Ammonoidea - Extinction

Extinction

The extinction of the ammonites, along with other marine animals and non-avian dinosaurs, has been attributed to a bolide impact, marking the end of the Cretaceous Period. Regardless of what effect an impact may have had, many of these groups, including ammonoids, were already in serious decline. Previously, ammonoid cephalopods barely survived several earlier major extinction events, often with only a few species surviving from which a multitude of forms diversified.

Eight or so species from only two families made it almost to the end of the Cretaceous, the order having gone through a more or less steady decline since the middle of the period. Six other families made it well into the upper Maastrichtian (uppermost stage of the Cretaceous), but were extinct well before the end. All told, 11 families entered the Maastrichtian, a decline from the 19 families known from the Cenomanian in the middle of the Cretaceous.

One reason given for their demise is the Cretaceous ammonites, being closely related to coleoids, had a similar reproductive strategy in which huge numbers of eggs were laid in a single batch at the end of the lifespan. These, along with juvenile ammonites, are thought to have been part of the plankton at the surface of the ocean, where they were killed off by the effects of an impact. Nautiloids, exemplified by modern nautiluses, are conversely thought to have had a reproductive strategy in which eggs were laid in smaller batches many times during the lifespan, and on the sea floor well away from any direct effects of such a bolide strike, and thus survived.