ABSTRACT

Introduction Fossils of myxinoids and other cyclostomes have wide ramications for our understanding of vertebrate evolution. Not only can they provide a timescale of events (e.g., calibrating molecular clocks, timing divergences, and deep splits), but they can also bring light to bear on the nature of evolutionary transitions. Have hagsh undergone some form of “degeneracy”? Do cyclostomes exhibit adaptations or radiations that might be associated with the complex sequence of genome changes? What are the relationships of cyclostomes to Early Paleozoic armored jawless vertebrates, or “ostracoderms”? The current phylogenetic interpretation of cyclostomes as monophyletic (Delsuc et  al., 2006; Peterson et  al., 2008; Heimberg et al., 2010) further underscores the need for a clearer picture of the fossil record. Although the gap has narrowed, the morphology of extant cyclostomes is not currently in accordance with interpretations of cyclostome monophyly as supported by extensive molecular evidence (Heimberg et al., 2010, but see reservations about the use of micro-RNA data expressed by Thomson et al., 2014; Dunn, 2014). This discord only serves to emphasize the necessity for closer scrutiny of the cyclostome

Introduction ...................................................................................................... 73 A framework for fossil cyclostome interpretation ...................................... 74 Fossil hagshes ................................................................................................. 77 Other possible stem and crown-group fossil cyclostomes ......................... 79 “Ostracoderms”: Bony cyclostomes or jawless stem gnathostomes? ....... 85 Conclusion ........................................................................................................ 87 References ......................................................................................................... 88

fossil record; the long evolutionary history of these groups (potentially the entire Phanerozoic) means that the morphology of extant forms might not be informative about deep splits. Fossils are needed to reconstruct the morphology of common ancestors and to overcome possible long-branch attraction. This problem is not trivial in the case of cyclostomes, however, as living hagshes and lampreys are completely devoid of a mineralized skeleton. It has been long debated whether this condition was primitive or derived, notably in the context of their presumed “degeneracy,” and their presumed relationships to “ostracoderms” (Stensiö, 1927). The absence of a mineralized skeleton also makes it highly improbable that they can be preserved as fossils.