St. Louis County, Missouri
Unknown concretion at base
This concretion is extremely hard and impossible to
remove even with an air abrasive unit. This concretion
is not unique to this specimen. It has been reported to
appear on other Archimedes from different localities.
1) Perhaps it is a parasite of some kind.
2) Perhaps it is a star fish. Commonly Mississippian star fish
appear as a bunched up mass/ball of very small plates that
could appear to be concretionary.
3) I have seen root holdfasts of Silurian crinoids appear to be
somewhat concretionary. The actual root ball is often times
a "glob" of calcite. Archimedies had to anchor itself to the
bottom somehow. Perhaps the "trunk" was much more robust
than the rest of the axis and thus fossilized as a glob/ball of
calcite. This is just another possibility and the last one
I can think of.
Mississippian Fossils of Missouri