Poles revise dawn of dinosaurs theory

Polish scientists posit that the first dinosaurs descended directly from quadrupeds and not from bipeds as has been assumed until today.

“We argue that the til now hypothesis on the dawn of dinosaurs is wrong,” said Mateusz Tałanda of the Faculty of Palaeobiology and Evolution of the University of Warsaw.

The Polish palaeobiologists made their claim following research on the ways the Late Triassic Silesaurus moved about and how these modes of ambulation looked in comparison to the early avian line of evolution.

Fossilized remains of Silesaurus have been found in the Keuper Claystone in Krasiejów near Opole, Silesia, Poland, which is also the origin of its name. The species, Silesaurus opolensis, was described by Jerzy Dzik in 2003. It is known from some 20 skeletons, making it one of the best-represented early dinosauriformes.

What makes Silesaurus special is that its remains do not look anything like other pre-dinosaurs’ remnants. Silesaurus stood out thanks to their long and slim front limbs. This is because it was reconstructed as a quadruped. The discovery caused a dramatic rift in the world of science. Most scientists believed that Silesaurus readapted to quadrupedal movement and treated the phenomenon as an exception from the general trend towards bipedality.

Meanwhile, after examining the body posture and the limbic range of movement of the reptilian, the Polish scientists aim to make the Polish dinosaur a reference model for dinosaur researchers.

“Until recently it was the American Tyrannosaurus rex… who served as a model reference,” said Rafał Piechowski of the Faculty of Palaeobiology and Evolution of the University of Warsaw. “We think, however, that our Silesaurus has high chances of bringing much newer insight into the research on the evolution of early dinosaurs than his American carnivorous end-stage dinosaur cousin… There’s much to be discovered,” said Mr Piechowski.

The Polish scientists’ research showed that the front limbs of Sileaurus were entirely straight and had shrunken muscles responsible for stretching out, retraction and bending. A similar anatomical solution is found among early sauropods, which were gigantic long-necked dinosaurs. Both Silesaurses and early sauropods’ front limbs acted to support the front part of the body.

The scientists also noticed that while sauropods did not have to worry about predatory attacks due to their imposing size, dog-sized Silesaurus had to make a run for it. That is why these animals relied on their rear limbs for the most part.

The Warsaw University researchers discovered that Silesaur’s thighbones were slightly bent, whereas its cotyloid cavity was directed downwards. That placed the legs of the dinosaur beneath its torso.

“Until now, this solution has been known only among members of the crocodile evolutionary line… Silesaurus had well-developed knee extensors and flexors. Its rear limbs were well-adapted to fulfilling main propulsion features, albeit in a way unknown until now,” said Mr Piechowski.

According to the authors of the research, whose results have been published in the “Journal of Anatomy”, this mode of ambulation might not have been exclusive to Silesaurus alone. The scientists posit that all pre-dinosaurs had their front limbs pulled underneath their torsos to serve as propers. Their theory is confirmed by Triassic dinosaur tracks.