New insights into the ultrastructure, permeability, and integrity of conodont apatite determined by transmission electron microscopy

Abstract

2007: New insights into the ultrastructure, permeability, and integrity of conodont apatite determined by transmission electron microscopy. Lethaia , Vol. 40, New crystalline structures have been observed in argon ion-milled conodont elements from a diverse suite of Ordovician taxa (' Cordylodus robustus ', Drepanoistodus suberectus , Panderodus gracilis , Plectodina ? sp., Aphelognathus sp., Periodon aculeatus ), using transmission electron microscopy (TEM). Electron diffraction patterns of albid tissue reveal that the component crystals are extraordinarily large, in the order of hundred(s) of microns. These large albid crystals show typical cancellate porosity, although a distinctly lamellar structure has also been observed within a large albid crystal positioned between hyaline lamellar and cancellate albid tissues. There is a distinct absence of 'interlamellar space' within all hyaline tissues examined, which are characterized by a polycrystalline matrix of micron-scale elongate crystals that are both strongly aligned and tightly bound within a broader lamellar structure. Optical opacity, caused by light scattering within large ( ≥ 0.5 µ m) pores, is also a feature of both albid and polycrystalline lamellar crown tissues. Accordingly, conodont hard tissues are differentiated by crystal size and shape, as well as inter-and intracrystalline porosity.These new observations highlight the structural complexities of conodont histologies and the need for more comprehensive investigations particularly of transitional crown tissues, which are not well defined by terms typically used in the literature. Their histological structures are interpreted to be a product of in vivo crystallization and thus provide new insights into the relative porosity, permeability, and inherent integrity of the tissues as well as their growth relationships. Accordingly, these data not only have implications for earlier histological and palaeobiological interpretations of conodont hard tissues but are also fundamental in determining their chemical integrity, which is crucial for characterizing palaeoseawater composition and palaeoenvironmental change. The potential for conodont apatite to retain primary chemical information depends on crystal size and permeability, so the large albid crystal domains are consistent with parallel geochemical studies that suggest that cancellate albid crown is more resistant to diagenetic modification. ᮀ Apatite, conodont, histology, TEM, transmission electron microscopy, palaeoseawater chemistr y.

False teeth: conodont-vertebrate phylogenetic relationships revisited

“…2E) has discussed the errors of Donoghue's (1998) interpretation of conodont morphogenesis providing clear morphogenetic diagrams for vertebrate odontodes and showing how the structure and growth of conodont elements does not match the Odontode Regulation Th eory (Reif 2002) in any way. As noted above, Trotter et al (2007) also showed that conodont element tissues are clearly distinct in crystal size from any vertebrate tissue. Th ere is also no mineralized keratin in conodont elements, and so affi nities with rasping teeth of lampreys and hagfi shes are also very unlikely.…”

“…Donoghue et al (2000Donoghue et al ( , 2006 considered conodont basal bodies to be formed of dentine, but the globular tissue they discuss cannot be homologous with vertebrate dentine, which grows centripetally (Gross 1954(Gross , 1957Schultze 1996;Reif 2006;Fig. 2E), and is added basally not topically (Trotter et al 2007). Even just considering the basal body structure, Carter & Lutz (1990: pl.…”

“…In Panderodus, the crystals are arranged obliquely or in line with the layer. Th e hydroxyapatite crystals in conodont hyaline tissue are exceptionally large, with no trace of prisms, unlike fi sh protoprismatic enamel, or the highly organized prismatic enamel of mammals (e.g., Kemp 2002a; Trotter & Eggins 2006;Trotter et al 2007).…”

“…Gross 1954). A major part of the argument for excluding all conodonts from craniates (and thus from vertebrates) and not interpreting their elements as teeth rests with the diff erences in hard tissues (Schultze 1996;Kemp & Nicoll 1996;Kemp e.g., 2002a, b;Reif 2006;Trotter et al 2007).…”

“…Th ere is no evidence that conodonts share any of the attributes of vertebrate hard tissues (Trotter et al 2007) and the morphogenetic system of formation of vertebrate hard parts (Reif 2006). Sansom (2006), in his review of book, corrected his own earlier work by stating "Conodonts did not possess cellular bone and probably did not possess cartilage within their oral-pharyngeal feeding elements, despite the interpretation of Sansom et al (1992), a point that has become clear from later papers and a deeper understanding of the constraints of tissue topology within conodont elements".…”