Two new theropods in November!

November's not quite done yet, and we've seen two important new theropod dinosaurs named this month alone. Both of these are scientifically significant, and both come from Utah, but they lived at different times and are from different groups. Here, I will briefly introduce you to these two new dinosaurs, Lythronax and Siats.

Siats meekororum
First up is Siats meekororum, from the Late Cretaceous [1]. It lived 98 million years ago, which makes it the youngest (geologically) known allosauroid (like Allosaurus) from North America. It's also the first time a dinosaur from the theropod group known as the neovenatorids has ever been found on this continent. Siats was a top predator, living long before tyrannosaurs, and may have been as much as 12m long. It is known from a single incomplete individual, that was skeletally immature, and about 9m long, suggesting a fully grown individual would have been longer. Siats helps to fill a 25 million year gap between larger North American predators like Acrocanthrosaurus and Tyrannosaurus, and palaeontologists were trying to understand why no large predator fossils had been found in between. They also found evidence of small bodied tyrannosauroids living alongside Siats, the first time these two groups have been found living together. We now know that there were large predatory theropods throughout the Cretaceous, and the presence of these other large predators likely prevented early success of they tyrannosaurs until after these allosauroids had disappeared. This was an important find by extending the evolutionary range of allosauroids, furthering our understanding of tyrannosaur evolution, and revealing a new coexistence between two groups of predators.

For a picture and another post on Siats, see Brian Switek's post here.

Lythronax argestes
The second big theropod news of November was Lythronax argestes, meaning 'southern king of gore' [2]. Lythronax is the oldest known tyrannosaurid known from a single adult individual with a partial skull and bits and pieces of the rest of the body. It would have been about 8m in length, and lived during the Late Cretaceous, about 80 million years ago. As there is a partial skull known, we know a bit about it's behaviour. Lythronax had a similar skull shape to Tyrannosaurus, with a short and narrow snout, wide back of skull, and forward-facing eyes allowing for depth perception, which is generally considered to be a characteristic of predators. This skull shape was thought to be a derived characteristic, but this find pushes the appearance of this skull shape back by several million years. It had large serrated teeth that were used for slicing through flesh, but also capable of crushing bones with its well built skull and jaw. Currently, Lythronax represents the largest predator from the ecosystem it lived in, which was cut off from the rest of North America from the Western Interior Seaway. This find has changed some of our understanding of tyrannosaur evolution as it has features that were not previously found so early, which has changed how we thought certain tyrannosaurs were related. Lythronax will be a very important find in the understanding of tyrannosaur evolution. 

For more details, check out Dave Hone's blog post in The Guardian about Lythronax, or download the paper itself, which is free and open access!

Pictures and skull reconstruction of Lythronax argestes from Loewen et al [2]

I hope you've enjoyed this short introduction to two important new theropod finds! These will both be very important in understanding theropod evolution in the future. 

References
1. Zanno and Makovicky. 2013. Neovenatorid theropods are apex predators in the Late Cretaceous of North America. Nature Communications 4.
2. Loewen et al. 2013. Tyrant dinosaur evolution tracks the rise and fall of Late Cretaceous oceans. PLoS ONE 8: e79420. --> Freely downloadable here doi:10.1371/journal.pone.0079420