![Geological Survey of Canada Geological Survey of Canada](/web/20061103035621im_/http://www.gsc.nrcan.gc.ca/esst_images/gsc_e.jpeg) Natural Resources Canada > Earth Sciences Sector > Geological Survey of Canada > Past lives: Chronicles of Canadian Paleontology
Past lives: Chronicles of Canadian Paleontology Hornby ammonites
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"
![Close-up of sutures of the Upper Cretaceous ammonite Pachydiscus, Hornby Island B.C. Vancouver Island Paleontological Museum Collections. (Photo by RL (c)) Close-up of sutures of the Upper Cretaceous ammonite Pachydiscus, Hornby Island B.C. Vancouver Island Paleontological Museum Collections. (Photo by RL (c))](/web/20061103035621im_/http://www.gsc.nrcan.gc.ca/paleochron/images/hornby1.jpg) Close-up of sutures of the Upper Cretaceous ammonite Pachydiscus, Hornby Island B.C. Vancouver Island Paleontological Museum Collections.
(Photo by RL (c)) |
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 hornbyense from the Lambert Formation of Hornby Island. Specimen is 15 cm high. Courtenay Museum Collections. (Photo by BDEC (c).) Nostoceras hornbyense from the Lambert Formation of Hornby Island. Specimen is 15 cm high. Courtenay Museum Collections. (Photo by BDEC (c).)](/web/20061103035621im_/http://www.gsc.nrcan.gc.ca/paleochron/images/nostoceras1.jpg) Nostoceras hornbyense from the Lambert Formation of Hornby Island. Specimen is 15 cm high. Courtenay Museum Collections.
(Photo by BDEC (c).) |
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. |
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