Sci-Day 7: Evolutionary History of Dinosaurs, part 3: Theropoda

Happy Sci-Day, fans! This week, I'm going to talk a bit about the only clade of dinosaurs with extant members - Theropoda.

I already said some of the basics about the group in last week's post, so I won't repeat those points. The theropods are an incredibly diverse group - they range in size from the monstrous Spinosaurus, weighing in at 12 to 20.9 metric tons (Therrien and Henderson, 2007) and over 15 meters long (Ibrahim et al., 2014) to the minute Mesigulla helenae, at only 5-6cm in length and weighing around 2 grams (Del Hoyo and Sargatal, 1999).

Theropods really came into their own at the dawn of the Jurassic - these early creatures were the Neotheropoda (though Coelohysoids existed at the close of the Triassic, Neotheropoda really cranked out some species when the Jurassic came around). These early Neotheropods include the Coelophysoidea and the Dilophosauridae. The forelimbs in these groups were relatively well-developed and robust, with four digits (though at least in some species such as Coelophysis bauri, only 3 were functional (Rinehart et al., 2007)) in contrast to many more derived groups that showed digit reduction (such as the Avialans), overall size reduction (taken to an extreme in some of the most derived Abelisaurs), or both (like in Tyrannosauroids).

The split in the Neotheropoda that led to Dilophosauridae also led to the Averostra - all members of this group have a promaxillary fenestra; this group is composed of the last common ancestor of Ceratosaurus nasicornis and Allosaurus fragilis, and all its descendants (Ezucrra and Cuny, 2007). This group split up into the Ceratosauria and the Tetanurae. The former of the two includes the Abelisaurs, and were the dominant dinosaur lineage on the Southern landmasses at the close of the Mesozoic. The Tetanurae would split again to form the Megalosauroidea (a group including the Spinosaurs), and the Avetheropoda.

The Avetheropoda split into the Allosauroidea and the Coelurosauria - the former of the two lasted until roughly 93 million years ago in South America (Coria and Currie, 2006), though if Megaraptorans are in the group that would mean they lasted to the close of the Mesozoic [though it is currently thought that Megaraptorans are actually tyrannosauroids (Porfiri et al., 2014)]. The Coelurosaurs, on the other hand, still live on today in the form of birds.

Coelurosaurs are an incredibly fascinating group of dinosaurs - this is the group that contains creatures as vastly different as Tyrannosaurus rex and Microraptor. Interestingly, most dinosaurs found with preserved feathers are Coelurosaurs, and it has been suggested that feathers were a shared character between all species in the group (Currie, 2005). The Coelurosaurs came to dominate the northern continents in the end of the Cretaceous, with Tyrannosaurs being the largest terrestrial carnivores. However, it was another Coelurosaur group, the Maniraptorans, that would survive through the K-T extinction in the form of the Avialae. If that name sounds familiar, then you've been paying attention - that is the clade that consists of all extant birds!

Well, I hope you enjoyed this week's Sci-Day! I did not go too in-depth on most of the groups, because I wanted to pay particular attention to those groups that were represented in the Hell Creek formation. As far as I know, the only theropods from Hell Creek were Coelurosaurs, with Tyrannosaurus, a few Oviraptorsaurs, some Ornithomimids, and a bunch of Maniraptorans of several affinities. I could go a bit more into those subgroups, but I also want to get a chance to cover other subjects in future Sci-Day posts. If you would like to learn more about the groups, I'd encourage you to search on the Internet for some information, there are plenty of great places to look! :)

Acknowledgements:
Therrien, F; Henderson, D.M. 2007. My theropod is bigger than yours... or not: estimating body size from skull length in theropods. Journal of Vertebrate Paleontology 27 (1): 108-115.
Ibrahim, N.; Sereno, P. C.; Dal Sasso, C.; Maganuco, S; Fabbri, M.; Martill, D. M.; Zourhi, S.; Myhrvold, N.; Iurino, D. A. 2014. Semiaquatic adaptations in a giant predatory dinosaur. Science 345 (6204): 1613-1616.
Del Hoyo, J. Elliott, A. and Sargatal, J. 1999. Handbook of the Birds of the World Volume 5: Barn-owls to Hummingbirds. Lynx Edicions, Barcelona.
Rinehart, Larry F.; Lucas, Spencer G.; Hunt, Adrian P. 2007. Furculae in the Late Triassic theropod dinosaur Coelophysis bauri. Paläontologische Zeitschrift 81 (2): 174-180.
Ezcurra M.D., Cuny, G., 2007. The coelophysoid Lophostropheus airelensis, gen. nov.: A review of the systematics of "Liliensternus" airelensis from the Triassic-Jurassic outcrops of Normandy (France). Journal of Vertebrate Paleontology 27 (1): 73-86.
Coria, R.A., Currie, P. J. 2006. A new carcharodontosaurid (Dinosauria, Theropoda) from the Upper Cretaceous of Argentina. Geodiversitas 28 (1): 71-118.
Porfiri, J.D., Novas, F.E., Calvo, J.O., Agnolin, F.L., Ezcurra, M.D., Cerda, I. A. 2014. Juvenile specimen of Megaraptor (Dinosauria, Theropoda) sheds light about tyrannosauroid radiation. Cretaceous Research 51: 35-55.
Currie, Philip J. 2005. Dinosaur Provincial Park: A Spectacular Ancient Ecosystem Revealed. Indiana University Press. p. 368