| This work explores cephalopod life history traits and the evolutionary consequences of those strategies. In a systematic note based on re-analysis of both morphological and genetic data generated by previous research of various authors, I propose a taxonomic revision of extant nautiloids wherein only one genus, Nautilus, and two species, N. pompilius and N. scrobiculatus, are given formal recognition.; Reproductive strategies, sexual maturity, sexual dimorphism, and heterochrony are investigated in the context of a case study of the Middle Carboniferous ammonoid Arkanites relictus from northern Arkansas. Analyses indicate that, depending on what trait is examined and what shell character is used as the proxy for age of individuals, the sexual dimorphism in A. relictus was produced by acceleration, neoteny, or hypermorphosis plus neoteny.; Analysis of intra- and inter-specific variation of embryonic morphology in Middle Carboniferous ammonoids from northern Arkansas reveals there is little variation in protoconch and ammonitella size within populations and species. In addition, data from four Carboniferous ammonoid lineages demonstrate that embryonic shell size within species is relatively stable through time. These results suggest that some embryonic features may be useful in phylogenetic analyses.; To explain the disparity in taxonomic durations and species richness between major cephalopod groups, I develop the hypothesis that species in clades characterized by short stratigraphic ranges and high total diversity had a semelparous reproductive strategy (parental mortality follows mass spawning event), whereas species in clades with long stratigraphic ranges and low total diversity had an iteroparous reproductive strategy (repeated, isolated breeding events). The relationship between reproductive strategy and embryonic shell size is discussed, and original data collected from the Carboniferous of the southern mid-continent as well as data compiled from the literature are analyzed. Compared to ammonoids, nautiloid embryonic shells are roughly an order of magnitude larger on average, and nautiloid taxonomic longevities are about three to four times greater. Differences between the groups are statistically significant across all taxonomic levels, and these results support the hypothesis that reproductive strategy has played a major role in shaping the evolutionary history of the Cephalopoda. |
