Ardnamurchan Ammonites

At several locations along West Ardnamurchan's shores there are outcrops of Jurassic-aged sedimentary rocks, and in some of these are to be found yet more of our local wonders - spectacular fossil ammonites.  To the untrained eye, their remains are no more than strange patterns in the rock, but these are representative of a hugely successful mollusc group which became extinct at the end of the Cretaceous.

Ammonites thrived in warm, shallow tropical seas.  Some 150 million years ago, the continental crust which was to become Scotland lay on the edge of a warm ocean, the Tethys - the map here shows the world in Jurassic times.

We would know relatively little about the ammonites were it not for a rare, living mollusc found mainly in the eastern Pacific, the Nautilus (above, and see earlier Diary post here).  Early versions of the Nautilus gave rise to the ammonites, which thrived through the Triassic, Jurassic and Cretaceous periods, at about the same time as the dinosaurs were ruling the land.  But, like the dinosaurs, the ammonites did not survive the Cretaceous-Tertiary extinction while the evolutionarily very conservative Nautilus family did.

Ammonites, like the Nautilus, seem to have hunted near the sea's surface during the night, and sunk into its dark depths during the day.  But they were preyed upon by swimming reptiles so, to avoid them, they evolved to sink deeper and deeper, into water which was under huge pressure and which, if they hadn't developed some remarkable features, would have crushed their shells.

Nautilus had concave transverse partitions to strengthen the thin shell wall, the equivalent of the modern submarine's bulkheads, as can be seen in the above photograph.  Ammonites went one better, crinkling the partition where it met the outer shell to give it more support against the huge outside pressures, using an idea which humans, belatedly, use in corrugated cardboard.

The crinkling effect can be seen in this picture, which shows part of an ammonite where the chambers have been infilled with sediment, preserving the intricate shape of the partitions.

Some of the West Ardnamurchan ammonites are, by ammonite standards, huge, being a good 30cm or more in diametre.  The Diary has only seen larger in the oolites of Portland Bill in Dorset.

Photo of Nautilus courtesy of J Baecker, link here.

Photo of sectioned Nautilus, Jose Luis Navarro Lizandra,link here.