JOURNAL OF MORPHOLOGY 260:1–12 (2004) Structure and Function of the Horn Shark (Heterodontus francisci) Cranium Through Ontogeny: Development of a Hard Prey Specialist Adam P. Summers,1* Richard A. Ketcham,2 and Timothy Rowe2 1Ecology and Evolutionary Biology, University of California, Irvine, California 92697 2Department of Geological Sciences, The University of Texas at Austin, Austin, Texas 78712 ABSTRACT The horn sharks (Heterodontidae: Chon- etal material they are capable of performing at func- drichthyes) represent one of four independent evolutions tional extremes — extant sharks are the largest of durophagy in the cartilaginous fishes. We used high- fishes ever to swim in the oceans (Gudger, 1941), resolution computed tomography (CT scanning) to visual- fast-swimming piscivorous species can likely move ize and quantify the mineralized tissue of an ontogenetic as rapidly as even the fastest bony fishes (Fierstine series of horn sharks. CT scanning of neonatal through adult California horn sharks (Heterodontus francisci) con- and Walters, 1968; Compagno, 1984), and certain firmed that this technique is effective for examining min- species are able to exert sufficient force with their eralized tissue in even small (Ͻ10 mm) specimens. The cartilaginous jaws to crush mollusks (Coles, 1910; jaw joint is among the first areas to become mineralized Summers, 2000). In the several radiations of carti- and is the most heavily mineralized area in the cranium of laginous fishes that have adopted a durophagous a neonatal horn shark. The hyoid is also well mineralized, diet there are likely multiple solutions to the prob- although the poorly mineralized molariform teeth indicate lem posed by orally crushing prey that is harder that the neonatal animal may be a suction feeder on softer than the skeleton of the jaws. prey. The symphysis of the jaws never mineralizes, in Previous studies have examined the mechanics sharp contrast to the condition in the hard prey-crushing and morphology of hard prey-crushing in the myli- stingrays. Digitally reslicing the CT scans along the jaws obatid stingrays (Summers et al., 1998; Summers, allowed measurement of the second moment of area (Ina). Assuming that the jaws are made of the same material at 2000), one of four independent acquisitions of a du- rophagous diet in the chondrichthians (Fig. 1). The all ages, Ina is an indicator of the flexural stiffness of the jaws. In all sizes of shark the lower jaws were stiffer than ten species of horn sharks (Heterodontiformes:Het- the upper and the stiffness increased in the area of the erodontidae) also crush hard prey between molari- molariform teeth. The central region of the jaws, where form oral jaw teeth, although these teeth are not the rami meet, support cuspidate grasping teeth and has nearly as specialized as those of the stingrays the lowest Ina. The spotted eagle ray (Aetobatus narinari), (Smith, 1942; Nobiling, 1977; Segura-Zarzosa et al., a hard prey-crushing stingray, shows a different pattern 1997). As their name implies, the genus Heterodon- of flexural stiffness, with the peak at the central part of tus is characterized by having more than one tooth the jaws where the prey is reduced between flattened shape — multicuspidate anterior teeth suited for tooth plates. Although the eagle ray jaws have a higher Ina than the horn shark, they are also far more heavily min- grasping, and rounded, broad molariform teeth for eralized. When the relative amounts of mineralization are crushing. Both types of teeth are present by the time taken into account, horn sharks do better with what min- the neonate shark hatches from its egg case (Gar- eral they have than does the eagle ray. With a tight jaw man, 1913; Smith, 1942). Examinations of the feed- joint and loose mandibular symphysis, as well as nearly ing mechanism of durophagous sharks have focused opposite patterns of stiffness in the jaws, it is clear that on muscular anatomy (Nobiling, 1977) and the kine- two of the clades of hard prey specialists use very different matics of feeding (Wilga and Motta, 2000; Edmonds methods for cracking the hard prey problem. J. Morphol. 260:1–12, 2004. © 2004 Wiley-Liss, Inc. KEY WORDS: CT scan; second moment of area; duroph- Contract grant sponsors: the McDowell Foundation (to APS), the agy; eagle ray; Aetobatus narinari National Science Foundation; Contract grant number: IIS-9874781 (to TR and RK). *Correspondence to: Adam P. Summers, Ecology and Evolutionary The cartilaginous fishes (Chondrichthyes), or rat- Biology, University of California, Irvine, CA 92697. fish, sharks, and rays, are a monophyletic, ancient E-mail: [email protected] assemblage of species found in all the oceans of the world, inhabiting a very broad range of ecological niches. In spite of, or perhaps because of, their skel- DOI: 10.1002/jmor.10141 © 2004 WILEY-LISS, INC. 2 A.P. SUMMERS ET AL. Fig. 1. Cladogram of the car- tilaginous fishes showing diver- sity of hard prey-crushing taxa and their relationship to other cartilaginous fishes. The num- ber of durophagous species / number of species in the clade is shown to the right. The horn sharks (Heterodontiformes) are the primary focus of this study, with some comparative data from a durophagous stingray. The topology of the cladogram is from Shirai (1996) with some clades collapsed for simplicity. et al., 2001), but there has been no examination of arranged in a series of thin, irregularly shaped tiles, the skeletal structure that allows this unusual be- called “tesserae,” on the surface of the skeletal ele- havior. ment (see Clement, 1992, for a review). The interior Cartilaginous fishes split from the lineage that of these skeletal elements is composed of a hyaline, gave rise to bony fishes (and tetrapods) over 320 unmineralized cartilage. The mineralized tesserae million years ago and experienced a significant Pa- are thin (ϳ1 mm) and form one or more layers of leozoic radiation represented today by two clades of tiles over the softer hyaline core. The mineralized Mesozoic origin — the chimeras and the elasmo- shell that surrounds a skeletal element is stiffer and branchs (Lund and Grogan, 1997; Coates and Se- stronger than the underlying cartilage and, for the queira, 2001). The completely cartilaginous skeleton purposes of this article, that core is ignored. The of Recent chondrichthian fishes reflects the loss of a stiffness of the skeletal element is presumed to come bony skeleton. The outgroup, placoderms, represent primarily from the tiled layer(s) of tesserae. the plesiomorphic condition, with normal dermal Three-dimensional reconstructions of mineralized bone, and there is clear evidence that some fossil tissue from computed tomography scans (CT scans) sharks had structurally significant endoskeletal are becoming increasingly available to morpholo- bone (Halstead, 1974; Smith and Hall, 1990; Coates gists (Rowe, 1996; Rowe et al., 1997; Cifelli et al., et al., 1998). Extant chondrichthians have aban- doned endoskeletal bone in favor of a cartilaginous 1999; Clark et al., 2002). These scans are visually skeleton that is mineralized to varying degrees engaging and provide a unique opportunity to un- (Ørvig, 1951; Applegate, 1967; Kemp and Westrin, derstand the positions of skeletal elements relative 1979). to soft, less radio-opaque tissues such as unminer- The mineralization of the shark endoskeleton, alized cartilage and tendon. CT scans also represent composed of calcium phosphate hydroxyapatite, a largely untapped source of data describing the takes several forms — the web-like calcified struc- structure of skeletal elements. For example, it is tures of the vertebrae (aereolar mineralization); the possible to “virtually section” a CT reconstruction prismatically mineralized crystals on the outer sur- along any arbitrary axis. These sections can then be face of skeletal elements; and a less dense, globular analyzed using traditional structural metrics such calcification usually associated with the prismatic as second moment of area. It is now possible to material (Ridewood, 1921; Ørvig, 1951; Summers, extract objective, numerical descriptors of the skel- 2000). The globular and prismatic mineralization is eton from digitally reconstructed skeletons. HORN SHARKS AND HARD PREY 3 The purpose of this article is threefold: 1) to ex- The juvenile horn shark (AL22) and the neonate (P1999) were plore the capabilities of high-resolution CT scanning scanned on the ultra-high-resolution subsystem. Both scans uti- Ͻ lized the microfocal X-ray source with settings 120 kV and 0.2 to assess mineralization in small ( 1 cm) chondrich- mA. Neither scan used beam filtration and both employed 160% thyan specimens; 2) to describe the ontogeny of the offset mode. X-ray intensities were measured using an image mineralization of the chondrocranium, jaws, and hy- intensifier connected to a 512 ϫ 512 CCD video camera. Both oid arch elements from a series of horn sharks; and specimens were scanned in 3-slice mode, in which data for three 3) to compare horn shark jaw morphology among slices are acquired simultaneously. Images for AL22 were ac- quired with 1200 66.7 ms views per 3-slice acquisition, for a scan three ontogenetic stages and another hard prey- time of ϳ28 sec per image. The slice thickness and interslice crushing elasmobranch, the spotted eagle ray (Ae- spacing was 0.151 mm and the field of view was 64 mm. P1999 tobatus narinari). was scanned using 1,800 66.7 ms views per 3-slice acquisition, for a scan time of ϳ41 sec per image. Slice thickness and interslice spacing were both 0.0698 mm and the field of view was 28 mm. The CT scan slices were processed with VoxBlast (VayTek MATERIALS AND METHODS Software, Fairfield, IA), which allows reconstruction of the three- Animals dimensional structure of the shark cranium. The cranium was virtually resliced in the sagittal, coronal, and frontal planes and Four freshly killed adult and juvenile California horn sharks movies of rotations and slicings of each specimen were generated.