There are a number of names floating around that are based on color or size
forms of N. pompilius; I think you will find that suluensis Habe & Okutani
is one of them. N. repertus is another. And until recently N. stenomphalus
was a doubtful species, but the animal is now known and is distinctly
different, as is the color pattern. Some specimens of N. pompilius resemble
N. stenomphalus in lacking umbilical plugs, either one or both sides. These
specimens may actually be hybrids. N. praepompilius is the only definite
fossil species yet known.
Allonautilus spp resemble Nautilus, but tend toward a tabulate venter and
have a much wider umbilicus with abutting whorl sutures; in N. macromphalus
the sutures are close to tangential. Allonautilus has a very strong and
undoubtedly not coincidental resemblance to Cenoceras, the
Triassic-Jurassic precursor, which is the founder-genus of Nautilidae.
There is one definite described fossil species of Allonautilus.
Siphuncular location varied among both nautiloids and ammonoids. Most
nautiloids have a siphuncle placed near or a bit below the whorl midheight,
but some (eg Solenochilus) were in contact with the ventral surface of the
whorl; that of Aturia, from Cretaceous to latest Pliocene, was dorsal,
almost touching the previous whorl. Most ammonoids had a ventral siphuncle,
touching or almost touching the venter; in the Devonian clymeniids it was
dorsal.
Planispiral cephalopod shell orientation terminology is thus:
-venter, ventral - the "outer" surface or edge of any whorl, full
circumference of every whorl; part of any whorl in direction away from
coiling axis (umbilical center)
-dorsum, dorsal - upper part of any whorl, ie part in contact with previous
whorl (if whorls are in contact!), full circumference of every whorl; part
of any whorl in direction toward coiling axis
The dorsal/ventral parts of the living animal/shell system have a definite
up/down orientation and are clearly NOT the same things as the
dorsal/ventral parts of the shell.
Evidence is building that ammonoids were not scavengers or predators, but
rather filterfeeders:
1) The lower mandible was modified into a hood, of either one unhinged
(anaptychus) or two hinged (aptychus) sections. This is known to have
sealed the shell aperture when the animal was retracted. Presumably the
upper mandible was lost. Therefore the animal had no hard jaws available
for biting.
2) Many genera, particularly in the Jurassic, developed highly modified
apertures at onset of full adulthood, sometimes constricting the aperture
down to two or three tight small openings (it is possible that these were
the females and that these stopped feeding at this point to brood eggs
within the modified shell, subsequently dying).
3) Heteromorphic ammonoids such as Hamites, Ptychoceras, Nipponites etc
were coiled in wierd shapes and could not possibly swim in any normal way
other than bobbing vertically; they must have been passive drifters.
Ammonoids, like most gastropods, had minute protoconchs (called
ammonitellas), and hatched as tiny larvae. Nautiloids, on the other
tentacle, begin life with a large cupshapedfirst chamber and grow to a
large size before hatching; Nautilus hatches, if I remember rightly, at
about 36mm diameter, with a multichambered shell. The large initial chamber
means that the first whorl cannot coil tightly enough to fully wrap around
the first chamber, leaving a small to large perforation in the center of
the umbilicus N. macromphalus may be imperforate). Those with very small
specimens at hand may note a slight constriction visible on both flanks,
strongest near the umbilicus; this is the nepionic constriction, formed at
time of hatching.
Both ammonoids and nautiloids seem to have had deep and shallow-living forms.
Nautiloids probably survived the end-Cretaceous event because they were
unspecialized in habit and could feed on carrion if necessary. Even if the
adults did not survive, their eggs, fixed firmly to sheltered seabottom,
develop very slowly; while all adults may well have been killed, enough
eggs remained incubating to allow a very few families to survive. The
specialized ammonoids, however, had planktic larvae which would have been
included in the plankton-dieoff; the specialized plankton-feeding adults
starved. Which is an awful shame, as these things, wierd as the animals
must have looked, must have been some of the most incredibly beautiful
things ever to have evolved. Amazing shapes with deeply-colored iridescence
not produced by any surviving thing. What I'd give for a time machine...
Andrew
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