Three-dimensional reconstruction of Shark Vertebrae: A technique with applications to age and growth studies

Shark vertebral centra show no histological evidence of resorption at any time in the animals' life. Deorganification of centra always reveals a large, residual, stable, three-dimensional skeleton. In contrast, the mineralized parts of other organs (e.g. claspers and jaws) crumble into their individual mineralized subunits, the tesserae, upon deorganification. In both cases, only appositional growth of cartilage on the pre-existing mineralized template is possible. The basic 'double-cone' shape of the vertebrae facilitates increases in body length simultaneously with an accompanying increase in girth. Once the initial shape of the mineralized portion of a vertebral centrum is fully established and hence can be described, then relatively simple mathematical models might be devised to predict future growth patterns. To advance this hypothesis, it has first been necessary to develop a method that can accurately record the sizes and shapes of complex three-dimensional anatomical structures. This paper describes a technique that is capable not only of recording and measuring the size and shape of shark vertebrae but also of predicting their subsequent growth. Furthermore, the technique enables reproduction of three-dimensional coloured and shaded stereoscopic images of vertebral structures, facilitating a better understanding of their intricate morphology. Three-dimensional coordinate data gathered from any shark vertebra can be manipulated mathematically to model future vertebral growth. Producing realistic images of vertebrae transformed in this way may allow the exploration of possibly unrealized taxonomic affinities.