The Origin and Relationships of Early Chondrichthyans

Chondrichthyan shes are probably the most successful of all shes if success is measured in terms of historical endurance. Indeed, they have survived the mass extinctions of the last 400 million years or so. They are essentially dened by a cartilaginous skeleton that is supercially mineralized by prismatic calcications (tesserae) and by the modication, within males, of mixopterygia (claspers) for the purpose of internal fertilization. It has been generally accepted that the Class Chondrichthyes is a monophyletic group divisible into two sister taxa, the Elasmobranchii and Holocephali, and that extant chondrichthyans (sharks, skates, rays, and chimaeras) are derivable from Mesozoic forms. Yet, how the extant forms relate to the distinctly more diverse Paleozoic forms and the relationship of the Chondrichthyes to all other shes are poorly resolved issues. Furthermore, some paleontologists currently question whether fossils attributed to chondrichthyans support a monophyletic class. The purpose of this chapter is to discuss the evidence for the origin, diversi-cation, and life histories of the early Chondrichthyes; to address trends in their morphological divergence and innovation; and to explore the possible relationships between fossil and modern forms. In a general discussion of relationships, we adopt the classication scheme for shark and shark-like shes put forth by Compagno (2001), as a consensus of the analyses of CONTENTS 1.1 Introduction ....................................................................................................................................................................... 3 1.2 On the Synapomorphic Chondrichthyan Characters: Tesserate Mineralization and Internal Fertilization by Male Claspers ...................................................................................... 4 1.2.1 Tesserate Mineralization ...................................................................................................................................... 5 1.2.2 Modication of Pelvic Girdle in Males to Generate Claspers ........................................................................ 5 1.3 Historic Evidence of Early Chondrichthyans ............................................................................................................... 6 1.3.1 Evidence from the Carboniferous ...................................................................................................................... 8 1.3.1.1 Carboniferous Communities and Chondrichthyan Adaptations ................................................... 8 1.3.1.2 Bear Gulch Limestone ........................................................................................................................... 9 1.3.1.3 Community Structure and Population Dynamics .......................................................................... 15 1.3.1.4 Segregation According to Age, Sex, and Reproductive Stage ....................................................... 16 1.3.1.5 Reproductive Strategies....................................................................................................................... 16 1.3.2 Upper Carboniferous and Permian Record .................................................................................................... 17 1.4 Theorized Relationships between Recent and Fossilized Forms ............................................................................ 18 1.4.1 On Holocephalan Origins ................................................................................................................................. 18 1.4.2 On Elasmobranch Origins ................................................................................................................................. 18 1.5 Cladistic Evaluation of Paleozoic Chondrichthyan Relationships and Comments on the Higher Systematic Groupings of Chondrichthyans .................................................................. 19 1.5.1 On the Higher Systematic of the Early Chondrichthyes ............................................................................... 20 1.5.2 Other Concluding Remarks on the Origins of Chondrichthyans, Trends in Chondrichthyan Evolution, and on Characters of the Class ...................................................... 20 Acknowledgments ................................................................................................................................................................... 22 References .................................................................................................................................................................................. 22 Appendix. Characters and States for the Cladogram of Figure 1.1 .................................................................................. 26 Compagno (1984), Shirai (1996), and de Carvalho (1996). The classication scheme used to describe the relationships of all Chondrichthyes is that originally developed in Lund and Grogan (1997a,b; 2004a,b) and Grogan and Lund (2000), but subsequently modied given new nds (Grogan and Lund, 2008, 2009, 2011). Details of this scheme have been rened based on discussions of higher chondrichthyan systematics with Dr. Joseph Nelson (University of Alberta), author of Fishes of the World (2006).

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