Ammonites of the Inner Hebrides

Ammonite, isle of Mull (c) Angus Miller
Ammonite, Isle of Mull (c) Angus Miller

Ammonites are one of the most recognisable fossils, and are found at many sites across Scotland from the Black Isle to the Inner Hebrides. On the Inner Hebridean islands of Skye and Raasay they are preserved in a nearly complete sequence of Jurassic-era sedimentary rocks. Rocks from this time are relatively rare in Scotland, and so to have a complete series of them on these isles makes the area particularly special. Ammonites, with their shell-shapes similar to the modern-day nautilus, are perhaps surprisingly more closely related to squid. Their curled spiral shells come in all sizes, with ammonite fossils up to half a metre across discovered on Skye. Different species of ammonite can be distinguished by the pattern on their shells, though little is known about the molluscs that lived inside the shells as preserved remains of their soft body parts have not, as yet, been found anywhere on Earth.

It is thought that most ammonites were open-sea dwelling creatures, as they are usually found in rocks without the remains of creatures from the sea-floor. The animal kept itself buoyant by regulating gas in the chambers of the shell, and probably used a jet of water for propulsion. The shells themselves were streamlined and often fairly flat and disc-shaped. Fossils found today are either the preserved shell in its entirety, or the imprint of a shell left behind in a rock such as mudstone, shale or limestone. Sometimes the fossil shells retain bite-marks from attacks by other marine creatures, such as reptiles.

AlAl McGowan, GES, University of Glasgow says:
Ammonoids were critical fossils for demonstrating that rocks widely separated in space were of a similar age. Their coiled shells are fine expression of the fact that much of the natural world is best understood through mathematics, as the famous Scottish morphometrician D’Arcy Thompson so convincingly demonstrated.