Disparities In The General Differences Between Nautiloids And Ammonoids

[howard_l]

Disparities in the General differences between Nautiloids and Ammonoids

Thanks to the amount of money I had to spend recently to keep my two cars running and my Lady bravely taking care of her ill Mother in Ohio, I am unable to do any field work, at least till I get paid again. I am trying to come up with things to keep busy. I started doing some additional research on my favorite fossils Nautiloids.

One of the main problems that occur in some specimen found in the ages between the Devonian and the Cretaceous is how to tell the difference between Nautiloids and Ammonoids since they sometimes have the same general shape. The three most commonly used features to tell them apart, (fig. 1), are septal curvature (Fig.2), simple vs. complex septa, (the wall in between the chambers), (Fig. 3), and the location of the Siphuncle, (the tube that goes through the septa and connects the chambers), (Figs. 4 & 5). I have two nautiloid genus from the lower Mississippian that not only are from the same age but can be found in the same outcrop that do not conform to these general differences.

The first nautiloid is Subclymenia in the family Trigonoceratidae. Subclymenia does not have the typical simple suture that nautiloids have owing to the name clymenia (Devonian Ammonoid), (Figs. 6, 7 & 8). Later nautiloids in the Mesozoic and Cenozoic also do not have a simple suture. However, the siphuncle in Subcylmenia is very prominently shown in the middle of the septal wall as is the typical nautiloid (fig. 9).

The other nautiloid, genus Solenochellus, that displays a difference also includes these difference with a number of genus under the Super family, Aipocerataceae. The genus Solenochellus does display the typical simple suture of nautiloids but the siphuncle is found on the outer ventral portion of the shell as found in Ammonoids (Fig. 10, 11, 12 & 13) .

Well this took care of the majority of this week, now I have to figure out what to do next week.

[Auspex]

Well, so much for rules of thumb...Nothing is quite that easy, is it?

Nice bit of work, this: thank you!

[fossilized6s]

Thank you for doing this. You've cleared up some of my questions.

[BobWill]

Yes and some endocerids and actinocerids also have the same marginal siphuncles. At least we're not confronted with any ammonites having central siphuncles, not that I know anyway!

Even though the suture on that Subclymenia shows a single saddle and lobe absent in most nautiloids there is still not much folding like what we see in ammonoids. Even more confusing is some of the overlapping complexity of the patterns found in ammonoids when you make comparisons among ceritids, goniatites and ammonites. Maybe a cladogram would explain some of this.

Edited June 7, 2015 by BobWill

[Al Dente]

Most of us have seen modern Nautilus shells cut open displaying the septa. The septa look like a simple curve but if modern Nautilus were found as fossils, few of us would recognize them as Nautilus because they have a more complex suture than what we see in cross section. Here is a modern Nautilus that is illustrated in AK Miller's "Tertiary Nautiloids of the Americas" He melted paraffin and poured it into the shell then dissolved the shell in acid. Here is the result.

[erose]

Nice work. This will be a go-to bookmark in the future.

[howard_l]

Most of us have seen modern Nautilus shells cut open displaying the septa. The septa look like a simple curve but if modern Nautilus were found as fossils, few of us would recognize them as Nautilus because they have a more complex suture than what we see in cross section. Here is a modern Nautilus that is illustrated in AK Miller's "Tertiary Nautiloids of the Americas" He melted paraffin and poured it into the shell then dissolved the shell in acid. Here is the result.

AKMillerNautilus.JPG

That is a true internal mold! Hercoglossa and many other Cenozoic Nautiloids Displays it as well.

[BobWill]

Love the modern example. Actually all of the nautiloids I've found so far in Texas are internal molds with sutures similar to Miller's. One saddle and one lobe on the flank with sometimes more on the venter, some more steeply folded than others but only the ceratites and ammonites I find here have more folds in the saddles and lobes. My collecting is mostly limited to north-central Texas though.

[Wrangellian]

Interesting... I learned something here too: I had not noticed that the shell of nautiloids was thicker than ammonoids! I suppose the complex sutures and ribbing of ammos gives strength that allows them to be thinner.

I think I see a mistake though: in the 1st pic, you say the naut. septa are convex toward the body cavity - shouldn't that be 'concave'?

I'm not sure how you can say what direction ammo septa are curved, though, considering how complicated they are. I would need to see more examples to judge that.

[howard_l]

Interesting... I learned something here too: I had not noticed that the shell of nautiloids was thicker than ammonoids! I suppose the complex sutures and ribbing of ammos gives strength that allows them to be thinner.

I think I see a mistake though: in the 1st pic, you say the naut. septa are convex toward the body cavity - shouldn't that be 'concave'?

I'm not sure how you can say what direction ammo septa are curved, though, considering how complicated they are. I would need to see more examples to judge that.

The figure in question was not of my design, as posted on the figure it was taken off the internet, I just included it as an example of the criteria typically used by geologist to categorize and discern nautiloids from Ammonoids. I am definitely not an Ammonoid person but I am very good friends with a Paleontologist who is. There is a septal curvature in all Ammonoids, as they get more complex toward the Cretaceous it would be much harder to determine the direction of the curvature, it would be for me

. Fortunately for me I only deal with the Paleozoic.