Scientists find the oldest tyreophoran track in the global south

Thyreophora are a suborder of herbivorous dinosaurs that inhabited the Earth from the early Jurassic period (about 200 million years ago) up to late Cretaceous (around 65 millions of years ago). Although there are records of their presence in both hemispheres, the fossil and ichnofossil findings that were known so far, allowed scientists to speculate that this group of animals were of Boreal origin and had arrived in the south shortly before the beginning of the Cretaceous.

“In South America, the oldest known footprints of thyrophores were found in Brazil and belonged to a limited stage between the Late Jurassic and the Early Cretaceous. Further south, the records of the presence of this clade corresponded to the Cretaceous period,” explains Pablo Pazos, CONICET independent researcher and director of the ‘Don Pablo Groeber Institute of Andean Studies’ (IDEAN, CONICET-UBA).

Recently, in Lajas Formation, Pazos and a group of collaborators found a Jurassic geological unit that is part of the Neuquén Basin –more specifically in Covunco (province of Neuquén) located in the north of the Huincul High – a footprint that corresponds to the foot of a Thyroforo of the Middle Jurassic. The discovery was published in Journal of South American Earth Sciences.

For Pazos, specialist in sedimentology and ichnology, apart from the paleobiological novelty, the finding makes experts rethink the existing interpretations of the ‘Lajas Formation’ (reservoir of gas and oil in the subsoil), whose localities are placed in the south of the Huincul High and were far more studied than those of the north of it.

“So far, the whole unit was considered to be part of a deltaic mega system that advanced on the sea (the paleo-pacific), so it was not expected to find remains of dinosaurs, much less traces. This forces us to review the geological hypothesis that suggested that the entire area was underwater, considering that the footprint appeared in the basal section within a rock profile of 500 meters. In the case of a large delta, as it happens in the south, we must have found underwater marine deposits,” explains the researcher.

The evidence that animals were walking on the basal section of the geological unit would imply that was not only marine but also was exposed to the air, what makes experts ask if the Lajas Formation has the same age in the north and the south of the Huincul High.

“In this sense, one of our collaboratos remembered an L.R. study of Lambert done in 1940s on the discovery of trigonias (an extinct genus of marine bivalves) in the area, which suggests a younger age. What was also indicated that localities of the north of the unit were newer than those located to the south,” Pazos comments.

The analysis of the researchers shows that the footprint is characteristic of the stegosaurus (a genus of tyrophoric dinosaurs) and it is undoubtedly the oldest in the Neuquén Basis and the oldest of a tireophore for the Southern Hemisphere and for all the territory of what used to be the supercontinent Gondwana, before the separation in deep waters of South America, Antarctica and Australia.

A particular characteristic of this finding is that, unlike what usually takes place, it is a single isolated footprint- the most frequent is to find them in pairs or composing a walk –and that is on an inclined plane and not in one horizontal as usual.

“The mark of the foot of the dinosaur is preserved in one sedimentary structure that is generated by fluvial currents and that produces the formation of the inclined plane. Probably, the surface on which the dinosaur stepped was submerged, not totally though, and that the humidity and the microbial bushes have favor its preservation. This is consistent with the hypothesis we found by reviewing the literature that stegosaurs could traverse small bodies of water,” Pazos says.

Although it is not possible to determine precisely the age of the site in which the discovery took place, the researchers conclude that it must be more than 163 millions years old and less than 170 million years old.

“Lajas ends in discontinuity –this means that in one temporal discordance with respect to the unity that is on it- that shows that what comes above is necessarily newer. We know that what comes up is part of the Callovian (between 166.1 and 168.3 million years ago), an early age of the Middle Jurassic. So the footprint, as very new could be from the first part of Callovian, but it is not possible to rule out that it can be a bit older. What we are sure is that until now, it is the oldest of a thyrophoros found in what was the supercontinent Gondwana,” concludes the researcher.