422px-Haeckel_Ammonitida

Ernst Haeckel’s ammonite forms

“The chambered part of the ammonite shell is called a phragmocone. The phragmocone contains a series of progressively larger chambers, called camerae (sing. camera) that are divided by thin walls called septa (sing. septum). Only the last and largest chamber, the body chamber, was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the coil. A thin living tube called a siphuncle passed through the septa, extending from the ammonite’s body into the empty shell chambers. Through a hyperosmotic active transport process, the ammonite emptied water out of these shell chambers. This enabled it to control the buoyancy of the shell and thereby rise or descend in the water column.”

(source)

You would think the nautilus is their closest living relative, but you would be wrong:  it’s the Coleoidea (octopus, squid, cuttlefish).  They would all be swimming together today, if the ammonites had survived the K-T event.  Medieval Brits thought fossil ammonites were snakes turned to stone by St. Hilda.  Judging from their global distribution, she was exceptionally generous with her time.

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