Palaeontologists discover of one of the oldest carnivorous dinosaurs of the Jurassic period which measures around 8 metres long. This discovery represents a moment of evolutionary explosion in which dinosaurs diversified.
The new fossil, found in the Argentine province of Chubut, has revealed a mixture of lineages in this species that leads scientists to reconsider its evolution.
"The study of the skeleton revealed a combination of very unusual characters that mixes peculiarities typical of different families of the group of tetanus, which is difficult to reconcile with the currently accepted image of the relationships between the three large groups of dinosaurs that comprise the tetanus: megalosauria, allosauria and celurosauria. This analysis leads us to rethink what we know so far about the early evolution of carnivores in the Jurassic".
explained Diego Pol, researcher at the Argentine organism CONICET and his colleague, Oliver Rauhut from the University of Munich.
The largest predator of the time
If we were to imagine this dinosaur in real life it would be a very intimidating one. The specimen can be assigned to the tetanurans, the most prominent group of bipedal dinosaurs, which includes such emblematic representatives as Allosaurus, Tyrannosaurus Rex and Velociraptor. The oldest Tetanurans are found in the earlier Middle Jurassic, but most of the known Middle Jurassic fossils are extremely fragmentary. Hence the importance of this discovery.
The fossil probably represents the oldest known representative of one of the main lineages of carnosaurian theropod dinosaurs: the alosauroids.
A new piece of the puzzle
The fossils represent a previously unknown gender. When the fossil was discovered, 50 % of the skeleton could be recovered, with the skull and complete jaws (almost two dozen teeth were found) and the complete vertebral column, including parts of the pelvis, all the bones of both anterior extremities and parts of the legs.
According to the results of the phylogenetic analysis of the new species, all the species of the two large groups allosauroids and megalosauroids have a common ancestor that they do not share with the coelurosaurus.
"The new taxon shows an unusual mosaic of tetanurian characters and highlights the large number of convergences in the early evolution of the group and, therefore, the resulting phylogenetic uncertainty in its early evolution," the authors explain.
Rauhut links the explosive evolution of the group with an episode of mass extinction that took place in the last stage of the lower Jurassic, some 180 million years ago. Therefore, the two experts interpret the parallel development of similar external traits in different species as an example of evolutionary experimentation during the subsequent rapid expansion and diversification of the tetanurans.
The previous extinction of potential competitors would have opened the way to new ecological niches for those who survived, and the tetanurans were apparently among them.
"This is a pattern that we also see in many other animal groups after mass extinctions. It is fulfilled, for example, for the expansion and diversification of mammals and birds after the extinction of dinosaurs at the end of the Cretaceous 66 million years ago, "concludes Rauhut.
Reference: Probable basal allosauroid from the early Middle Jurassic Cañadón Asfalto Formation of Argentina highlights phylogenetic uncertainty in tetanuran theropod dinosaurs, Oliver W. M. Rauhut & Diego PolScientific Reports 9, Article number: 18826 (2019). Nature DOI: https://doi.org/10.1038/s41598-019-53672-7