NAUTILOID MACROEVOLUTION: AN INTERPLAY OF ADAPTIVE SHELL FORM, PREDATION, AND SPECIES SELECTION

Adaptive breakthroughs in shell morphology established the course of nautiloid evolution early in the Paleozoic Era. Nautiloid mollusks probably arose via the paedomorphic retention of an ancestral monoplacophoran's planktonic larval stage later in ontogeny. The chambered phragmocone initially solved the problem of buoyancy control and allowed a rapid Cambro-Ordovician expansion into a largely empty adaptive zone above the seafloor. Straight or curved shells, possessing heavy apical counterweights in the form of calcareous siphuncular or cameral ballast, represent an evolutionary grade that improved upon the mobility of these archaic nautiloid stocks by assuming a horizontal, aperture-first swimming orientation. By Late Devonian time, coiled shells predominated and conferred superior maneuverability and mechanical strength, as well as more efficient buoyancy regulation, compared to non-coiled forms. This remarkable proliferation of planispiral nautiloids resulted from the great radiation of Nautilida, the polyphyletic origins of coiling in Tarphycerida and Oncocerida, and the slow decline of other nautiloid orders. Only two living genera, Nautilus and its descendant Allonautilus, survive in deep fore-reef slopes away from shallow marine habitats inhabited by modern teleost fishes. Transitions from curved to compactly coiled shell types are best interpreted as abrupt saltations without any intermediates, while reversions from coiled shells to secondarily uncoiled shells, such as in the Lituitidae, are typically gradual. Most explanations for these patterns invoke interspecific competitive interactions or large-scale tectonic alterations of sea level and crucially overlook their intimate relation to species selection, the primary source of morphologic trends in phylogeny. Favored species flourish, but those with particularly vulnerable character states experience lower rates of speciation and/or shorter species longevities than average. Evidence from the fossil record, including direct empirical documentation of punctures and crush marks in both straight and coiled conchs, suggests that durophagous predation was likely the major ecological agent of species selection, which persistently shaped the functional morphology of the nautiloid shell over the past half billion years.