The Nautilus Ammonite Fossil

Paleoecology, the study of ancient plantlife fossils, has interpretations of the Ammonite mode of life that are based on the behavior of Nautilus. However, Ammonite shell morphology is extremely diverse. The effect of shell shape on lifestyle has been investigated using numerical modeling to assess the stability of the shell within the water column and by observing the hydro-dynamic properties of different shells in the laboratory.


Any body immersed of floating in a liquid is acted on by a buoyant force equal to the weight of liquid displaced. The point through which this force acts is the center of buoyancy. The point through which the resultant force of gravity passes is the center of gravity.

In a stable orientation, the center of gravity lies directly below the center of buoyancy. The greater the distance between the centers, is the greater the stability. By estimating the volume and weight of Ammonites during life, the center of gravity of buoyancy of the animal can be calculated. This work has established the probable orientation of Ammonites with differing shell morphologies within the water column, helping to identify their likely mode of life.

The Oldest Fossil

It is probable that all living multicellular animals shared a single common ancestor. Molecular data provide widely divergent results, but suggest that this common ancestor arose between 1 and 1.6 billion years ago. The fossil record of metazoans is more recent, and older fossils are sometimes regarded with suspicion by experts. The oldest common and widely accepted metazoans are members of the Ediacaran fauna, found in rocks dated at between 565 and 543 million years old.

The Ediacaran fauna represents the oldest, well known, diverse set of multicellular organisms, They are found almost worldwide, distributed in rocks from just above the tillites of the last Precambrian glaciation to the Precambrian - Cambrian boundary, and possibly beyond!

The fauna is entirely soft bodied and contains up to 30 species of organisms. The lack of hard parts means that the animals were probably fossilized close to where they lived, as the dead bodies would have been too frail to have been transported for any significant time or distance. If this is, indeed, the case.. then the fossil assemblages represent the beginning of ecosystems.