A nautilus at the Birch Aquarium. Photo by the author.

The Secret of Ammonoid Success

ByRiley Black
May 02, 2012
7 min read

A few weeks ago, during a trip to the Scripps Institution of Oceanography’s Birch Aquarium, I bumped into a nautilus. More or less. Admittedly there was an aquarium pane and many gallons of water between me and the cephalopod, but I was still surprised to see the archaic creature bobbing along in the dark tank.

I had seen photos of the shelled mollusks plenty of times, but seldom the actual animals. I hate the term “living fossil” – the phrase is more of a subjective judgment of a creature’s place in nature than a useful descriptor – but, I have to admit, the nautilus did look as if it was a visitor from a more remote time. And, as I stood watching the creature, the coil-shelled cephalopod reminded me of its extinct cousins, the ammonoids. Nautiloids survived to the modern era as a shadow of their former diversity, but the last of the equally strange ammonoids were wiped out in the same extinction which eliminated the non-avian dinosaurs. Such a shame. Ammonoids had a long and successful tenure in the world’s oceans, spanning about 300 million years, and a new paper by Kenneth De Baets and colleagues outlines the evolutionary conditions of the great ammonoid explosion.

A prolonged explosion – how different forms of marine life evolved, leading up to the Devonian Nekton Revolution. From Klug et al., 2010.

Chances are that you have heard of the Cambrian Explosion before – the fantastic, rapid flowering of animal body forms that started around 530 million years ago. But this wasn’t the only major evolutionary radiation in the deep past. During the Great Ordovician Biodiversification Event, which kicked off around 488 million years ago, many of the weirdo Cambrian lineages disappeared while what we think of as “modern” groups of animals became established and took up open-ocean niches. And then, around 416 million years ago, there was another evolutionary pulse. Recently dubbed the Devonian Nekton Revolution, this drawn out series of changes saw numerous free-swimming organisms take up residence in the water column – creatures referred to as “nekton” by marine biologists. Along with jawed fish and mollusks with planktonic larvae, ammonoids were a major part of the nekton revolution. They floated and jetted through the world’s oceans until about 66 million years ago.

But what allowed ammonoids to proliferate so rapidly and hang on for so long? These cephalopods survived three mass extinctions, including the worst ecological disaster of all time at the close of the Permian, around 250 million years ago. De Baets and co-authors hint that ammonoid success may have been tied to the way the cephalopods reproduced and grew up.

Among the earliest forms, the shells of embryonic ammonoids were large and were not tightly coiled. Gravid females of the earliest ammonoid species probably had only a few, relatively vulnerable offspring at a time. As ammonoids evolved, however, multiple lineages were adapted to have more tightly-coiled shells, to the point where even the embryonic creatures began to be more snugly coiled. More than that, De Baets and colleagues found that there was an increasing disparity in size between embryonic and adult ammonoids. As adult body size increased, embryos became smaller and smaller. These changes allowed female ammonoids to carry ever-more offspring. Based on shell size and aggregations of embryonic ammonoids found in the fossil record, paleontologists have estimated that some Devonian ammonoids may have carried more than 220,000 eggs at a time.

An outline of how ammonoid shell shape changed during the Devonian. As adult body size increased, embryonic ammonoids became smaller and more tightly coiled. Modified from De Baets et al., 2012.

Tracking a pattern is one thing. Explaining why the changes occurred is quite another. For the moment, exactly why embryonic ammonoids downsized and became more closely-coiled is a mystery. Predation is one possible answer. As De Baets and colleagues point out, ammonoids weren’t the only creatures undergoing an evolutionary explosion during the Devonian. New forms of jawed fish, crustaceans, and even other cephalopods might have fed on ammonoids at various life stages. (And parasites also made themselves comfortable in ammonoid shells.) A more tightly-coiled shell is stronger and provides better protection than a loosely-coiled one. In the eat-or-be-eaten world of the Devonian seas, lovely ammonoid shells might have been one result of an evolutionary arms race.

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Unfortunately, the idea that predation drove changes to early ammonoid shells is impossible to directly test, but, whatever the cause, the selection pressure was powerful and widespread. Several ammonoid lineages independently evolved small, tight-coiled embryos, and more distantly-related Devonian mollusks also developed tighter coils. There seems to have been a common reason for snug shells in the Devonian.

Whatever pressures altered the natural history of ammonoids, however, the end result may have made the cephalopods more resilient to major catastrophes. By the late Devonian, just before one of the Big Five mass extinctions, ammonoids were capable of producing massive amount of tiny, strong-shelled offspring. While many individual animals might die during a mass extinction, any survivors would be able to rapidly fill the vacated habitats with swarms of offspring, forming the foundation for future evolutionary alterations. Ammonoid evolution during the Devonian may have made the creatures extinction-resistant.

But there are no ammonoids left. The last disappeared around 66 million years ago. Ammonoids persisted through the worst of the world’s extinctions, yet ultimately succumbed to the last great catastrophe on record. And nautiloids, with a less prolific reproductive strategy, somehow outlasted their cousins. The nautiloids have some survival secret we don’t yet fully understand. As I looked at the pinhole eye of the Scripps nautilus, I wished that I could ask how its kin persisted while the ammonoids died, but I knew that the cephalopod had nothing to say to me.

References:

De Baets, K., Klug, C., Korn, D., & Landman, N. (2012). EARLY EVOLUTIONARY TRENDS IN AMMONOID EMBRYONIC DEVELOPMENT Evolution DOI: 10.1111/j.1558-5646.2011.01567.x

KLUG, C., KRÖGER, B., KIESSLING, W., MULLINS, G., SERVAIS, T., FRÝDA, J., KORN, D., & TURNER, S. (2010). The Devonian nekton revolution Lethaia, 43 (4), 465-477 DOI: 10.1111/j.1502-3931.2009.00206.x

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