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Later evolution of primitive agnathous fishes
Although much has been written about possible descendants of the early agnathans, additional discussion of the interrelationships of these primitive groups, mainly because authorities disagree as to where the relationships lie. Different authors consider different characters as ancestral, derived, or convergent, and consequently arrive at different conclusions about relationships between and among jawless and jawed forms. One interpretation gaining acceptance, and the one presented here, is summarized in Fig. 11.6. For an historical overview of this controversy, the reader is referred to Jarvik (1980), Carroll (1988), Forey and Janvier (1993), Long (1995), Janvier (1996, 2001), Maisey (1996), Forey (1998), Donoghue et al. (2000), Clack (2002), Pough et al. (2005), and Nelson (2006).
One view of relationships among early agnathous fishes, modern jawless forms, and jawed vertebrates. Notable here is the stem or sister position of cephalochordates relative to all craniates, of conodonts relative to all jawless vertebrates except lampreys, and of osteostracomorphs (Osteostraci, Galeaspida, Pituriaspida) relative to jawed fishes. Major geological time periods are given at the top of the figure, with abbreviated subdivisions immediately below. The time scale is millions of years before present. Most groups depicted are discussed in the text. From Donoghue et al. (2000), used with permission.
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