Phil said:Anyway, I don't see any reason why these giant ammonites could not have had thick arms like this, after all, the living chamber would be quite massive and able to support a large bulky animal. It's hard to imagine something this massive feeding on plankton, though one can imagine it hopping along the seabed hunting crustaceans. I'm sure the smaller forms were drifters feeding on plankton with thin delicate arms as you say, Jean.
Phil said:Andy, that is a fantastic specimen. Looks like you've got a very interesting collection back there at home. What is the plant stem fossil behind the ammonite you have there, a horsetail perhaps?
Phil;30212 said:I wonder if the thing was fully grown? The outermost chambers seem to be somewhat worn away which would make maturity a difficult thing to determine. Perhaps a fully mature specimen was even larger????
The specimen is fully adult. Note that the ribs coarsen and the last section of bodychamber has developed flanges; these are all that attach the last (detached) part-whorl to the pharagmocone. The width of these flanges is not known for certain; Graeme Stevens, the paleontologist who described the species, reconstructs them as being quite wide. The fossil is a steinkern; the fact that the flange bases are preserved AND that the edges of the flange-infillings are broken-off would indicate that Stevens is correct and that the flanges must have been wide; I'd estimate that they would have extended at least 15cm from the shell surface, and may well have wrapped at least halfway around the previous whorl.
I think that the detached part of the whorl may well be a true shaft, as with scaphitids. This would make the species a true heteromorph. No other such specimens are known, but then, though other L. taharoaense have been collected, I don't think that any are over 35cm diameter ie they are all microconchs (that is, If Lytoceras has micro/macros) and/or or juveniles.
Lytoceratids tend to be serpenticones ie they have subcircular to circular whorl-sections and the whorls only just touch one another; they are as evolute as possible.
I can't understand why the Museum of NZ ("Te Papa", translated from Polynesian as "the mud", is a typically irrelevant "Maori" title, the like of which every NZ governmant department gets... and which are NOT translations of the English titles) is calling it "the largest ammonite in the world"; certainly the paleontologists involced in their fossil display know that this is an outright untruth. But the museum PR people are interested in generating public interest, not in facts.
Like the Otago Museum in Dunedin, specimens in this display are not Museum of NZ specimens, but are on loan from elsewhere, in this case the Institute of Geological & Nuclear Research (IGNS).
I have seen this specimen, and it is impressive. It's unfortunate that a bit of steinkern has been lost from the point where the detached shaft of whorl meets the rest of the shell (it's been infilled with filler).
I don't think that any ammonoids had internal shells. Certainly the changes in ribbing and the shapes of heteromprphs would reinforce this. Scaphitids have a swollen final chamber which some workers believe to be brooding-chambers (what's the bet that some forms not only hatched their eggs in there, but brooded their young).
There is some evidence to suggest that the ammonite may have born a much closer resemblance to the coleoids (esp the octopus) than the nautiloids based on similarities in the radula. Personally, I suspect that there there was a great variety in size and shape of the soft-bodied parts adapted to differing lifestyles, but all based on the ten-armed plan. But no-one really knows for sure........yet.
Certainly reconstructions using Nautilus as a basis are fatally flawed. The shells are superficially similar... but that of Spirula is far closer to Nautilus than are ammonoids. Ammonoids may well turn out to be coleoidlike... and advanced at that. They cannot be coleoids, as they evolved from the nautiloids long before the coleoids did ie they are separate clades. The first known ammonoid is believed to be the orthoconic form Bactrites (which would indicate that anything NOt straight should be called heteromprph!). The first accredited heteromorphs were around in the late Triassic, and were unrelated to the later ones. Some heteromorphs belonged to genera which were comprised mainly by "ordinary" species ("orthomorphs"?). Coiling iteslf is not of generic importance in gastropods, and there is no reason why it should be with ammonoids either. Ammonoid taxonomy is a nightmare of genus-group synonyms in use; the Triassic is probably the worst, with no two authorities agreeing on supraspecififc taxonomy.
Well, I'm off in to-and-a-bit weeks to collect ammonoids (and other living & fossil molluscs) on Vancouver & Strait of georgia Islands... wish me luck!!
Oh, there's that giant "Titanites" near Fernie, BC... it would benefit esthetically by removal of the bodychamber, no? Anyone interested in helping me carry this part to the car...? (I'm not entirely joking here). I haven't seen the specimen, and it's a bit out of the way of my route... but anyone know how to get to it? I wouldn't mind seeing it. It really should have a shelter built over it, or be removed and placed in some institution.