Abstract
Conodont elements are important archives of sea/pore water chemistry yet they often exhibit evidence of diagenetic mineral overgrowth which may be biasing measurents. We decided to investigate this phenomenon by characterising chemically and crystallographically, the original biomineral tissue and the diagenetic mineral nature of conodont elements from the Ordovician of Normandy. Diagenetic apatite crystals observed on the surface of conodont elements show distinctive large columnar, blocky or web-like microtextures. We demonstrate that these apatite neo-crystals exhibit the same chemical composition as the original fossil structure. X-ray microdiffraction has been applied herein for the first time to conodont structural investigation. Analyses of the entire conodont element surface of a variety of species have revealed the existence of a clear pattern of crystal preferred orientation. No significant difference in unit cell parameters was documented between the newly formed apatite crystals and those of the smooth conodont surfaces, thus it emerges from our research that diagenesis has strictly replicated the unit cell signature of the older crystals.
Highlights
Conodonts, for a long time considered enigmatic, represent an extinct group of jawless vertebrates, that were the first among the group to experiment skeletal biomineralization with tooth-like elements in their feeding apparatus[1]
Peculiar apatite neo-crystal overgrowths on the oral surface of conodont elements were revealed during detailed scanning electron microscopy investigation
These crystals were seen to occur on elements representative of different morphologies belonging
Summary
X-ray microdiffraction was used to characterise various points of the surface of the conodont elements. Comparison between μXRD data measurements carried out in different points of the conodont element at the same Phi and Omega angles highlights that the polycrystalline structure does not maintain the same orientation along the surface of the conodont This conclusion is supported by Beta angle pattern of the (300) reflection which varies both in intensity and in Beta position (Fig. 4c). Single crystals of apatite were detected in another analysed ramiform element of the same conodont genus In this latter case it was not possible to show the patterns of Beta angle as the strongest XRD reflections were always revealed both as diffraction arcs and spots.
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