ZOOLOGY Zoology 109 (2006) 164–168 Mineralized C artilage in the skeleton of chondrichthyan fishes Mason N. Dean, Adam P. Summers Ecology and Evolutionary Biology, University of California – Irvine, 321 Steinhaus Hall, Irvine, CA 92697-2525, USA Received 10 February 2006; received in revised form 2 March 2006; accepted 3 March 2006 Abstract The cartilag inous endoskeleton of chondrichthyan fishes (shark s, rays, and chimaeras) exhibits complex arrangements and morphologies of calcified tissues that vary with age, species, feeding behavior, and location in the body. Understanding of the development, evolutionary history and function of these tissue types has been hampered by the lack of a unifying terminology. I n order to facilitate reciprocal illu mination between disparate fields with convergent interests, we present levels of organization in which crystal orientation/size delimits three calcificati on types (areol ar, globular, and prismatic) that interact in two distinct skeletal types, vertebral and tessellated carti lage. The tessellated skeleton is composed of small blocks (tesserae) of calcified cartilag e (both prismatic and globular) overlying a core of unmineralized cartilag e, while vertebral cartilag e usually contains all three types of calcification. r 2006 Elsevier GmbH . All rights reserved. Keywords: Elasmobranch skeleton; Mineralization; Calcified cartilage; Tesserae Introduction The breadth of morphological varia tion of the mineralized cartilag e of the endoskeleton of chon- drichthyan fishes (shar ks, rays, and chimaeras) has led to a confusion of terminology in the literat ure. Confli cting descriptive terms make it difficul t to define, let alone answer, questions of the evolution, homology and function of the skeleton and skeletal tissues. Here we lay out the most accepted terminology for miner- aliz ed tissue, sometimes called ‘cal cified cartilag e’ in the literature, in carti laginous fishes and propose a hier- archical fram ework for future descriptive work. To integrate the convergent evolutionary ( Smith and Hall , 1990; Sansom et al., 1992; Hall, 2005), paleonto- logical ( Coat es et al., 1998; Janvier et al., 2004), developmental ( Davi s et al., 2004) and biomechanical ( Summers, 2000; Schaefer and Summers, 2005; Dean et al., 2006) interests in cartilag inous skeletons, we need a common language. Furtherm ore, the current termi- nology masks unappreciated complexity in morphology that will fuel future research in several of these fields. Classification of elasmobranch cartilage tissue types The entire endoskeleton of sharks, chimaeras and rays is cartilagino us, composed of chondrocytes in an extracellular matrix (ECM ) surrounded by a fibrous perichondri um. The ECM may be mineralized to varying degrees with crystals of calcium phosphate hydroxyapat ite ( Apple gate, 1967; Dean et al., 2005). Ther e are no nanostructural data on the orientation or size of these crystals , but the higher-level organization of the mineral has inspired several useful descriptive terms. ARTICLE IN PRESS www.elsevier.de/zool 0944-2006/$ - see front matter r 2006 Elsevier GmbH. All rights reserved. doi:10.1016/j.zool.2006 .03.002 Correspond ing author. E-mail address: asummers@uci.edu (A.P. Summers).