In 1686 the great English philosopher and mathematician, Robert Hooke (1635-1703) perceptively concluded that ammonites were the fossil remains of "some kinds of Nautili"

Ammon, the ancient Egyptian god, was usually depicted in human form with a ram's head. The similarity of the spiral fossils to the coiled ram's horn provided the name to ammonites. During the Renaissance the organic nature of fossil clams, snails and shark teeth was readily accepted by naturalists because they were so similar to existing forms, but ammonites were so unusual and so confusingly preserved that most scientists doubted that they were ever living things or, if they were organic, then they were much more likely to be snake-like.

A great variety of ammonites have been collected from the Cretaceous rocks exposed on Vancouver Island and the Gulf Islands. Two types of ammonites are found in concretions weathering out of the Upper Cretaceous shales at Collishaw Point on Hornby Island in the Strait of Georgia -- planispirals and heteromorphs. Planispirals are "normal" ammonites that are coiled in a single vertical plane -- each whorl in contact with and overlapping previous whorls. Heteromorphs depart from this compact plan. In some heteromorphs, the coiling of the initial shell is snail-like followed by an upwardly-recurved portion. In others, the initial few coils are followed by a straight shell, or a shell with two, three or four parallel shafts -- like gigantic paperclip. Planispirals are symmetrical across the midline and they obviously maneuvered through the water column in a vertical stance. Most heteromorphs, on the other hand, lack planes of symmetry and it is not clear how they were oriented in the water column if, indeed, they swam at all. Some paleontologists have suggested that some heteromorphs lay immobile on the bottom, or even buried in the sediment as immobilize infaunal organisms.

The presence of an ammonite is generally disclosed by a bit of iridescent shell material visible on the edge of the concretion. Those that contain fossils tend to be of the size and shape of the fossil within. A concretion the size and shape of a kaiser bun might include a specimen of the planispiral Pachydiscus; one that is sausage-shaped could contain the straight heteromorph Baculites; and a large oblong concretion might enclose cordwood-sized fragments of the ribbed heteromorph Diplomoceras.

Nostoceras is the most sought-after ammonite prize on Hornby Island. This heteromorph comprises a spirally-coiled turret that is followed by a downwardly hanging living chamber that recurves upward -- hence the name, Greek for "the horn that returns home". Questions abound about this ammonite. Some turrets are dextrally-coiled, others coil sinistrally -- the reason is not clear. Some specimens are half the size of others. Could this be sexual dimorphism? This has been suggested for other ammonites -- with the female being much larger than the male.

Further reading:

Ludvigsen, R. and Beard, G.
1997:  West Coast Fossils: A Guide to the Ancient Life of Vancouver Island. Harbour Publishing, 216 p.