Studying Stegosaurs from the Arctic Circle

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Earlier this spring, a team of researchers led by Pavel Skutschas (St Petersburg University, Russia), published a paper reporting on the study of 63 stegosaur teeth that had been found in Lower Cretaceous deposits in south-western Yakutia, located in eastern Siberia. Just like today, during the Early Cretaceous this part of Siberia was at a high latitude (palaeolatitude estimate of N 62°- 66.5°) and within the Arctic circle. The scientists conclude that these stegosaurs may have had some special adaptations to help them survive their harsh environment, such as a rapid tooth replacement rate to permit them to cope with a diet mainly consisting of conifer needles and branches.

Examples of very worn stegosaur teeth from the Teete locality (Siberia).
Maxillary teeth with two (A-D), and with three wear facets (E-H) of Stegosauria indet. from the Teete locality, Yakutia, Russia; Batylykh Formation (Lower Cretaceous). Specimens ZIN PH 80/246 (A-D), ZIN PH 57/246 (E-H) in labial (A, E), lingual (B, F), occlusal (C, G) views, and interpretive drawings (D, H) in occlusal views showing the wear facets with scratches. Picture credit: Skutschas et al.

Stegosaurs Probably Present All Year

There has been much debate amongst palaeontologists as to whether large herbivorous dinosaurs such as stegosaurs were permanent residents of high latitude palaeoenvironments, or whether they migrated up to these latitudes in the summer to take advantage of the long periods of daylight. In the height of summer, there would have been around twenty-two hours of daylight at this latitude presumably providing very favourable conditions for plant growth.

The researchers, writing in the on-line, academic journal PLOS One report the discovery of smaller stegosaur teeth in the excavations, which took place in 2012 and then in the summer months from 2017-2019. As it is thought that the teeth represent a single species of stegosaur, this indicates that both adults and juveniles were present at this site.

Lead author Pavel Skutschas an Associate Professor in the Department of Vertebrate Zoology at St Petersburg University commented:

“We have found teeth of animals of different ages. This suggests that the polar stegosaurs are most likely to have been sedentary that they reproduced and raised offspring on the same territory all year round”.

Small stegosaur tooth from the dentary.
Small, relatively unworn tooth of an indeterminate stegosaur from the Teete locality in (A) occlusal, (B) labial and (C) lingual views. This suggests that both adult and juvenile stegosaurs were present. Picture credit: Skutschas et al.

A Diverse Prehistoric Fauna

The researchers from St Petersburg University worked together with colleagues from the Zoological Institute of the Russian Academy of Sciences, the Borissiak Paleontological Institute of the Russian Academy of Sciences, the University of Bonn (Germany) and the Diamond and Precious Metal Geology Institute of the Siberian Branch of the Russian Academy of Sciences to examine, excavate and sieve material from the Batylykh Formation, Sangar Series (Lower Cretaceous, Berriasian–Barremian faunal stage) along the banks of the Teete River in Suntar Ulus, Yakutia, Eastern Siberia.

This location has yielded dinosaur fossils (including theropods as well as ornithischian dinosaurs), turtles, salamanders and early mammals.

The stegosaur teeth excavation site
Researchers working at the excavation site. Picture credit: University of St Petersburg.

Stegosaurian remains are the most abundant and consist of numerous isolated teeth, vertebrae, ribs, pelvic elements and occasional cranial material. The stegosaur teeth were all retrieved by screen washing and sieving samples. Most exhibit a high degree of wear and indicate that these animals fed on very abrasive plant material. In addition, study of the tiny scratches on the teeth suggest tooth on tooth contact and precise dental occlusion in the Teete River stegosaurs.

Studying Stegosaurs

The microwear examined suggests that these animals had a more complex jaw movement to help them process food in their mouths, the tooth wear observed could not have occurred if these dinosaurs were only capable of moving their jaws up and down in a simple scissor-like action.

Examples of the very worn stegosaur teeth from the Teete locality
Maxillary teeth with a single apical facet of Stegosauria indet. from the Teete locality, Yakutia, Russia; Batylykh Formation (Lower Cretaceous). Specimens ZIN PH 7/246 (A–D), ZIN PH 11/246 (E–H), ZIN PH 20/246 (I–L), ZIN PH 6/246 (M–P) in labial (A, E, I, M), lingual (B, F, J, N), occlusal (C, G, K, P) views, and interpretive drawings in occlusal (D, H, L) and lingual (O) views showing the wear facets with scratches. Picture credit: Skutschas et al.

Rapid Tooth Replacement

Under the microscope, the researchers made another surprising discovery. The Teete stegosaurs are characterised by their relatively short tooth formation time. The teeth were rapidly replaced and the replacement teeth were formed in a relatively short time (95 days). This might have been an adaptation to the particularly abrasive diet of these herbivores which probably fed on conifers. Furthermore, the scientists identified the presence of a “wavy enamel pattern” on the teeth.

This type of enamel has also been found on the teeth of Psittacosaurus, a basal member of the horned dinosaurs (Ceratopsia) and within the Ornithopoda. The researchers conclude that this feature of teeth is a shared trait amongst bird-hipped dinosaurs. Whether it was present in the ancestor of the ornithischian dinosaurs or whether this histological feature is an example of convergent evolution in different types of plant-eating dinosaur is not known.

The scientific paper: “Wear patterns and dental functioning in an Early Cretaceous stegosaur from Yakutia, Eastern Russia” by Pavel P. Skutschas, Vera A. Gvozdkova, Alexander O. Averianov, Alexey V. Lopatin, Thomas Martin, Rico Schellhorn, Petr N. Kolosov, Valentina D. Markova, Veniamin V. Kolchanov, Dmitry V. Grigoriev, Ivan T. Kuzmin and Dmitry D. Vitenko published in PLOS One.

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