Sci-Day 6: Evolutionary History of Dinosaurs, part 2: Evolution and Diversification of Dinosauria

In last week's Sci-Day, I did an overview on the origin of Dinosauria - what they branched off from, and a few of the characters that are indicative of that ancestry. This week, I will be continuing from where we left off - the dinosaurs just appeared, and we're going to look at what happened in the next ~180 million years. If you have not read the last Sci-Day post, I would recommend that you do so now.

As I said before, the earliest true dinosaurs known date from the Carnian stage of the Late Triassic. In this time, Dinosauria diverged into two lineages. These two lineages, Ornithischia and Saurischia, can be identified by the position of the pubis and ischium. In Saurischians, the ischium points backwards while the pubis points forward; in Ornithischians, both pubis and ischium point backwards (as in modern birds). However, the similarity between the hips of birds and those of Ornithischian dinosaurs is not indicative of shared ancestry, but rather is a result of convergent evolution.

After this split between the aforementioned lineages, Saurischia diverged into another two lineages - Theropoda and Sauropodomorpha. Theropoda is composed of bipedal (with the possible exception of Spinosaurus (Ibrahim et al., 2014)) dinosaurs - many lineages were carnivorous, though there were several groups that evolved omnivorous and herbivorous diets such as Therizinosaurs and some Maniraptorans (Longrich and Currie, 2009). Modern birds are the only living descendants of this group.

Sauropodomorpha consisted of bipedal and quadrupedal herbivorous dinosaurs with long necks, proportionally small heads, and long tails for counterbalance. The synapomorphies for this clade can be found in Martin, 2006. While the earliest members of this lineage were relatively small (Langer et al., 1999), through time they would grow to be the largest animals to ever walk the planet - the largest known sauropod, Argentinosaurus huinculensis, has been estimated to be up to 30 meters (98 feet) long (Carpenter, 2006) and weighed up to 83.2 tonnes (91.7 tons) (Sellers et al., 2013)!

Now, because there are so many lineages to talk about within the aforementioned 3, I will be using subsequent Sci-Day posts to go into more detail about some of those lineages so I can do them justice. Plus, if I were to write all of it in a single post, it would take a LONG time to read! I hope you have learned something from this post, and I look forward to delving into the history of one of these lineages in the future Sci-Days!


Acknowledgements:
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.
Longrich, Nicholas R.; Currie, Philip J. 2009. Albertonykus borealis, a new alvarezsaur (Dinosauria: Theropoda) from the Early Maastrichtian of Alberta, Canada: Implications for the systematics and ecology of the Alvarezsauridae. Cretaceous Research 30 (1): 239-252.
Martin, A.J. 2006. Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300.


Carpenter, Kenneth. 2006. Biggest of the Big: A Critical Re-Evaluation of the Mega-Sauropod Amphicoelias fragillimus Cope, 1878. In Foster, John R.; Lucas, Spencer G. Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin. pp. 131-138.
Langer, M.C., Abdala, F., Richter, M., and Benton, M. 1999. A sauropodomorph dinosaur from the Upper Triassic (Carnian) of southern Brazil. Comptes Rendus de l'Académie des Sciences, 329: 511-517.
Sellers, W. I.; Margetts, L.; Coria, R. A. B; Manning, P. L. 2013. Carrier, David, ed. March of the Titans: The Locomotor Capabilities of Sauropod Dinosaurs. PLoS ONE 8 (10): e78733.