Ammonites Survived Three Mass Extinctions Before Going Extinct at End of Cretaceous
Posted on April 28, 2012
Ammonites lived on Earth for 300 million years. They successfully negotiated three mass extinctions, only to die out eventually at the end of the Cretaceous along with the dinosaurs. Scientists believe these marine cephalopods believed to be related to today's squid and nautiloids.
Ammonoids changed their reproductive strategy early on in the course of evolution. However, what was once a successful initial strategy may well have proved to be a fatal boomerang at the end of the Cretaceous, as an international team of researchers headed by paleontologists from the University of Zurich demonstrate in a study recently published in the science journal Evolution. Kenneth De Baets, a paleontologist at the University of Zurich, is pictured holding a fossil of a fully grown manticoceras (Late Devonian Period, Morocco), which was one of the largest ammonites of the Devonian Period.
At the beginning of their evolution, ammonoids had straighter shells, which, like other mollusks, they began to coil during the Devonian Period. Scientists can not yet explain this change, but the selection pressure in favor of more tightly coiled shells is believed to have sprung from the ammonoids' natural predators. As the scientists have now discovered, the shell change also affected the ammonoid embryos. A more tightly coiled shell may have provided increased defensive benefits against certain predators.
De Baets says, "In the oldest ammonoids, the embryonic shells were considerably bigger and coiled less tightly than in later forms."
Two other evolutionary trends that coincided with the more coiled shell trend. As the size of the embryonic shells shrank increasingly over time, the hatchlings also became smaller and smaller. In parallel, the shell size of fully grown animals increased and the animals became increasingly bigger as adults. Based on this, the researchers deduced that the number of offspring in ammonoids rocketed during the Devonian Period. This is confirmed by discoveries of substantial clusters of fossilized embryonic shells at the end of the Devonian Period and more recent deposits.
De Baets says, "The large number of offspring could have been the key to the rapid proliferation of the ammonoids in the aftermath of each mass extinction."
De Baets hypothesis is supported by the fact that precisely the groups with smaller, loosely coiled embryonic shells and proportionately fewer offspring died out in certain Devonian extinction events. This once successful reproductive strategy of many offspring appears to have turned against the ammonities at the end of the Cretaceous Period, because they all died out. Only nautiloids - which are characterized by large young and a small number of offspring - are still with us today.
A lack of food resources is another explanation given for the great ammonite extinction.
A research paper on the ammonite extinction was published in Evolution, International Journal of Organic Evolution.