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SACRUM AND COCCYX

     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.

yunnanozoan      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).

lancelet

lancelet

fish embryo

fish embryo

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. 

 

alligator

alligator

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.

dinosaur hips

emu hip and sacral vertebrae

emu hip and sacral vertebrae

great blue heron

great blue heron

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)

platypus sacrum

platypus tail

cat

cat

mink

mink

rhesus monkey

monkey

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.

human

human

human

human

human human