Evolutionary trait of dinosaurs discovered
Perhaps the most famous of all dinosaurs is the Tyrannosaurus rex; this large carnivorous dinosaur with its huge teeth would have cut a fearsome figure. If it came to boxing however - with its disproportionately, almost comical, small arms - it would have come up short. However, the Tyrannosaurus wasn't the only dinosaur to have such small arms; a whole family of dinosaurs known as the abelisaurids also shared this trait, and according to new discoveries, may have actually began with them.
Fossil material of the abelisaurid family of dinosaurs discovered in the southern-most tip of South America, Patagonia, is showing a timeline of the evolution of this characteristic. The specimen in particular dates from the Jurassic age and is 40 million years older than any other known abelisaurid. It clearly shows that arm reduction began at an early stage in the group's evolutionary history.
This discovery was made by Ludwig Maximilians University Munich (LMU) palaeontologist Dr Oliver Rauhut, who is also affiliated with the Bavarian State Collections for Palaeontology and Geology in Munich and his Argentinian colleague Dr Diego Pol. 'Abelisaurids were a highly diverse and widely distributed group during the Cretaceous [period],' says Rauhut, 'but their origins have so far remained enigmatic.'
The abelisaurids were bipedal carnivores approximately seven to nine meters in length and feared predators in their day. They roamed the area of Patagonia and other areas of Gondwanaland, the southern-most continent which was comprised of what is now Africa, Antarctica, Australia and South America. In characteristics they closely resembled the tyrannosaurids, the main difference being their unusually short and high skulls which were unique. This feature suggests that they had an extremely powerful bite.
Rauhut and Pol have named the new abelisaurid species Eoabelisaurus mefi, or 'dawn Abelisaurus of the Museo Palentológico Egidio Feruglio (MEF)', in recognition of the Museum's support for their research collaboration. 'The new find reveals that the abelisaurid lineage is more ancient than we thought,' says Rauhut. 'The reason why so little is known about it is that the fossil record of predatory dinosaurs in the Southern hemisphere is very incomplete, especially for the period from the Middle Jurassic to the Early Cretaceous.'
While they too had short arms like the Tyrannosaurus, they are different in that according to some research, the Tyrannosaurus was able to use its arms like powerful hooks to keep its prey immobile. The abelisaurids arms appear to be much weaker and simply vestigial remnants. Their finding however has allowed researchers to make a clearer picture of how these arms evolved.
'The trend apparently set in early on, and began at the distal end,' says Rauhut. 'In Eoabelisaurus, the upper arm is of normal size, but the lower arm is much shorter in comparison; the hand is very stunted and the fingers and claws are tiny.' This finding appears to confirm suggestions from other palaeontologists that the reduction in abelisaurids began with the hand.
What surprised the researchers with their find was that while the Eoabelisaurus had evolved on the supercontinent Pangaea, they are not found on every continent following its breakup. Pangaea was the supercontinent that consisted of every continent we know now of today and existed some 300 million years ago. 'One possible explanation is that a huge desert in Central Pangaea prevented dispersal of the group to the North, confining the evolution of the lineage to the Southern hemisphere,' says Pol.
The hypothesis of the existence of such a geographical barrier is supported by recent geological studies and the results of climate modelling.
Despite their significant findings there is still a lot to discover. 'Our picture of dinosaur evolution in the Jurassic is largely based on fossils from Northern sites,' says Rauhut. 'The Southern hemisphere surely still has many surprises in store for us.'
The scientists hope to continue their research into the Jurassic dinosaurs of Patagonia, which has been supported by the Deutsche Forschungsgemeinschaft (DFG) and the Argentinian Agencia Nacional de Promoción Científíca y Tecnológica.
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