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SACRUM
AND COCCYX
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In most coelomates
(worms, insects, or the fossil hemichordate pictured below) the digestive
tract stretches the length of the body and the body ends with the anus. |
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In chordates, there is a muscular tail which
exists behind the anus through which the digestive tract does not pass.
It seems that the zinc finger transcription factor Manx was critical in the evolution of chordate
tails, given that its mutation can cause tailless conditions in both urochordates and vertebrates (Satoh, 1995). |
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lancelet |
fish embryo |
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Hox gene
cluster. In vertebrates, Evx
was moved from the cluster but is still linked to it.
In Drosophila, the even-skipped gene was shown to control a number of steps in development.
Vertebrate homologs Evx1 and Evx2 resulting from a gene duplication in
early vertebrate ancestry also function in development. In all bilateral animals studied to date, Evx functions during gastrulation,
which may the original role for the gene.
In Amphioxus, Evx plays a role in the development of the post-anal tail,
one of the major characteristics shared by all chordates. Later vertebrates recruited Evx in limb development as well (Ferrier, 2001). The vertebrae which develop in the region
of the hip are referred to as sacral vertebrae and the vertebrae of the
tail are called coccygeal vertebrae. Sacral vertebrae were modified for their attachment
to the hip in early amphibians. In
the most primitive known amphibian, Acanthostega, the pelvis is not
attached to the vertebral column and, as a result, the leg could not support
the weight of the body. In later
amphibians and reptiles, the vertebrae with which the pelvis interacted
fused to form a solid bone called the sacrum which attached to the pelvis
at the sacroiliac joint. |
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alligator |
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The first dinosaurs had two vertebrae
contributing to the sacrum which attached to the pelvis. These first two sacral vertebrae are indicated
by the red and purple bones of Herrerasaurus. The number of sacral vertebrae increased in
later dinosaurs. |
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emu hip and sacral vertebrae |
great blue heron |
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In humans embryos, the sacral vertebrae begin their development
as individual vertebrae which then fuse. The size of the tail was reduced in both therapsids and cynodonts (Kemp,
1982, p. 124) (Carroll). Cynodonts incorporated a third vertebrae
into the sacrum (Kemp, 1982, p. 34) |
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platypus sacrum |
platypus tail |
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cat |
mink |
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rhesus monkey |
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As
apes became larger and spent at least some time erect (either walking or
hanging), the pull of gravity on the urinary and reproductive organs became
problematic: there was originally no reinforcement of the body wall in this
region. Apes solved the problem by tucking their tail
between their legs (refer to the following photo of the human pelvis). The tail was reduced to form a bone a fused
vertebrae called the coccyx (in humans, the tail is much more significant
earlier in development) and positioned to help support these pelvic organs.
The muscles which used to move the tail are still present but they
now reinforce the body wall, forming the pelvic diaphragm. |
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human |
human |
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