Ammonite And Nautilus Question

[Bad Posture]

I am curious as to the reason why ammonoids went extinct and the nautiloids did not. I always hear that ammonites were rapidly evolving creatures that were everywhere during the Cretaceous.

Does anyone have any insight into this? What traits do the nautlioids have that enabled them to survive until today? How were the ammonoids ill-prepared for the event or events that caused the K/T mass extinction?

[Wendell Ricketts]

If you want the longer and more complex answer to the question, which I don't think I'm smart enough to sum up here in any way that would make sense, I'd suggest a book I just finished: In Search of Nautilus: Three Centuries of Scientific Adventures in The Deep Pacific to Capture a Prehistoric, Living Fossil by Peter Douglas Ward. In several chapters and passim he reviews that question in a variety of contexts, including an analys of how what we don't know about the evolution of these creatures complicates matters considerably. (Hint: it apparently had a lot to do with reproductive strategies.) One detail I remember is that ammonoids had been in bad shape for a long time, and only a handful of species made it as far as the Cretaceous (and none made it out). There are several used copies of the book on amazon for not much, btw, or your local library might have it.

W.

[DLB]

Maybe the depth that the animal lived had something too do with it! I know a lot of the liveing species of nautilus live in pretty deep water:)

Edited April 10, 2012 by DLB

[Paleoc]

I suspect that has much to do with it. Many of the shark species that survived the KT event were deeper water species, while many of the near shore species went extinct.

[BobWill]

Maybe there's several factors invloved. Here's a couple possibilities to add. During the Mesozoic Nautiloids were less diverse than Ammonoids and perhaps this allowed more opportunities for reproduction when things got tough. Another idea I've considered is thickness of shell. Simpler sutures suggests less need for re-inforcement by septa folds. Thicker shell might mean surviving bites from smaller predators.

Edited April 10, 2012 by BobWill

[Bad Posture]

Interesting viewpoints from everyone.

I took a conservation biology class last year and the professor hammered down the idea that genetic diversity leads to a more healthy and well-equipped species that is more apt to withstand environmental change. Or, a species with more genetic diversity has a better chance of surviving when compared to a similar species that is very specialized and homogenized genetically.

The shallow water theory makes a lot of sense though, seeing as how most marine creatures in Texas from the Mesozoic thrived in warm shallow seas. These shallow waters would be the first to experience environmental change, and would feel the effects most prominently. A large asteroid impact could have rendered these seas anoxic, or the gradual tectonic drift of the continents could have just wiped out the habitat that is crucial to the ammonites.

Most likely, Bob is right, and there are several factors involved.

[mikeymig]

Not everything that has ever lived on this planet is preserved in its rocks. We have no idea what Ammonites ate and it may have been something that died out or became so few in mass as to not being able to sustain a viable population. It’s true that the number of Ammonite species was on the decline but, their numbers in the fossil record during the Cretaceous suggest that they were still a dominate force in the sea. I’m one of those people who believe that MOST extinction is a result of climate change that occurs to fast for some species to adapt.

I love your user name...Bad Posture.

mikey

[Ludwigia]

Another factor to add to the above mentioned ones is that the Nautiloids were not so specialized as the Ammonoids and therefore were more flexible and more able to adapt to a continually changing environment.

[Auspex]

Here is my take on it:

The ammonite's adaptation that allowed it to out compete the nautiloids in the paleozoic was faster reproduction and faster growth (making their population less vulnerable to recently evolved predators (the fish). The strong implication is that ammonites had a higher metabolic rate, which is also supported by their supercharged rate of speciation.

When predators evolved in the Mesozoic that could easily crack the shells of adults, both ammonites and nautiloids responded initially by fortifying their armor, but it was an arms race that could not be won. A retreat to the dark depths was the only refuge, and only the nautiloids, with their slower metabolism, were able to take advantage of the sparse resources there.

[mikeymig]

I dont know about that last point you made Auspex "A retreat to the dark depths was the only refuge, and only the nautiloids, with their slower metabolism, were able to take advantage of the sparse resources there." We know today that nautiloids and other cephalopods (squids) hangout in deep water then come to the surface at nite to feed. I cant see a nautiloid living in the depths of the ocean forever like some exile feeding off detritus. I think they would have evolved some bizarre form by now like the vampire squid if they stayed in the depths. The red markings on modern nautilus shells may be an adaptations to deep water living but these animals are predators and need to go into the shallows every nite to feed.

mikey

[Northern Sharks]

I write a column for my club's newsletter about a different type of fossil every month. In the November 2011 issue, I wrote about ammonites (and ammolite) and wrote about why ammonites died out and nautiloids didn't. While it isn't the most scientific, it is more general info for non-fossil people, here is a link to the article

http://www.kawartharockandfossilclub.com/news.htm

[Auspex]

I dont know about that last point you made Auspex "A retreat to the dark depths was the only refuge, and only the nautiloids, with their slower metabolism, were able to take advantage of the sparse resources there." We know today that nautiloids and other cephalopods (squids) hangout in deep water then come to the surface at nite to feed. I cant see a nautiloid living in the depths of the ocean forever like some exile feeding off detritus. I think they would have evolved some bizarre form by now like the vampire squid if they stayed in the depths. The red markings on modern nautilus shells may be an adaptations to deep water living but these animals are predators and need to go into the shallows every nite to feed.

mikey

That might have been a lifestyle whereby the ammonites could not sustain their higher metabolism. The Nautilus grows very slowly (especially for a marine organism), and invests heavily in a small number of large eggs, while the ammonites had enjoyed ascendancy by casting vast numbers of eggs to the sea winds (insuring that more survived the most vulnerable juvenile stage). That particular adaptation probably worked against them when they were forced into areas that provided less opportunity to fuel up, while the pre-adapted nautiloids were able to squeak by.

[FossilDAWG]

My understanding of this issue is in agreement with the reproductive strategy discussed by Auspex, and by Northern Sharks in the article he linked. I'd just add that we can get an idea of reproductive strategies by comparing the size of the initial chamber of nautiloids and ammonites. In Mesozoic nautiloids the initial chamber (which represents the shell the animal had when it hatched from the egg) was relatively large, suggesting that these animals invested in producing relatively few eggs that were well provisioned, resulting in large juveniles with a high survival rate (classic K-selection strategy). This is the reproductive strategy of modern Nautilus as well. While we can't directly know the reproductive strategy of the ammonites, the initial chamber is invariably small, suggesting a strategy of producing a lot of small eggs (R selection). These juveniles would have had less reserves carried over from the maternal provisions, and so would have had to be more active in foraging for themselves. These differences probably explain why nautiloids are never as abundant as ammonites where they occur together. On the other hand, the slow growth/slower metabolism/greater investment in provisioning eggs seems to have been enough to get them through the ecosystem crash at the terminal Cretaceous catastrophe. This is similar to other extinction patterns at the same time, where predators and animals that required a lot of resources (large herbivorous dinosaurs) didn't make it, but species with slow metabolisms or that could survive as scavengers (such as turtles, crocodiles, etc) show little extinction across the boundary. I'd guess that birds and mammals that made it did so because their food resources (insects, seeds, etc) were not greatly affected either.

Don

[FossilDAWG]

Oh and everybody should check out the excellent series of articles Northern Sharks has written in the newsletter for his paleo/mineral club (http://www.kawarthar...ub.com/news.htm). Great job!

Don

[Northern Sharks]

Oh and everybody should check out the excellent series of articles Northern Sharks has written in the newsletter for his paleo/mineral club (http://www.kawarthar...ub.com/news.htm). Great job!

Don

Why thank you kind sir. I started with the trilobite article in Mar 2011

Edited April 10, 2012 by Northern Sharks

[Wrangellian]

I would add that the diagram at the top should say "Ammonitic - Upper Triassic to Cret." The way I understand it is the ammonites appeared in the Upper Tri in North America and in the Lowermost Jur in Europe.

BTW I think I know where that large ammo featured in the article that follows NS's article ended up.. see the wikipedia article on 'ammonite'!

Edited April 11, 2012 by Wrangellian