Divergence time estimates for major cephalopod groups: evidence from multiple genes

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Abstract
This is the first study to use both molecular and fossil data to date the divergence of taxa within the coleoid cephalopods (octopus, squid, cuttlefish). A dataset including sequences from three nuclear and three mitochondrial genes (3415 bp in total) was used to investigate the evolutionary divergences within the group. Divergence time analyses were performed using the Thorne ⁄ Kishino method of analysis which allows multiple constraints from the fossil record and permits rates of molecular evolution to vary on different branches of a phylogenetic tree. The data support a Paleozoic origin of the Orders Vampyromorpha, Octopoda and the majority of the extant higher level decapodiform taxa. These estimated divergence times are considerably older than paleontological estimates. The major lineages within the Order Octopoda were estimated to have diverged in the Mesozoic, with a radiation of many taxa around the Cretaceous ⁄ Cenozoic boundary. Higher level decapodiform phylogenetic relationships appear to have been obscured due to an ancient diversification of this group. � The Willi Hennig Society 2006. The fossil record is inevitably an incomplete account of life that once existed on earth. Due to the nature of fossilization, organisms that possess hard parts are better represented in the fossil record than those that are soft-bodied. As a result, most coleoid cephalopod taxa rarely fossilize well. The Subclass Coleoidea is divided into two Superorders: Decapodiformes, currently con- taining the Orders Teuthoidea (most squids), and Sepioidea (cuttlefishes, Ram's Horn, pygmy, bottletail and bobtail squids) and the Octopodiformes, containing the Orders Vampyromorpha (vampire ''squid'') and Octopoda (octopuses). Most fossil coleoid cephalopods are known only from their highly reduced internal shell (gladius (chitinous and feather shaped) or phragmocone (conical and chambered)), which is often only partially preserved and very difficult to classify (Bandel and Leich, 1986;