![]() Based on this, the researchers deduced that the number of offspring in ammonoids rocketed during the Devonian Period. In parallel, the shell size of fully grown animals increased and, on the whole, the animals became increasingly bigger. There were two more evolutionary trends that coincided with the increasingly more tightly coiled shells: The size of the embryonic shells shrank increasingly over time - the hatchlings became smaller and smaller. "In the oldest ammonoids, the embryonic shells were considerably bigger and coiled less tightly than in later forms," explains Kenneth De Baets, a paleontologist at the University of Zurich, summing up the latest findings. As the scientists have now discovered, the shell change also affected the ammonoid embryos. The selection pressure in favor of more tightly coiled shells is believed to have sprung from the ammonoids' natural predators. The precise reason behind this change is unknown. 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.Īt the beginning of their evolution, ammonoids had straighter shells, which, like other mollusks, they began to coil during the Devonian Period. ![]() ![]() Ammonoids changed their reproductive strategy early on in the course of evolution. They successfully negotiated three mass extinctions, only to die out eventually at the end of the Cretaceous along with the dinosaurs: Ammonoids, or ammonites as they are also known, were marine cephalopods believed to be related to today's squid and nautiloids. For 300 million years, they were the ultimate survivors.
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