Wednesday, 16 September 2015

A New Species Of Australian Theropod

Australian dinosaurs are uncommon. While Australia has rocks of the right age for dinosaur fossils, it is a vast continent with virtually no habitation at its desert core, making exploration difficult. As such, most Australian fossil sites are restricted to its accessible regions. Roughly 12 dinosaur species are known - a small number compared to the hundreds discovered in the USA, which is of a similar size and has a similarly arid core. Yet a new species has been discovered. What is more is is a theropod. Theropods were carnivorous, and carnivores due to their higher positions in food webs, are less common than herbivores, making this find all the more exciting.

The opalised section of claw, displaying its blue colouration
The fossils, which consisted of a fragment of the lower leg, the hip, a rib, metatarsal and forearm - and a section of a 25 centimetre claw - were originally collected in the 1990s by miners working at the world famous opal mines in Lightning Ridge.

Unusually, a few of the fossils were opalised, giving them a pale blue colour. Analysis of the specimens involved simple ratios to extrapolate the creature's original size. Incredibly it came in at seven metres, making it the largest species of carnivorous dinosaur yet found on the continent.

The specimens have yet to be assigned a new species name, but their morphology demonstrates that the species belongs to the megaraptorid group of theropod dinosaurs. The megaraptorids lived on the southern continent of Gondwana. Yet they were particularly prevalent in South America, so it was long thought that they originated there and then migrated across Gondwana, resulting in the handful of Australian known specimens. This new species suggests the opposite: that the megaraptorids originated in Australia.

An artist's impression of the as-of-yet unnamed megaraptorid dinosaur
The evidence is the age of the fossils. At 110 million years old, they are ten million years older than the previous place-holder Australovenator.

Phylogenetic analysis suggested an Asian origin of the wider megaraptora group in the latest Jurassic around 150-135 million years ago, an early Cretaceous divergence of the Gondwanan lineage leading to the megaraptorid group at around 130 to 121 million years ago, and ultimately an Australian root for megaraptorid radiation. The products of this radiation would have subsequently spread across Gondwana from their Australian homeland to future South America.

'This specimen provides new evidence that Australia played an active role in the evolution and radiation of at least one group of apex theropods. Significantly, the Australian origin of megaraptorid theropods is echoed by eusuchian crocodylomorphs wherein Isisfordia duncani from the Lower Cretaceous of Queensland suggests the origin of this clade also has an Australian root.'