EVOLVING TEETH · Uncovering developmental and functional constraints on the occupation of conodont tooth morphospace

Uncovering developmental and functional constraints on the occupation of conodont tooth morphospace

Introduction: Conodonts were a successful and important group of early fish, with an excellent fossil record spanning over 300 million years. Their skeleton formed the first vertebrate dental apparatus. The elements of this feeding apparatus display extensive variation; however, our understanding of how such morphological differences influenced their function as teeth has been limited, and the occupation of morphospace – the representation of element morphological variation – has never been investigated in conodonts. Previous qualitative analyses of individual elements have revealed that their growth and basic occlusion differed fundamentally from the teeth of jawed vertebrates, reflecting independent origin. Moreover, at approximately 0. 2-2 mm long, these elements are at least an order of magnitude smaller than many teeth and were not anchored in jaws. Thus, it is unclear how effectively elements could have function as teeth. Additionally, the dental tools on the elements responsible for food fracture – cusps and denticles – were often composed of an enigmatic tissue called "white matter"; a tissue unlike any other found in animals. The functional significance of this tissue is uncertain. Our aim in this project was to address these issues, to gain a better understanding of conodont functional morphology, and so unlock their potential for elucidating broader questions about dental evolution in animals. Objectives 1. To acquire the first clade-wide database of high resolution 3D models of conodont dental structures, using the SLS (Switzerland) and SPring-8 (Japan) synchrotrons 2. To derive functional morphometric data from these models, and in conjunction with finite element (FE) modelling and observations of wear, interpret likely dietary characteristics and occlusal mode 3. To compare the occupation and disparity patterns within a morphospace defined by these functional morphometric variables to those within a morphospace circumscribed by cladistic characters and surface complexity. Results 1. We have demonstrated that combined application of FE analysis, functional morphometric analysis and observations of wear provides a useful and informative approach to understanding conodont element function 2. Using this approach, we have revealed that conodonts possessed the sharpest dental apparatus of any animal group thus far measured; supported by other functional data, this shows that conodonts optimised their dental morphology to maximise the limited muscular force they were able to exert owing to their small size and lack of jaws 3. Based on this information, we have produced the first rigorous four dimensional reconstruction of conodont element occlusion. 4. FE analysis has further demonstrated that constructing cusps and denticles from white matter reduces stress during function and minimises the probability of element failure 5. Based on this evidence, variation in conodont element morphology and tissue composition should reflect variation in dietary properties and occlusal mode 6. We reveal that explorations of functional morphospace mirrored broad scale evolutionary radiations, suggesting that these radiations were linked to the exploitation of new trophic niches 7. Patterns of variation in element surface complexity appear to be decoupled from those in other morphological metrics, demonstrating the presence of multiple signals in different morphological data from these dental structures 8. We have scanned articulated conodont skeletons embedded in rock for the first time using synchrotron radiation, which has allowed us to digitally dissect the skeletons and reconstruct the first virtual 3D model of a conodont dental apparatus Impact: Integration and analysis of these data have produced the first comprehensive conodont morphospaces and provide a macroevolutionary prospectus of morphological diversity and function in conodonts for the first time. It will also provide the first quantitative description of conodont element function in a broad range of taxa, allowing us to understand how the earliest dental apparatus functioned and evolved in unprecedented detail. The results of the project will form the foundation for all future analysis of conodont function and provides a fresh perspective on the evolution of teeth more generally, establishing conodonts as a phylogenetically independent comparative system for examining constraint, complexity and function within and between dental apparatuses.

Data: CORDIS, © European Union