippopotamus, an artiodactyl. Not all too surprisingly, hippopotamus is a semi-aquatic
land animal.
And here are the details about the anatomical evidence for cetaceans being artiodactyls:
- the double pulley joints anatomy of the ankle, a trait unique for artiodactyls (mentioned in the video)
- a hooked knob pointing up towards the leg bones in the astralagus, unique for artiodactyls (also mentioned in the video).
To make the evidence complete, the next step is to prove that Dorudon (after all it's an extinct animal so we only have its fossils to establish this) should be classified as a
cetacean:
- alignment of the upper incisors with the cheek teeth (typical for cetaceans)
- the nostril is not in the tip of the snout but has travelled halfway its head (the blowhole in whales)
- the ear region is surrounded by a bony wall (the thickened involucrum mentioned in the video)
- reduced pelvis and hind limb size (I come back to this!)
- particular structure of the tympanic bone
- the anatomy of the teeth...
… and a few other traits that are unique for cetaceans.
But the most telling evidence for cetaceans having evolved from land animals are the pesky hind limbs of Dorudon.
Dorudons and also another later, extinct cetacean, Basilosaur (mentioned in the video), have
fully developed hind limbs, attached to a pelvis and,
another specimen (both linked examples are of the species Dorudon atrox).
Those hind limbs were still fully developed according to basic amniote anatomy. Amniotes, meaning "membrane surrounding the fetus", are a clade of tetrapod ("four footed") vertebrates comprising the reptiles, birds, and mammals that lay their eggs on land or retain the fertilized egg within the mother, both of which are made possible by the membrane - the anamniotes like fish and amphibians don't have such a membrane and need to lay their eggs in water. Amniotes can lay their eggs on the dry land which enlarges their habitats greatly.
The typical amniote hind leg anatomy is:
- femur including patella
- fibula and tibia
- tarsals and metatarsals
- digits
- neatly attached to a pelvis.
But perky those hind limbs were indeed:
- first of all, they were extremely small for such rather large animal (Dorudon was ~ 5 meters tall and weighted some 2 tons). The size of Dorudon’s hind limbs was about a modern housecat’s ones. I don’t think an animal that long and heavy could have walked with such small hind limbs
- but, moreover, the pelvis was detached from its spinal cord. You just can’t walk with hind limbs detached from the spinal cord
- also much of the ankle bones and carpals were fused as well, again making walking impossible.
Now the next, profound question here is: what was a
full-blown marine animal doing with
fully developed, amniote type of hind limbs which were
detached from the spinal cord and
too small for such a large animal and
whose ankle bones and carpals were fused, making walking entirely impossible. In other words, what was a fully marine animals, that used front flippers and a tail fluke for propelling, doing with hind limbs in the first place but also ones it couldn’t walk with?
Well, it's because those hind limbs are vestiges and point out to the terrestial origine of cetaceans.
It's immediately here important to tell that vestiges are
not necessarily functionless. The vestigial pelvises in cetaceans still in some species attach to the male sexual organs. That indicates some function (a kind of anker of the penis). I don't believe it is but let's say it actually does. That still makes the cetacean hind
legs and pelvises vestiges:
- it's still vestigial since the pelvis isn't doing "pelvis things" like providing a joint for leg bones, and anchor points for leg and back muscles
- the "function" it still may have for sexual reproduction is that it's attached to the penis via a ligament. It actually is useless in females (and I add that in all vertebrates the sexual organs are fixed to the pelvis, so this might as well count as evidence for evolution)
- the femur often in modern whales isn't attached to anything or the pelvis is even entirely lacking. So how are in these species the males performing?
- some dolphins have gotten rid of it entirely and have any pelvis or hind leg structure left at all and still manage to reproduce. Which is typical of vestigial structures, they are sometimes completely absent in some individuals, some humans don't have wisdom teeth, for example.
Now why did cetaceans end up in the oceans. We have a good explanation for that: cetaceans evolved in the aftermath of the demise of the non-avian dinosaurs. These were wiped out ~65 millions of years ago mostly due to an asteroide impact (that also caused numerous other groeps of animals to go extinct).
Among those extinct dinosaurs there were species living in the oceans in the same ecological niches cetaceans today dwell. So when those dinosaurs went extinct, they left niches open to be occupied.
Moreover also on land mammals were capturing the open niches left by the extinct dinosaurs. This is called the mammalian radiation because it happened rather fast. This might have caused severe competition for food and habitat among those, especially in estuaria, river deltas or coast line, where a lot of food is found. Apart from finding open niches in the oceans, the early cetaceans also were escaping fierce competition on land by conquering the oceans or even the threat by predators.
