Natural History Museum: Ammonite

I'm always captivated by this fossil ammonite: it is in the "Lasting Impressions" gallery which contains objects which bear an imprint of their past... This ammonite is included because as they grew they would create new, larger chambers in their shell: they lived only in the outermost and largest chamber, and would seal them off consecutively as they grew. The past chambers weren't non-functional, because the ammonite was still connected to them all by a tube of tissue called the siphuncle which ran through them all, and using which the animal could control its buoyancy by altering the relative amount of water and gas in the chambers... SO, an ammonite's shell is a lasting impression of its growth and life history.

This ammonite is an example of Asteroceras stellare from the early Jurassic period, so this animal lived 175 - 200 million years ago.

I walk past this on my way in and out of the museum, and what really captivates me is the satisfyingly spiralled shape of the cross-section: the shell spirals outwards logarithmically... Each section is larger than the previous one, so the diameter increases exponentially as the animal grew around. The tiny chambers in the middle were the young ammonite's shell: tiny ammonite.

I've blipped an ammonite before here.