Nautiloid? Ordovician, NT, Australia

[JesseKoz]

Hello everyone! I've been examining a fossil I found a short while back and wanted to try and confirm my suspicions that it is a Orthoconic Nautiloid. Interested to hear some opinions from those more knowledgeable than I. So far I've been struggling to find good resources describing the different species found in this formation, it seems the work of John Laurie should possibly be my focus. The diameter of the possible siphuncle seems unusually large and positioned in very close proximity to the outer shell. I seem to remember reading something about the siphuncle moving closer to the outer shell as the nautiloid ages.

 

The fossil was found in Maloney Creek, NT, Australia and comes from the Horn Valley Siltstone, early to middle Ordovician (487 Ma - 468 Ma).

 

Feel free to ask for any additional photo angles, measurements or further information on the location. I also have a number of specimens from the same location that are clearly straight-shelled nautiloids, but likely another species.

[Herb]

cool looking, might be a cephalopod

[JesseKoz]

Thanks @Herb, I now see I should have said class Cephalopoda rather than Nautiloid as the details I've picked up from the fossil could easily relate to a number of their subclasses.

 

Anyone have input as to what the extra chambers could be? I'm unable to find any examples of this feature being present in Ordovician Cephalopods. Possibly what appear to be extra chambers is just a change in density of the shell material at these locations? Also the third and fourth images display a lengthwise section of the fossil and along the length I see no evidence of individual septa. Possibly simply due to the fact only 2cm of material is exposed.

 

Hoping to get past just the class of the fossil.