Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
John A. Musick
The interrelationship of the chondrichthyan subclasses is evaluated based on divergence in the nature of the suspensorium, the preorbital cranial anatomy, the distribution of major venous sinuses and localization of hematopoietic tissue. The anatomy of representative extant taxa was examined by radiography and/or dissection. Fossil selachians, paraselachians, and holocephalans of the Bear Gulch of Montana, U.S.A. (Mississippian, Namurian E2B) were studied for evidence of vascular pigmentation, suspensorium, and cranial, branchial, and pectoral anatomy. These studies validate the suspensorial condition of autodiastyly and suggest autodiastyly is a fundamental condition involved in the basic radiation of Chondrichthyes. The plesiomorphous condition of all gnathostomes is proposed to be sutodiastylic, with the hyoid arch modified for the support of an opercular covering. A precerebral fontanelle is primary within Chondrichthyes, being lost in Holocephali as cranial remodeling induces ethmoid canal formation. The holocephalan pattern of cranial vascularization is based on the more general selachian plan. Thus, given the formulation of a morphocline based on selachian, paraselachian, and holocephalan data, seemingly distinct selachian and holocephalan vascular elements are shown to be analogous. Similarly, the unique patterns of lymphomyeloid tissue distribution identified for each subclass may also be explained on the basis of general plan which has been subject to relocalization stresses. Finally, both the morphocline and a cladistical analysis of the data support a cochliodont ancestry for extant holocephalans.
© The Author
Grogan, Eileen D., "The structure of the holocephalan head and the relationships of the Chondrichthyes" (1993). Dissertations, Theses, and Masters Projects. Paper 1539616674.