Unusual replacement of biogenic apatite by aluminium phosphate phases in dinosaur teeth from the Early Cretaceous of Spain

Altered iguanodontian dinosaur teeth have been studied to analyse the unusual replacement of biogenic apatite and to deduce how the alterations affect the preservation of these fossils. The fossils were recovered from alluvial lacustrine facies (Barremian, Early Cretaceous) in Teruel (Spain). Mandibular teeth and sediments were studied in hand sample, by X-ray diffraction (XRD) and by scanning electron microscopy (SEM). The combination of the different techniques shows that the teeth have undergone at least three pseudomorphic stages: (1) replacement of the organic component of the dentine and enamel by fluorapatite; (2) replacement of the fluorapatite by aluminium sulphate phosphate phases (crandallite–woodhouseite series); and (3) replacement of part of the tooth by gypsum and Al- and Fe-rich phosphates (vauxite) as a consequence of the superficial weathering of the fossil-rich outcrop. The presence of organic matter and sulphides in the host rocks was the main factor promoting the dissolution of the previous phases and crystallization of the new ones. This is the first description of the replacement of biogenic apatite in dinosaur teeth by aluminium and iron phosphates as a result of fossil diagenetic processes; these minerals are scarce in sedimentary environments and may have economic interest because they contain rare earth or radioactive elements. Additionally, our results indicate that, to ensure proper curating of the specimens from this fossil site, it is crucial to completely remove the host rock and strictly control storage humidity.