On ammonite maturity

From
Date
Reply-To
Dear All,

Heinz Hilbrecht is absolutely right about the way modern cephalopods
migrate to 'breeding grounds'. In my opinion, some of the belemnite
battlefields are a result of exactly this. Jacques Cousteau gives a
splendid description of a breeding event for, I think, Loligo peali in his
book Kingdom of the Octopus. In that he describes how the squid are
oblivious to the predators, such as sharks. Perhaps the well known fossil
Hybodus from Solenhofen with a belly full of belemnite guards was one such
predator from long ago...

Back to ammonites: it is not easy to define the "best" shell shape. It
depends on how the animals lived. If ammonites swam about like cuttlefish
do today, the scaphitiform (if there is such a word!) shell would not be
helpful, since it diverts the exhalent current away from the presumed
direction of travel. As the final whorl grows, there is a change in the
angle of the aperture from the sea-bed, so it is unlikely the animal took
food from the bottom.

Suppose that the animals only fed as sub-mature individuals, laid down fat
reserves, and then changed shape for breeding. This is not far fetched:
salmon, butterflies, and mayflies all do this. It would imply, of course,
that the animals died after breeding. This matches coleoid ecology
perfectly. All coleoids live for one to four years, and then die after
mating.

However, see Hewitt, Westermann, and Checa (1993- Geobios MS 15: 203-208
"Cephalopode actuels et fossiles"). They write about the growth rates of
ammonites, using APTYCHI. Breeding events appear to be related to bunching
of growth bands on the aptychi. Presumably food/energy is converted to
gonad growth rather than somatic (body) growth. Interestingly, while the
microconchs (males?) do die after one breeding event; macroconchs appera to
recover and breed again. Then they die, having bred twice.

So in some ways it is very coleoid like, but this female-breeds-twice bit
is a new twist.

Anyway, thanks Heinz.

Neale

Neale Monks, Department of Palaeontology,
Natural History Museum, London, SW7 5BD
Internet: N.Monks@nhm.ac.uk
Telephone: 0171-938-9007

"...now Nature is having the last laugh. The freaky stuff is turning out to
be the mathematics of the natural world"

from 'Arcadia', by Tom Stoppard