Ontogenetic variability in crystallography and mosaicity of conodont apatite: implications for microstructure, palaeothermometry and geochemistry

Abstract

X-ray diffraction data from Silurian conodonts belonging to various developmental stages of the species Dapsilodus obliquicostatus demonstrate changes in crystallography and degree of nanocrystallite ordering (mosaicity) in both lamellar crown tissue and white matter. The exclusive use of a single species in this study, combined with systematic testing of each element type at multiple locations, provided insight into microstructural and crystallographic differentiation between element type ( S a , S b - c , M ) as well as between juveniles and adults. A relative increase in the unit cell dimensions a / c ratio of nanocrystallites during growth was apparent in areas demonstrating single-crystal behaviour, but no such relationship was seen in dominantly polycrystalline areas. Systematic variations in mosaicity were identified, with mosaicity (as a proxy for disorder) increasing during growth, as well as along elements from tip to base. These results provide potential insight into the integrity of conodont apatite as a recorder of palaeoseawater chemistry, as well as demonstrate the need to consider the influence of ontogeny and element type on the use of conodonts in palaeothermometry and geochemical investigations.

X-ray nanotomography and electron backscatter diffraction demonstrate the crystalline, heterogeneous and impermeable nature of conodont white matter

“…This challenge is made all the greater since conodont elements are composed largely of brittle enamel-like tissues that contain few organics [ 3 , 5 , 7 ] and so their analysis is not readily amenable to decalcification and microtomy. Thus, conodont element structure has hitherto been limited largely to traditional invasive methods such as thin-sectioning for optical microscopy, ground sections for scanning electron microscopy (SEM), ion-milling to create single sections for TEM [ 3 , 5 , 7 ], or two-dimensional X-ray diffraction [ 10 ]. More recently, non-invasive tomography has been introduced through srXTM exploiting a high-brilliance synchrotron source to provide coherent X-ray beam for absorption [ 11 ], phase-contrast tomography [ 12 ] and ptychographic nanotomography [ 13 , 14 ], which is a coherent diffractive imaging technique that does away with imaging lenses and delivers three-dimensional volumes, yielding definitive data on the physical structure of conodont elements.…”

“…scanning electron microscopy (SEM), ion-milling to create single sections for TEM [3,5,7], or twodimensional X-ray diffraction [10]. More recently, non-invasive tomography has been introduced through srXTM exploiting a high-brilliance synchrotron source to provide coherent X-ray beam for absorption [11], phase-contrast tomography [12] and ptychographic nanotomography [13,14], which is a coherent diffractive imaging technique that does away with imaging lenses and delivers threedimensional volumes, yielding definitive data on the physical structure of conodont elements.…”

Nanomechanical variability in the early evolution of vertebrate dentition

“…The majority of each conodont element is composed of crown tissue with basal tissue limited to the region that was presumably permanently attached to the surrounding soft tissue 4 , 12 , 13 . Crown tissue is composed of hyaline lamellar crown tissue and white matter (albid), each of which have their own microstructure and crystallographic properties 6 , 8 , 21 . Conodont elements grow through the apposition of lamellae with nanocrystallites typically arranged perpendicular or parallel to the growth lamellae, although a variety of growth patterns have been observed 8 .…”

“…All of the specimens (Fig. 1 ) studied here were previously characterized utilizing micro-X-ray diffraction (μXRD) techniques 21 , which allows us to directly relate the measured nanomechanical property to crystalline structure of the materials in each element. The new AFM data presented here provide the first documentation of this important mechanical property (E) of conodonts and demonstrate a close relationship between variations in Young’s modulus, ontogenetic development, and crystallographic structure of conodont bioapatite.…”

“…All scale bars are 100 μm and all images are scaled to the same size. These are the same specimens previously characterized by micro-X-ray diffraction (μXRD) 21 . …”