The teeth of gnathostomes (jawed vertebrates) show rigidly patterned, unidirectional replacement that may or may not be associated with a shedding mechanism. These mechanisms, which are critical for the maintenance of the dentition, are incongruently distributed among extant gnathostomes. Although a permanent tooth-generating dental lamina is present in all chondrichthyans, many tetrapods and some teleosts, it is absent in the non-teleost actinopterygians. Tooth-shedding by basal hard tissue resorption occurs in most osteichthyans (including tetrapods) but not in chondrichthyans. Here we report a three-dimensional virtual dissection of the dentition of a 424-million-year-old stem osteichthyan, Andreolepis hedei, using propagation phase-contrast synchrotron microtomography, with a reconstruction of its growth history. Andreolepis, close to the common ancestor of all extant osteichthyans, shed its teeth by basal resorption but probably lacked a permanent dental lamina. This is the earliest documented instance of resorptive tooth shedding and may represent the primitive osteichthyan mode of tooth replacement.

The dentitions of extant fishes and land vertebrates vary in both pattern and type of tooth replacement. It has been argued that the common ancestral condition likely resembles the nonmarginal, radially arranged tooth files of arthrodires, an early group of armoured fishes. We used synchrotron microtomography to describe the fossil dentitions of so-called acanthothoracids, the most phylogenetically basal jawed vertebrates with teeth, belonging to the genera Radotina, Kosoraspis, and Tlamaspis (from the Early Devonian of the Czech Republic). Their dentitions differ fundamentally from those of arthrodires; they are marginal, carried by a cheekbone or a series of short dermal bones along the jaw edges, and teeth are added lingually as is the case in many chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods). We propose these characteristics as ancestral for all jawed vertebrates.

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The ontogenetic trajectory of a marginal jawbone of Lophosteus superbus (Late Silurian, 422 Million years old), the phylogenetically most basal stem osteichthyan, visualized by synchrotron microtomography, reveals a developmental relationship between teeth and dermal odontodes that is not evident from the adult morphology. The earliest odontodes are two longitudinal founder ridges formed at the ossification center. Subsequent odontodes that are added lingually to the ridges turn into conical teeth and undergo cyclic replacement, while those added labially achieve a stellate appearance. Stellate odontodes deposited directly on the bony plate are aligned with the alternate files of teeth, whereas new tooth positions are inserted into the files of sequential addition when a gap appears. Successive teeth and overgrowing odontodes show hybrid morphologies around the oral-dermal boundary, suggesting signal cross-communication. We propose that teeth and dermal odontodes are modifications of a single system, regulated and differentiated by the oral and dermal epithelia.

Deep groundwater characteristics provide valuable information on oil and gas extraction and evolution of hydrosphere, and nonmetallic and metallic elements in deep groundwater are raising industrial interest. There is therefore a need for a better understanding of the origin and evolution of deep groundwater in large sedimentary basins, e.g., by using non-traditional isotopes. Here, we review the constraints of isotopes of chloride (Cl), bromine (Br), boron (B), lithium (Li), helium (He), neon (Ne), and argon (Ar) on the origin and evolution of deep groundwater in large sedimentary basins. In deep groundwater, δ 37 Cl ranges from −1.96 to + 2.07‰, δ 81 Br from −1.50 to + 3.35‰, δ 11 B from + 1.10 to + 39.99‰, and δ 7 Li from −1.00 to + 31.80‰. These values either overlap or are different compared to those in freshwater, e.g., meteoric water, river water and shallow groundwater, hydrothermal fluid, seawater, subsurface brine, lake sediment, or mineral. Noble gas isotopes such as 3 He/ 4 He, 4 He/ 20 Ne, and 36 Ar/ 40 Ar are also effective tracers for deep groundwater evolution. Integrating multiple nontraditional isotopes allows to study dissolution, sedimentation, evaporation, and mixing of different waters in deep aquifers.