9614285 Saunders Nautilacean cephalopods include more than a score of genera and hundreds of named species spanning almost 230 million years (Triassic-Recent). Their history and phylogeny have been thought to be well understood, and their systematics have remained relatively unchanged for ~50 years, during which three tenets have remained unchallenged: 1) nautilaceans were evolutionarily conservative, showing little morphological change since the end of the Triassic; 2) most genera can be differentiated by only one or two characters; and 3) the last-evolved member is extant Nautilus, having evolved in the Pleistocene or Pliocene. In the last decade, however, new information suggests that a radical re-evaluation of both the phylogenetic interpretation and the classification of the group are in order. It now appears that post-Jurassic nautilacean species have been vastly over-slit; new fossil finds show that the genus Nautilus dates back at least to the middle Cretaceous; recent investigations demonstrate several new characters that will aid in the cladistic evaluation of the group; and preliminary phylogenetic analysis of the Nautilaceae stands in marked contrast to past interpretations. We propose to develop a suite of morphologic characters that will permit us to undertake cladistic analysis of the group, in order to develop understanding of evolutionary relationships, and to assess the validity of named fossil taxa in the light of our understanding of variation and species-and generic concepts in extant taxa. We will use the presence of shared derived characters to develop a classification system of the group, based on evolutionary relationships, rather than morphological similarity. In addition, we propose to use principal components analysis (PCA) in order to: a) quantify and to track morphologic variation and diversity through time; b) determine whether (which) particular shell morphotypes were preferred and the relationship between theoretical and occupied morphospace over an extremely long time span (>200 my); and c) assess how these morphologies responded to biotic crisis, focusing on the terminal Triassic- and Cretaceous extinction events. The results of our investigation will bear directly on understanding the evolutionary history of this particular lineage, one of the longest ranging of molluscan clades, extending from the Recent to the Triassic (~230 million years). The results will have bearing on the interpretation of macroevolutionary change in morphologic diversity and richness, and to morphologic responses to biotic crisis, on a scale of nearly 200 million years.
