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Author Topic:   Creationist response to cetacean femur, leg atavism, and limb bud.
Aaron
Member (Idle past 3959 days)
Posts: 65
From: Kent, WA
Joined: 12-14-2010


Message 1 of 2 (617618)
05-30-2011 1:53 AM


In another thread, a few others and myself engaged in a discussion about whale evolution and morphology. The issue of vestigial pelvises and limbs arose. I took some time to research these issues further. I’d like to lay out a summary of my findings — a more detailed version will be forthcoming in a paper. In the midst of this I will try and answer some direct questions posed to me by Granny Magda.
These questions include:
1.) How much of a role does sexual dimorphism play in the size and shape of cetacean pelvises?
2.) Why do some whales have femur remnants and/or tibia remnants while others don’t?
3.) How similar is the whale’s pelvis to its ancestors?
4.) Why are some modern whales born with protruding rear legs?
5.) What about the molecular and DNA basis for limb loss?
Part of my discourse will be to show how dramatically changed these structures are from their proposed ancestor — much more dramatic than a bone that has slowly been reduced in size. In examining the unique structure of the pelvis, Struthers mentions adaptations that have led to its current shape and function. His 30 pages of analysis of the actual form and function of the pelvic, femur, and tibia bones are rarely mentioned in evolution literature. This includes an extensive account of the unique muscular and tendinous attachments to these bones and the function they perform.
I’ll start with the question: How similar is the whale pelvis to its proposed ancestors?
While the pelvis of land mammals up to Basilosaurus are composed of three distinct bones fused together, the pelvis of modern whales is only composed of one single bone — as noted by several authors who note a single ossification center found in cetacean embryos. This single bone is thought to be the ischium.
Granny Magda brought to my attention the picture of the fin whale pelvis with a few small bone nodules fused to it, labeled the femur. This example was different from the other fin whales in Dr. David Taylor’s pelvis gallery. It is difficult to say for certain if this is the femur or not. The femur of whales is always connected to the pelvis via a ring of cartilage or short tendon strands — not fused directly to it — which is why none of the other fin whale pelvises show an attached femur. The bony nub could also be a bone spur or bone tumor. If the nub represents a femur fused to the pelvis, this would highlight another departure from the typical pelvic structure. In other mammals, the leg bones develop and ossify at different developmental stages than the pelvis does - eliminating the possibility that the femur could fuse to the pelvis.
The other obvious structural departures of the modern whale pelvis include the lack of an acetebular cavitity in which the head of the femur typically resides. Struthers notes the head of the cetacean femur is in the position which in human surgery we would call dislocation backwards. The other point of difference is the lack of an obturator foramen in — or the hole in the pelvis through which nerves and muscles pass.
This illustration attempts to illustrate how the transition in position must have taken place.
Since the shape of particular bones and the attachment points of specific muscles can differ between families of mammals, the nerve structure of the surrounding muscles are often examined to try and determine homologies between the bones of different animals. However, in the case of whales, the nerves associated with the pelvic musculature are so greatly altered, that it is utterly impossible yet to homologize them with muscles of terrestrial mammals according to author Alfred Brazier Howell, a professor of comparative anatomy.
There is a significant sexual dimorphism in the structure of the male and female pelvis — a fact that Granny Magda tried to downplay saying individual variation is at least as big a variable.
While it is true that individual body part variation is probably higher in whales than in other mammals — this is not a reflection of the necessity of those body parts. An example of this is the fluke. Individual variations in fluke shape is one-way researchers can easily tell one specific whale from another.
Even though the pelvic bones can differ between individuals, definite generalizations can still be made. Struthers notes:
In the female the pelvic bone is shorter, more bent, broader at the angle, and, above all, thinner at and towards the hinder end,
But the chief difference in the sexes is on the posterior part of the bone, which is so thick and narrow in the male as to be almost rounded in appearance, while in the female it is thin, and may be also broad. The adaptation here is seen by referring to the attachment of the interpelvic ligament to the bone. In the male the thick rounded ligament, supporting the crus penis, is attached to the hinder end of the bone, while in the female the more expanded ligament reaches forwards along the inner margin and upon the bone.
Differences within a species can also be age related. The pelvis undergoes a dramatic growth spurt around the age of sexual maturity — which varies from 6-20 years old, depending on the species. The pelvis is the last bone to ossify in whales — the same for humans. Therefore, in younger whales, large portions of the pelvis may be completely cartilaginous. These cartilaginous parts would not be preserved in the museum samples recorded in Dr. Taylor’s gallery. gallery
Atavistic Hind Limbs
Yablakov questions whether any structure in whales can be considered vestigial if it performs a necessary function. Indeed, he suggests that the term vestigial should only be attached to structures that are not universally shared by all members of a single species. The only example in this category he can give is the occasional presence of hind protrusions in whales. Yablakov observed cases of external cetacean hind limbs during his experiences with the whaling industry.
Granny M. points this out as well:
some sperm whales have been observed to have protruding limbs, actual mini-legs, quite visible outside the body. Not all sperm whales have these; indeed most do not. You could argue that they serve a purpose, but this is obviously not an important factor, since so few sperm whales have them. You could argue that the normal sperm whale pelvis/femur arrangement serves a purpose, but this is rather undermined by the existence of perfectly healthy sperm whales with these more developed limbs.
Here is an example of a hind limb from a sperm whale:
Interestingly, the x-ray reveals a series of bones that very closely match the bones of the front fin. In A.A. Berzin’s account of this example he writes:
The section of the left protrusion that extended from the body had the appearance of the rounded blade of a propeller, while the right one looked like a fin with finger-shaped processes.
The similarity of the rear protrusion to the front fins is an important detail that cannot be overlooked.
Similarly, Roy Andrews wrote about a 1919 instance of a humpback with protruding hind limbs that extended 4 feet 2 inches. It’s clear this isn’t a simple atavistic reversion, for if it were a reversion to an ancestral growing pattern the length of the bones should be much closer to the proposed ancestral form. The hind limbs of basilosaurids were much smaller than 4 feet. On the other hand, the length of the protrusion more closely matches that of the humpback front flipper.
Andrews makes several comments on the homology of the hind limb to the front flipper.
I infer from Mr. Ruck’s description that the connective tissue and blubber were essentially the same as in the flipper, or fore limb, of cetaceans.they are homologous in many respects to the flippers, or fore limbs.
Edited by Aaron, : Eliminating potential issues for publication.

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Message 2 of 2 (617642)
05-30-2011 8:51 AM


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