Growth, form and function in problematic Cambrian paraconodonts

Paraconodonts represent the earliest candidate vertebrate feeding structures in the fossil record. However, critical questions regarding individual element growth and function, and apparatus structure remain unconstrained. To address these questions, large assemblages of elements from the Deadwood Formation, Canada and Alum Shale Formation, Sweden were examined. The new assemblage of SCFs from the Deadwood Formation has yielded over 1800 paraconodont elements, including 536 Westergaardodina spanning six species, two of which are new. Biological taxonomy based upon ontogenetic development of elements has been presented in Westergaardodina for the first time. Taphonomic examination reveals that elements were flattened and partially demineralised during preservation. Histological examination of phosphatic coniform paraconodont elements using tomographic imaging has revealed four distinct internal grades of basal accretion that are highly variable among genera, supporting a polyphyletic Euconodonta. Observations under transmitted light, scanning electron, and tomographic microscopy reveal that Westergaardodina elements grew through at least two distinct modes of morphogenesis. The first is modified basal accretion with distinct lateral allometry, and the second involves the incorporation of distinct units into a united element superstructure. This mode of development is observed in derived euconodont elements and other basal vertebrates but is the earliest known example of this in vertebrate feeding structures. Finally, quantitative functional analyses of damage patterns and sharpness of Westergaardodina preclude it from an external body function and indicate it was used directly in food processing as a slicing surface. The apparatus structure of Westergaardodina remains unknown but is clearly separate from those currently known in conodonts, further adding to the known diversity of the earliest vertebrates. The combined results of this work have highlighted the hitherto underappreciated diversity present within the skeletons of the earliest conodonts and vertebrates extending to both their growth, and function. Paraconodonts were clearly a key component of late Cambrian ecosystems.