ABSTRACT

Introduction Hagsh are regarded as the most primitive of the vertebrates. Current thinking places them at the beginning of the vertebrate phylogenetic tree and indeed places them as prevertebrate chordates rather than true vertebrates (Rasmussen et al., 1998; Cobb et al., 2004; Janvier, 2007), although recent molecular work has reopened this debate (Heimberg et  al., 2010). In particular, Heimberg et  al. (2010) have indicated that the cyclostomes (hagsh and lampreys) are a monophyletic group and commented that hagsh show a loss of vertebrate characters. This is a signicant divergence from the accepted philosophy because much of the research on hagsh hearts has focussed on the premise that hag-sh are at the very beginning of the vertebrates and that any changes (especially between hagsh and lampreys) represent forwards rather than backward steps in vertebrate evolutionary history (Forster, 1998; Farrell, 2007).