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Funky foram, center left, 330 myo Mississippian Bangor Ls, thin-section photo, 2.5 mm wide.

A Mississippian carbonate mound thin section

What is it? Suggestions include a tubular calcimicrobe or encrusting foraminifer.

let's get small

An ooid, a concentrically laminated calcium carbonate spheroid, formed inorganically by wave action around a pre-existing nucleus. The nucleus in this case is the microscopic shell of an amoeba called a foraminiferan. Just so you know.

A tiny box without key or lid
inside a chambered treasure hid
its maker dead and long forgot
yet carbon saved so we aren't hot

Doggerel again, I'm afraid.