6. p. 99Origin and Radiation of Chondrichthyans
- F. Harvey Pough, F. Harvey PoughProfessor Emeritus, Rochester Institute of Technology
- William E. Bemis, William E. BemisProfessor of Ecology and Evolutionary Biology and Faculty Curator of Ichthyology, Cornell University
- Betty McguireBetty McguireSenior Lecturer, Cornell University
- , and Christine M. JanisChristine M. JanisProfessor Emerita of Biology, Brown University, USA, Honorary Professor of Earth Sciences, University of Bristol, UK
Abstract
This chapter looks at chondrichthyans, which are not primitive fishes, but are highly developed. The first definitive chondrichthyan fossils came from the Early Devonian period, although chondrichthyan-like scales are known to have existed from the early Silurian, and possibly the Ordovician periods. The chapter highlights the two main branches of chondrichthyans which reflect a division which had occurred by the start of the Devonian. Elasmobranchii includes the extant neoselachians and extinct relatives, while Holocephali includes the extant chimaeras and related extinct forms. The chapter shows that chimaeras have a single gill opening covered by soft tissue and an immobile upper jaw fused with the skull. It then considers the ecological implications of declining populations of the elasmobranch species, referring to how the life-history characteristics of neoselachians make them vulnerable to overfishing. Policies to protect these specieis revolve around sustainable exploitation of individual species.
