Tooth histology in the cretaceous ichthyosaur Platypterygius australis, and its significance for the conservation and divergence of mineralized tooth tissues in amniotes

Abstract

Ichthyosaurs are an extinct group of secondarily aquatic reptiles that show ligamentous tooth attachment to the jaw in some derived forms. Here, we provide a modern description of tooth histology in ichthyosaurs, using Platypterygius australis, a large ichthyosaur from the Cretaceous of Australia. Our study supports evolutionary conservation of the principal mineralized tooth tissue types in amniotes with ligamentous tooth attachment: enamel, dentine, cellular, and acellular cementum. This is the first time that the latter tissue has been located in ichthyosaurs. Vascularized cementum (osteocementum) is reduced or absent in amniotes in which the teeth are ankylosed to the jaw bone, such as basal ichthyosaurs, and raises questions regarding the function of this tissue and the potential developmental or selective conditions leading to its convergent evolution.

Survival of the thinnest: rediscovery of Bauer’s (1898) ichthyosaur tooth sections from Upper Jurassic lithographic limestone quarries, south Germany

“…7), a very fine short period line set where the spaces between lines varies around 2-3 lm (=0.002-0.003 mm) and a coarser, long period line set where spaces vary around 15-20 lm (=0.015-0.02 mm). This is in contrast to the findings of Maxwell et al (2011), At the root the orthodentine (blue colours) is embedded in vascularised pulpal and external cellular cementum of the osteocementum type. Note the differences in structural orientation between the ordered upper pulpal cementum (yellow and blue colours) and the irregular external and basal root cementum (pink colours).…”

“…Tomes 1876; see also Peyer 1968;Dean 2000), is present. Furthermore, a thin layer of acellular cementum, recently identified for the first time in ichthyosaur teeth, i.e., in Platypterygius australis from the Cretaceous of Australia (Maxwell et al 2011), is also identifiable between the orthodentine and the outer cellular osteocementum. Slide ''8'' (Fig.…”

“…Following Bardet and Fernández (2000), the transversely sectioned tooth (slides 1-''8''; BSPG AS XIX 504a-h) belongs to the ophthalmosaurid Aegirosaurus leptospondylus, whereas the longitudinally sectioned tooth is derived from an indeterminate ichthyosaur close to the ophthalmosaurid Nannopterygius. Studying the slides with up-to-date knowledge of ichthyosaur dentition and tooth histology further led to the recognition of an exposed layer of acellular cementum (sensu Maxwell et al 2011) at the tooth neck, and daily and long period growth lines in the orthodentine in both specimens. Some slides of the tooth of A. leptospondylus furthermore revealed a more oval than rectangular shape of the root and the presence of peculiar vascular canals, interpreted as secondary osteodentine deposition, in the peri-pulpal orthodentine.…”

Survival of the thinnest: rediscovery of Bauer’s (1898) ichthyosaur tooth sections from Upper Jurassic lithographic limestone quarries, south Germany

“…Tomes 1876; see also Peyer 1968;Dean 2000), is present. Furthermore, a thin layer of acellular cementum, recently identified for the first time in ichthyosaur teeth, i.e., in Platypterygius australis from the Cretaceous of Australia (Maxwell et al 2011), is also identifiable between the orthodentine and the outer cellular osteocementum. Slide ''8'' (Fig.…”

“…6b). The layer extends from the proximal tips of orthodentine up to the neck of the tooth, at which it is exposed as in P. australis (Maxwell et al 2011). The enamel and associated part of the orthodentine of the crown are not preserved on the slide.…”