taxonomic rank of ammonoidea

John M

Registered Senior Member
Perhaps this belongs in Earthscience / paleontology, but anyway.

Should ammonoids -- externally shelled cephalopods with convex septa and sometimes intricately crenelated sutures - be given the same taxonomic rank (Subclass) as the once diverse Nautiloidea --older shelled cephalopods with concave septa and generally smooth septa, or should ammoids be consisdered simply a highly modified superorder within the nautiloids. :shrug:
Both are in different subclasses now, Ammonoids (Subclass: Ammonoidea) and Nautiloids (Subclass: Nautiloidea), what are your arguments for reclassifying them?
classification of ammonoids

As a researcher of fossil cephalopods -- nautiloids and ammonoids I'm aware of their respective rank as subclasses, which seems quite reasonable. I'm not proposing ammonoids be down graded, just bringing up what might be a taxonomic dilemma; are they sufficiently different and evolved to be of equal rank as their ancestors or -- following the limitations of cladistics -- are they simply a group within the nautiloids. While I can see an argument for the latter, to me ammoids with their convex septa are sufficiently different to be a separate subclass, leaving nautiloids quite acceptably a paraphyletic group.
Nautiloid article in TONMO

Not really, Walter, it's a pretty good article that gives a good summary of early nautiloids with good illustrations. Keven's pictures remind me of some specimens I've worked with at the New Mexico Museum of Natural History and Science. Reminds me, I need to get back on TONMO and reestablish contact.
From a dim memory early ammonites had very simple sutures, as simple or almost as simple as the nautiloids. If that is correct would it not remove that distinction between the two
I learned something new, too. The article referenced the faster spin rate of earth circa 500 mya (21 hours versus 24 hours nowadays), but it also referenced a much shorter orbital period for the moon (8 days, versus 28 days nowadays), based on striations for each monthly cycle in ammonoid fossils. How huge would those tides have been with the moon so much closer?

And, what was the moon's orbital period some 3,000 mya during the prokaryotic era? This would have been a major daily event, with the earth's spin rate even faster also (on the order of a dozen hours?)

In any event, calamari is a great food these days, so I'm glad it came about as it did. I really like the cuttle-fish which have those rapidly changing skin colorations. Can you imagine what that would be like with vertebrates had that capability been evolved! But no, we got scales, feather and hair.