http://standardbredhorses.yuku.com/topic/464
Page 21 Vol 1 of Dr. Deb's POCA - both the Poebrotherium and the Mesohippus both have this protrusion.
Go back further to page 14 - so did the Hyracotherium - 55 million years ago. This animal looks like it can sit like a dog. I wonder if these protrusions were "sitting bones"?
"Slope" is measured in bones like the shoulder in the front, and the pelvis in the back. (Some people put a lot of emphasis on the "slope of the pastern" but I don't - the pastern slope is the result of a well-balanced foot and nothing more, it doesn't need to "match" the slope of the shoulder). The reason the slope is important in bones is because it's an indicator of motion. For example the more upright the shoulder is, the more room is has to move (rotate) back. If it were laid back, it would have less room to move (rotate back). This impacts how high a horse can raise his front legs. The more room he has to rotate his shoulder blade back, the higher he can raise his knees..
(Same theory holds true with the pelvic slope, although there are other points of discussion about that one.)
The question came up about the "bony extension" -- we call the 'point of buttock' -- that sticks out from the back of the pelvis. No, you don't have to thank me. I sent a message to Dr. Deb and asked her about it. Here's the question and her answer verbatim. I have bolded what was good for me.
Dr. Deb
I am having a discussion with friends about the purpose of the point of buttock and why it extends beyond the pelvic-femur joint. Looking at the diagram on page 14 of the POCA Volume I, it appears that the Hyracotherium would be able to "sit like a dog".
Is this bony protrusion an evolutionary left over of "sitting bones"? And if so (or not) what is the purpose of this protrusion in the modern horse?
Many thanks.
Dr Deb wrote:
It isn't a "protrusion", either in Eohippus or in the living horse. Let's first get some terminology straight:
(1) The "point" of buttock is an area on the SURFACE of the body which is underlain, or structured, by the blunt rear end of the ischium bone.
(2) The ischium bone is one of three bones that make up each side, or half, of the pelvis. In other words, the left side or left half of the pelvis is made up of an ischium, an ilium, and a pubis bone, and ditto for the right side or right half, so that the pelvis as a whole is made of six bones. The ischia (plural) are the rearmost bones of the six, one on the left that underlies the left point of buttock, and one on the right ditto.
(3) All animals that have a pelvis have a pair of ischia. Primitively all mammals have a pelvis, and unless something has gone on to cause a certain lineage of animals not to have a pelvis, then they will have a pelvis. So therefore, both Eohippus and Equus have a pelvis and they also both have ischia. In other words, the horse lineage has always had a pelvis, including ischia, and it still has them today.
And yes, Eohippus could indeed sit like a dog. And it's also true that when dogs sit, they sit on the rear ends of their ischia, i.e. they sit on the points of buttock. Horses can also sit down like dogs, i.e. as Allen Pogue's horses when they sit on a bale of hay, and when horses sit on a bale of hay, they also sit on their points of buttock. Where else would they sit?
But "sitting" is not really the main thing that the points of buttock are for. People ask animals to sit and they do sit because they can and because they want to please us. But to the animal himself, the purpose of the ischium is to provide an area of attachment for the upper ends of the hamstring muscles, a root-area for some of the muscles that move the tail, and a firm anchor for some important sheetlike ligaments that pertain to the inner aspect of the pelvis.
All bones of the body have "purposes" that are more of this nature.
Hope this clarifies matters and actually answers the question you meant to ask.
Best wishes -- Dr. Deb
