Hungarian, English and German researchers were first to identify features similar to the Hunter–Schreger bands in the tooth enamel of a non-mammal prehistoric animal. The discovery was made during the teeth examination of the Iharkutosuchus makadii, a former herbivore crocodile, and it suggest that similar structures may have formed independently in the tooth enamel of mammals, dinosaurs and prehistoric crocodiles.

The Iharkutosuchus makadii lived approximately 85 million years ago in the late Cretaceous period, and currently is known only from the site of Iharkút in Veszprém County. The fossils found in the former bauxite mine located there have contributed to several internationally significant paleontological results in the past decades.

In the tooth enamel of most mammals, under polarizing microscope, alternating dark and light coloured bands can be observed, which are referred to in the literature as Hunter–Schreger bands. In mammals, these result from the bundling and decussation of enamel prisms. The structure is going to increase the resistance to mechanical forces of the tooth and prevent further growth of cracks.

However, the HSB-like pattern from the Iharkutosuchus’ tooth enamel is not connected to these prism bounds. By using X-ray diffraction computed tomography (XRD-CT), researchers revealed that the banded appearance is of crystallographic origin that is, the orientation of the crystals creates the optically similar pattern. Therefore, the similar appearance arose from different microstructural bases.

Polarized microscope image of a section of one of the Iharkutosuchus teeth examined. The alternating light and dark curved bands indicate an HSB-like structure, while the light and dark waves running parallel to the tooth surface indicate the presence of wavy enamel. (Photo: Krisztián Horváth)

During examinations researchers identified the wavy enamel in the prehistoric crocodile’s teeth. This structure is well-known in case of the herbivore ornithopoda dinosaurs where it plays a role in the shear-type chewing motion.

The fact that, the HSB pattern and the wavy enamel is present at the same time in Iharkutosuchus’ dentition, indicates that these microstructural solutions supported high efficiency plant procession.

The animal had strong heterodonty - teeth of different shapes and functions - and complex chewing mechanism, supported by previous anatomical and tooth wear studies. (The actively chewing nature of Iharkutosuchus is known from the anatomical and tooth wear studies of Attila Ősi and his colleagues.)

According to researchers, the mammals’ Hunter–Schreger bands, the dinosaurs’ wavy enamel and the Iharkutosuchus’ HSB-like structure could have developed independently of each other. Solutions that appear on different evolutionary lines yet are functionally similar are examples of evolutionary convergence, and they clearly show the significant role that biomechanical challenges related to chewing played in the evolution of the dentition of amniotes.

The leader author of the study, Edina Prondvai (School of Life Sciences, University of Warwick), the research’s Hungarian participant Krisztián Horváth, a doctoral student at ELTE’s Faculty of Science; both of them had previously worked at Department of Palaeontology at ELTE.

Source: ELTE Faculty of Science Cover image: Reconstruction of Iharkutosuchus makadii

(graphic: Tibor Pecsis)