Loïc Ségalen

A carbonate-fluoride defect model for carbonate-rich fluorapatite

Abstract We propose a microscopic model of the dominant carbonate for phosphate substitution in fluorapatite. A well-crystallized sedimentary fluorapatite sample containing ~2.3 ± 0.8 wt% of carbonate was investigated using Fourier transform infrared spectroscopy (FTIR) and 13C and 19F magic angle spinning nuclear magnetic resonance (MAS NMR). About 75% of the carbonate groups replace the phosphate group (“B-site”), whereas a lesser contribution from carbonate groups located in the structural channels (“A-site”) is observed. Beside the dominant 19F NMR signal of channel ions at ~ -102 ppm, an additional signal corresponding to ~8% of fluoride ions is observed at -88 ppm. 19F double quantum-single quantum (DQ-SQ) MAS NMR and 13C{19F} frequency-selective Rotational Echo DOuble Resonance (REDOR) experiments prove that this additional signal corresponds to isolated fluoride ions in the apatite structure, located in close proximity of substituted carbonate groups. Density functional theory (DFT) calculations allow us to propose a composite carbonate-fluoride tetrahedron defect model accounting for these experimental observations. The planar carbonate ion lies in the sloping face of the tetrahedron opposite a fluoride ion occupying the remaining vertex, together replacing the tetrahedral phosphate ion. This “francolite-type” defect leads to a diagnostic narrow IR absorption band at 864 cm-1 that could be used as a guide to, e.g., detect the incipient transformation of fossil bone and teeth samples.

Evidence for High Bone Growth Rate in Euparkeria Obtained Using a New Paleohistological Inference Model for the Humerus

ABSTRACT The study of bone growth rate and metabolic rate evolution in archosaurs (crocodiles, dinosaurs including birds, and pterosaurs) and close outgroups has become a subject of major interest among paleontologists in recent years. In this paper, we estimate the bone growth rate of Euparkeria using a new statistical inference model for the humerus. We modified the taxonomic range of extant species used in previous studies, on which we performed quantitative measurements of histological features and bone growth rates. Bone growth rate values estimated for Euparkeria are crucial in understanding the ancestral condition for archosaurs because this taxon is considered the closest relative to the archosaur crown group. We obtained an instantaneous growth rate of 6.12 µm/day, suggesting that Euparkeria shared with other non-archosaurian archosauromorphs (Prolacerta, Proterosuchus, and Erythrosuchus) a condition of high growth rate compatible with endothermy. This derived state may have been inherited by some Triassic crurotarsans, as suggested by the high instantaneous bone growth rate (14.52 µm/day) estimated in this study for Postosuchus. Jurassic crurotarsans may have lost endothermy during the transition from terrestrial habitats and active predation to aquatic habitats and sit-and-wait predation behaviors, so that Cretaceous crocodiles may be secondarily ectothermic, as suggested by &dgr;18O values. In conclusion, we provide new evidence for the hypothesis of an ancestral endothermic state for the last common ancestor of archosaurs, and show that non-archosaurian archosauromorphs and Triassic crurotarsans may have been characterized by a thermometabolism more similar to that of dinosaurs than to that of lepidosaurs and turtles.

Late Miocene karst system at Sheikh Abdallah, between Bahariya and Farafra, Western Desert, Egypt: Implications for palaeoclimate and geomorphology

The extensive spelean deposits in the Western Desert of Egypt at Crystal Mountain (Gebel Bellorat) are considered to be 11-10 Ma, confirming the ‘Vallesian’ estimate made by Heissig (1982). Several new faunal elements have been discovered including anurans, snakes, soricids, bats, galagids, hystricids and glirids. This fauna indicates that the region was appreciably more humid 11-10 Ma than it is today, with at least 750 mm and possibly as much as 1,200 mm mean annual rainfall. The role of karst processes in the development of the oases of the Western Desert that may have been underestimated by previous researchers is emphasized.

Modeling the attenuated total reflectance infrared (ATR-FTIR) spectrum of apatite

Abstract Attenuated total reflectance (ATR) infrared spectra were measured on a synthetic and a natural fluorapatite sample. A modeling approach based on the computation of the Fresnel reflection coefficient between the ATR crystal and the powder sample was used to analyze the line shape of the spectra. The dielectric properties of the samples were related to those of pure fluorapatite using an effective medium approach, based on Maxwell–Garnett and Bruggeman models. The Bruggeman effective medium model leads to a very good agreement with the experimental data recorded on the synthetic fluorapatite sample. The poorer agreement observed on the natural sample suggests a more significant heterogeneity of the sample at a characteristic length scale larger than the mid-infrared characteristic wavelength, i.e., about 10 micrometers. The results demonstrate the prominent role of macroscopic electrostatic effects over fine details of the microscopic structure in determining the line shape of strong ATR bands.

Probing atomic scale transformation of fossil dental enamel using Fourier transform infrared and nuclear magnetic resonance spectroscopy: a case study from the Tugen Hills (Rift Gregory, Kenya).

A series of fossil tooth enamel samples was investigated by Fourier transform infrared (FTIR) spectroscopy, (13)C and (19)F magic-angle spinning nuclear magnetic resonance (MAS NMR) and scanning electron microscopy (SEM). Tooth remains were collected in Mio-Pliocene deposits of the Tugen Hills in Kenya. Significant transformations were observed in fossil enamel as a function of increasing fluorine content (up to 2.8wt.%). FTIR spectroscopy revealed a shift of the ν1 PO4 stretching band to higher frequency. The ν2 CO3 vibrational band showed a decrease in the intensity of the primary B-type carbonate signal, which was replaced by a specific band at 864cm(-1). This last band was ascribed to a specific carbonate environment in which the carbonate group is closely associated to a fluoride ion. The occurrence of this carbonate defect was consistently attested by the observation of two different fluoride signals in the (19)F NMR spectra. One main signal, at ∼-100ppm, is related to structural F ions in the apatite channel and the other, at -88ppm, corresponds to the composite defect. These spectroscopic observations can be understood as resulting from the mixture of two phases: biogenic hydroxylapatite (bioapatite) and secondary fluorapatite. SEM observations of the most altered sample confirmed the extensive replacement of the bioapatite by fluorapatite, resulting from the dissolution of the primary bioapatite followed by the precipitation of carbonate-fluorapatite. The ν2 CO3 IR bands can be efficiently used to monitor the extent of this type of bioapatite transformation during fossilization.

Neogene palaeoenvironments of the Namib Desert: a brief synthesis

The Namib Desert which is the oldest in the world, was occasionally more humid during the Neogene than it is today as evidenced by the palaeontological record and geochemical analyses of fossil struthioniform eggshells and enamel of large mammals. The biochronological framework was established by comparison with other sites in Africa and on the basis of the fossil mammal remains, which allows the reconstruction of the broad environmental history of the area during the Neogene.

Palaeoenvironments and the origin of hominid bipedalism

Abstract It has long been accepted that hominids emerged during the Pliocene in a savannah environment in which a terrestrial quadruped gradually developed bipedal adaptations. However, data from the Late Miocene (i.e. 7–7.5 Ma), including detailed palaeontological and biogeochemical studies, suggest that our earliest Upper Miocene ancestors inhabited well-wooded to forested environments where they could have spent a certain amount of time in the trees. A plausible type of ecosystem in which upright posture and bipedal locomotion could have emerged is represented by Miombo Woodland, in which vertical arboreal supports predominate and trees are separated from each other by gaps. Subsequently hominids dispersed into the Savannah as accomplished bipeds, but retained the ability to climb trees. This scenario is compatible with the postcranial anatomy of Australopithecus, including its femoral elongation, body proportions, manual precision grip (also present in 6-million-year-old Orrorin) and a non-prehensile hallux.