FUNCTIONAL MORPHOLOGY AND EVOLUTION OF SWIMMING ABILITY IN PLANISPIRALLY COILED NAUTILOID MOLLUSKS

Both theoretical and experimental considerations suggest a strong relationship between shell form and swimming ability by jet propulsion in coiled cephalopods. The basic geometry of planispiral shells can be described by three dimensionless parameters: the whorl expansion rate (W), the umbilical diameter (D), and the shape of the aperture (S). Streamlining, the reduction of drag to improve velocity in capturing prey or avoiding predators for example, should be superior in tightly coiled involute conchs (low D) with narrow compressed apertures (low S). Stability and orientation of the conch, on the other hand, depend on the spacing between the centers of buoyancy and mass. If these regions are closely spaced, as in loosely coiled evolute conchs (low W and high D), low rotational and lateral stability and susceptibility to pinwheeling motion can result due to thrust generated by the expulsion of water from the mantle cavity. Among coiled members of the order Nautilida (Subclass Nautiloidea), highly streamlined taxa (D ≤ 0.33; S ≤ 1.0) and involute taxa (W > 2.5; D < 0.30) increased in incidence from approximately 10-20% of nautilid species in late Paleozoic time to 70-80% in the Cenozoic Era. In contrast, the incidence of evolute taxa (W < 2.5; D > 0.30) decreased from over 70% of species in the Devonian Period to less than 10% by mid-Mesozoic time. A more modest decrease is seen in the incidence of comparatively unstable, slowly expanding forms (W < 2), from about 27% of Devonian species to under 10% of Cenozoic species. These trends, along with the prevalence of large retractor muscle scars in fossil nautiloids, reinforce the view that propulsive capability is a significant adaptive theme in nautiloid evolution and that nautiloids maintained a scavenging or predatory lifestyle. Analysis of morphologic patterns in the Nautilida also confirms previous results that coiled nautiloids originated near a high D, low drag coefficient (C_D) adaptive peak and underwent a late Paleozoic radiation associated with the rise of the major nautilid superfamilies. After nearly dying out during the terminal Triassic mass extinction, nautiloids re-expanded somewhat in morphospace; their descendants, however, became generally stereotyped into involute lineages that differed only in ornamentation, suture complexity, and a few other features.