Eucalyptocrinus Stem

Here is a Silurian fossil crinoid stem section of the Eucalyptocrinus. Fossil was found in the Waldron Shale of Clark County, Indiana, USA. It will be the trunk section of my fossil Christmas tree for this year. Visit the December 25 post to see it. Section is about 1.5 cm in length.
The subrings still have matrix on them but I liked the limestone patterns on the larger rings that showed up after I polished it. I wonder the alternating sized discs evolved that way? Maybe helped with water flow around the stem or increased flexibility of the stem in the water currents.