"To see a world in a grain of sand,
And a heaven in a wild flower,
Hold infinity in the palm of your hand
And eternity in an hour."

From "Auguries of Innocence"

by William Blake

Thursday, 11 November 2010

Fossils - Part 1: Ammonites

I've been fascinated with fossils and the history of life on Earth for probably as long as I have been interested in natural history. Although my son went through the usual stage of being engrossed in dinosaurs and collecting fossils, its only in the last ten years or so that I have started to build up my own collection.

I find it absolutely amazing to be able to handle the remains of an organism that is millions of years old and its even more astounding when you find your own fossil and realise that you are the first person to have set eyes on the organism since it died all that time ago. Its estimated that between several hundred million and a few thousand million species have existed on Earth during the past 540 million years and of this number only a few hundred thousand species have been discovered as fossils. By comparison the number of species living today could range from between 5 million to 10 million species. The vast majority of species that have existed are now extinct.

Here's a selection of some of the ammonites from my collection

Ammonites which belong to the Phylum Mollusca and Class Cephalopoda were confined to the Mesozoic Era which included the Triassic, Jurassic and Cretaceous Periods (248 to 65 million years ago).

They were entirely marine and most were pelagic (living above the sea floor) and the majority were nektonic (active swimmers). It is thought many species were predatory feeding on plankton whilst others could have been scavengers. Ammonites ranged in size from 20 millimetres in diameter to well over 2 metres.

The ribs (and in some species spines and knobs) on the shell which can be seen on the photos below may have helped to strengthen it, provide protection against predators and may also have been used in sexual display.

The shell which was made of aragonite was a coiled tube which was divided into many chambers, the animal living in the outer open end of the shell. Chambers in the shell which had formed earlier would have been filled with gas and water acting as a buoyancy aid.

Ammonites were widely distributed throughout the oceans and evolved very quickly with each species having a fairly short life span and are therefore often used as zone or guide fossils to date Jurassic and Cretaceous sedimentary rocks.

Ammonites became extinct at the end of the Cretaceous 65 million years ago during the same mass extiction event that killed off the dinosaurs. Representatives of Cephalopods which survive today include squids, cuttlefish and octopuses. The nautilus is probably the nearest living relative to ammonites.

The following are photos of polished cross sections and in the first photo the frilled patterns are sutures which reveal where each individual chamber wall joined the inside wall of the coiled shell.

Sixteenth century natural history books mentioned similarities between the ammonite's coiled shell and snakes or serpents and ammonite fossils were once believed to be the petrified remains of snakes and were called "snakestones".Snakes heads were often carved on ammonites by fossil collectors at this time.


The Wessex Reiver said...

A very interesting post. I've spent many an hour down in Lyme Bay hunting for fossils, to no avail. I made a radio programme with a certain famous wildlife icon last year on fossils, he just brought the whole process to light recounting his Leicestershire boyhood rambles.

Ragged Robin said...

I'm so glad you enjoyed the post. To be honest I have only found a small amount of the fossils myself - the rest I have bought usually whilst on holiday. I am exceedingly envious of your encounter with a certain icon !! - have you by any chance got a link to listen to the programme again? His new series "First Life" is superb.