Why all the Ammonoid Fossils


Staff member
Moderator (Staff)
Nov 19, 2002
somewhere under the desert sky
Alot of ammonoids died in an ancient sea that is now the desert of the western US. I have gathered some of the evidence and posted it to my website.

Anasibirites Beds

More information about ammonoids can be found in Phil's article


Have a look at the evidence and help me try to figure out why they all got where they are. There are alot of pictures so unless you have a fast internet connection it may take awhile.

Have Fun!
Hmm........if they died during spawning then I doubt you would see such disparate sizes of ammnoids as they should all be adults. Do you think you may have juveniles in there?

Toxic environment? Possibly. I wonder what could have caused it?

Washed in? Possibly. If so, I would anticipate that fragments of other creatures might be washed in too. Any traces of these?

What do you think, Kevin? (After the poll finishes, that is!)
This is fascinating. While I love the landscape of the southwestern US, I never imagined that articles about rocks would interest me. I was tempted by "condensed beds" only because it calls up a picture of bunk beds collapsing, but I voted for the toxic environment because it is a very wide brush: colossal meteors, algae, and bad weather could all qualify. Could climate change create a "toxic" environment? If you've had heatstroke, you might think so.

Kevin, can you tell us about what else you've found at this site? We there other kinds of animal or plant life? Any theories about what might have been there but which didn't fossilize? Why am I surprised that this is more interesting to me than the work I should be doing!?

The only other fossils I can see in the beds are small gastropods less than 2mm high. Other fossils found in other strata of the thaynes formation in the study area are gastropods, bivalves, scaphopods, echinoids, ophiuroids, crinoids, and sponges.

Most of the grains in the Meekoceras beds and other limestone beds in the area are small broken pieces of mollusc shells and echinoderms, the matrix is a sparite, making a Bio-sparite.
The grains in the anasibirites beds are alot smaller, the matrix is a micrite, making a Bio-micrite.

Ammonoids are usually dimorphic, a small microconch (male) and a larger macroconch (female), sometimes ornament on the shell differentiates male and female, a robust, strongly ornamented (male), and a gracile, smooth (female) shell, both about the same size. But as Phil mentioned the adults would all be about the same relative size.

But what if you had a polymorphic specie? Just a thought! I have to do a lot more cutting and sectioning to see if all the fossil ammonoids in the beds are adult, or a family grouping (theres another thought, ammonoids rearing their young).

It seems that any kind of toxic environment would not only kill the ammonoids, but every other critter swimming in the sea at that time and that place. Why only ammonoids? Why only ammonoids fossilized? Why only one basic shape of ammonoid? Some of the ammonoids in the lower beds occur in beds above the anasibirites beds, so they would have been around at that time, maybe just not that place.

Remember, this took place right after the End-Permian Mass extinction event (3my is a very short time geologically). At least 80% of all life on earth was gone. What ever caused that extinction could still be affecting the worlds environment. At any rate there was a great diversification taking place, all those empty niches being filled.
Some new evidence has come to my attention about the condition of another form of life that lived at the same time as these ammonoids. It seems that gastropods were having a tough time of it.

Early Triassic Gastropods (abstract)

Sinbad Limestone microgastropods

If ammonoids made it through the End-Permian mass Extinction "semi-easy" (easier than the gastropods) I wonder why they didnt get through the KT event (or interval), the gastropods did.

I still havent voted in this poll as I am still trying to figure it all out :heee:
the more I study earth events, the more confusing it becomes...so many wheels within wheels...
The toxic bloom theory is good,as is the washed in, but like Phil said, where are the rest of the other animals?
( Perhaps the old gods had a ammonite-boil, and threw all the shells into one spot?)
If the cause was riverine, it seems that more shells would be crushed...
Of course, I really know little about ammonites, but the arguments and discussions attended regarding archosaurs are somewhat similar..
Haven't voted yet, either. Confusing in a great way..
Perhaps a massive bloom of red algae? This could have been caused by an earth trauma that overloaded the water in certain, specific areas...
Good luck Kevin. You'll be just great. I hope you enjoy the experience as much as the audience enjoys listening to you!

Pity we can't pod-cast your lecture here. Or can we, Tony?

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