DEFENSIVE ADAPTATIONS, TRENDS, AND SURVIVAL IN POST-PALEOZOIC NAUTILOID MOLLUSKS

Spectacular adaptive radiations of durophagous predators, such as bony fishes, brachyuran crabs, and carnivorous snails, during the Mesozoic Era fundamentally altered the marine ecosystem. A major consequence of this so-called Mesozoic marine revolution involves escalatory trends in the architectural elaboration of defensive morphologies among prey. By analogy with gastropod mollusks, nautiloid conchs with strong external sculpture should be superior in resisting attacks by predators compared to smooth conchs. To determine if trends in increasing sculpture in post-Paleozoic nautiloids exist, species of the order Nautilida were sorted according to grades of ribbing and ornamentation. Although predation intensified during the Jurassic and Cretaceous periods, the percentage of armored forms decreased over the course of post-Paleozoic time, from 50% of Early Triassic species to less than 10% of Cenozoic species. Moderately to strongly ribbed species also decreased in frequency. Despite a modest Jurassic radiation, nautiloids failed to re-evolve ornamentation such as nodes, clavi, tubercles, and spines after the extirpation of the heavily sculptured tainoceratids during the terminal Triassic mass extinction. Nonetheless the incidence of fine to moderate ribbing increased in the Mesozoic Era, from under 10% of Triassic species to about 40% of Cretaceous species. Furthermore, very strongly ribbed species were present only during the Jurassic and Cretaceous periods. Possible explanations for the lack of a pronounced trend in increasing sculpture may indicate adaptive conflicts that offset its advantages. Strong sculpture, for instance, increases drag and weakens the shell against implosion, and is thus disadvantageous both for fast swimmers and deep-water dwellers. The rarity of heavily sculptured forms in the Cenozoic Era could therefore reflect a post-Cretaceous shift to deeper water refugia from shallow water habitats among the Nautilidae as well as the differential extinction of the morphologically specialized, rib-bearing cymatoceratids.