An oldie but a goodie

We don't have to go to the beach to find intriguing marine invertebrates. Last Sunday, we went to an antique fair, and I came home with an ammonite from Madagascar, split in two to show the chambers.

The two halves, face to face.

The ammonites were prehistoric crustaceans, living at the same time as the dinosaurs, and going extinct with them. They are related to our modern squids and octopuses, and more remotely, to the nautilus, which they looked like.

Artist's reconstruction of Astroceras. Wikipedia, art by Nobu Tamura (http://spinops.blogspot.com)

Fossilized ammonite shells can be found all over the world, with an interesting lode occurring just "next door", in Alberta; these can be highly iridescent, and are sold as gemstones. Others may be filled with chalk, sandstone, pyrite; whatever sediment and minerals are prevalent in the layer where the ammonite is found. My son has an uncut ammonite he found on a mountainside, a charcoal grey, fist-sized, coiled lump of granite. The fossils show up in all sizes, from 1 mm. to over 6 feet across. Mine measures just over 3 inches.

The animal lived in the front end of a series of chambers (camerae), divided by curved walls from the older ones. As the creature grew, always moving to the new room on the front, it sealed off the inner chambers, except for a thin connecting siphon, creating a sort of flotation device that could be filled with water or gas as needed to rise or drop in the water column.

The meaty part of the animal was not often fossilized; what we find are the empty shells, now filled with an assortment of minerals.

Beyond a tentative ink sac and possible digestive organs, no soft parts are known at all. They likely bore a radula and beak, a marginal siphuncle, and ten arms. (Wikipedia)

Sometimes the fossils contain smaller fossils: the animals that the ammonite had eaten, giving us a clue to their life style.

Ammonites were the predators of their time, feeding on most living marine creatures including molluscs, fish and even other cephalopods. By analogy to modern cephalopods, their method of attack probably comprised of silently stalking their prey, then rapidly extending their tentacles to grasp the target. Once caught the prey would be devoured by the ammonite's powerful jaws, located at the base of the tentacles, between the eyes. (From Discovering Fossils.)

This I learned from the web. What can I learn from the critter in hand, itself?

Closer view, showing half the diameter of the shell.

The outer chambers are filled with fine sandstone, with a few larger grains spotted about. Each chamber is divided off by a wavy wall (septa). As we move to the internal sections, the filling changes to crystallized stone, in varying tones of red, and translucent. (That inner spiral looks like sugared fruit; appetizing, but hard on the teeth!) Several chambers are hollow, lined with sharp crystals, slightly purplish, a pale amethyst hue. There seem to be the remains of communication pipes through the walls.

Zooming in on a pair of chambers. Two tubes enter or leave each side of the chamber.

I wondered why there was such a difference in the filling material. Why are the larger chambers filled with sand, and the inner ones with crystal? Back to the web . . .

The body chamber is the final, longest chamber, in which the ammonite animal actually lived. It is not divided by sutures and is often fossilised in a different colour to the phragmocone chambers, as sediment readily filled it after the creature died and its soft parts rotted away. The chambers of the phragmocone however, are largely sealed off from the body chamber, and because of this they are usually mineralised over a longer period, due to percolation of mineral rich water through the shell. (From An Introduction to Ammonites, UK Fossils.)

There is still more variation; each of the larger chambers has collected its own load of sediment, and seepage has tinted the outer sections, creating beautiful patterns.

Detail of first and second chamber filling.

Ammonites from Madagascar and Alberta can be iridescent. This one is from Madagascar. Unfortunately, it is badly worn, and only hints of the iridescent coating are left on the outside of the shell; glints of red and green along the broken edges.

Iridescent ammonite, from Wikipedia.