- Joined
- Jun 29, 2003
- Messages
- 22
Phil,
I must confess I only just noticed your question about siphonal notches in Ordovician nautiloids and their relevance to jet propulsion in ammonites.
The bottom line is that while siphonal notches, that is, embayments in the anterior margin to allow the siphon to protrude freely, do exist in various fossil (and living) nautiluses, they aren't known from ammonites. Quite the reverse. Many ammonites have modified apertures sometimes with constrictions and collars (very common with heteromorph ammonites) or rostra (many Jurassic and Cretaceous true ammonites). The rostra are rather like long spikes sticking out the front. These are distinct from the well known "lappets" that form on the sides of the aperture, rather like ears.
In some species the rostra points forwards, curling slightly in keeping with coil of the shell (e.g. Diploceras) while in others it curves backwards and underneath like a mini-spiral hanging under the shell (e.g. Mortoniceras). Either way, if there was a siphon it might have laid across the rostrum but couldn't have curled underneath the animal to allow it to swim forwards (as Nautilus does today). One solution is that these animals had two siphons, one on either side of the rostrum. The alternative is that the animal extended far further from the shell than the nautilus does, and so the rostrum could have been a support for the mantle, with the siphon well ahead of it.
Among the heteromorphs, constrictions that narrow the aperture are frequent, while among some like Sciponoceras the thing develops a curved hood that folds over the aperture so that if the head stuck out it would point at ninety degrees from the long axis of the shell. Clearly, such a thing couldn't have swum by jet propulsion.
Cheers,
Neale
I must confess I only just noticed your question about siphonal notches in Ordovician nautiloids and their relevance to jet propulsion in ammonites.
The bottom line is that while siphonal notches, that is, embayments in the anterior margin to allow the siphon to protrude freely, do exist in various fossil (and living) nautiluses, they aren't known from ammonites. Quite the reverse. Many ammonites have modified apertures sometimes with constrictions and collars (very common with heteromorph ammonites) or rostra (many Jurassic and Cretaceous true ammonites). The rostra are rather like long spikes sticking out the front. These are distinct from the well known "lappets" that form on the sides of the aperture, rather like ears.
In some species the rostra points forwards, curling slightly in keeping with coil of the shell (e.g. Diploceras) while in others it curves backwards and underneath like a mini-spiral hanging under the shell (e.g. Mortoniceras). Either way, if there was a siphon it might have laid across the rostrum but couldn't have curled underneath the animal to allow it to swim forwards (as Nautilus does today). One solution is that these animals had two siphons, one on either side of the rostrum. The alternative is that the animal extended far further from the shell than the nautilus does, and so the rostrum could have been a support for the mantle, with the siphon well ahead of it.
Among the heteromorphs, constrictions that narrow the aperture are frequent, while among some like Sciponoceras the thing develops a curved hood that folds over the aperture so that if the head stuck out it would point at ninety degrees from the long axis of the shell. Clearly, such a thing couldn't have swum by jet propulsion.
Cheers,
Neale