Sci-Day 5: Evolutionary History of Dinosaurs, part 1: The Evolutionary Origin of Dinosauria

Happy Sci-Day, everyone! I'll admit it took me a long time to decide what today's topic would be, especially given that it is following the single most important post that I could ever make on any Sci-Day. That said, today I will be trying to give you a bit more info on what we know about the evolutionary history of dinosaurs. I will not be going too in-depth on specific families within dinosauria, though perhaps later sci-day posts will cover that information on lineages that were present in some form in the Hell Creek formation. This will be a two-part blog - today, I will simply cover the origins/roots of the dinosaurs, with the second post covering the evolutionary history of the group itself.

Before I delve in to their origins and evolution, let's answer an important question: what is a dinosaur?

A dinosaur is any species within the clade Dinosauria. I have already explained cladistics in my first Sci-Day post, so if you haven't read that, you may want to do so now it will help you understand this a bit easier. There are several characteristics that all members of Dinosauria share - these derived characteristics are what distinguish them from other groups, and are known as synapomorphies. I will not list these here, as they require a deep understanding of skeletal anatomy, but it is important to understand that these characteristics are shared by dinosaurs and are indicative of their shared evolutionary history.

Now, onto the question at hand - where did dinosaurs come from?

Well, when a mommy and a daddy dinosaur love each other very much.... Oh, whoops, we're talking about where they come from in an EVOLUTIONARY sense! My mistake. ;)

The evolutionary path that would eventually lead to dinosaurs started in either the Permian or Early Triassic, with the first Archosaurs (Gower and Sennikov, 2003). These creatures had several synapomorphies that dinosaurs would also inherit (Dinosauria is placed within Archosauria, which also includes the lineage that consists of modern crocodilians and their extinct relatives) - their teeth were set in sockets, they had two additional openings, or fenestrae, in their skull (antorbital and mandibular, specifically) (Dyke and Kaiser, 2011), and a prominent ridge on the femur called a fourth trochanter (Khanna, 2004). The last of these was particularly important, as it provided a large site for muscle attachments that allowed for an upright gait in these primitive archosaurs.

By the middle of the Triassic period, archosauria split into several separate lineages - the largest two were Pseudosuchia (those archosaurs more closely related to modern crocodilians than to birds), and Avemetarsalia (those archosaurs more closely related to birds than to modern crocodilians) (Benton, 1999). The dinosaurs are included in the latter of these two, linked by the structure of their ankle joint. Interestingly, this clade also includes Pterosauria - I think you can guess what prehistoric creatures that lineage led to :)

Dinosauromorpha split into three lineages, with dinosaurs being placed in the aptly-named dinosauriformes. From this clade came Dinosauria (as well as several other taxa, but let's stay on topic here). The earliest true dinosaur known is Eoraptor - a small, bipedal predator that lived approximately 231 million years ago. It is thought that this genus bears resemblance to the common ancestors of all dinosaurs (Sereno, 1999). Throughout the rest of the Triassic, dinosaurs were not the dominant terrestrial animals; that role fell to the archosauromorphs. It is also believed that dinosaurs competed with the pseudosuchians of the time  (Brusatte, 2008).

However, towards the end of the Triassic, many of the early archosauromorphs started to disappear, and many lineages went extinct during the Triassic-Jurassic mass extinction event. As the archosauromorphs began to die out (during the Carnian and Norian stages of the Triassic), the early dinosaurs began to diversify - it is hypothesized that this diversification was due to the vacant niches left by the extinct archosauromorphs (Langer et al., 2010).

Hopefully, this has given you a bit more information on how dinosaurs came to be. Check in next week to find out more about the evolution of dinosaurs themselves!

Acknowledgements:
Gower, D.J.; Sennikov, A.G. 2003. Early archosaurs from Russia. In Benton, M.J.; Shishkin, M.A.; and Unwin, D.M. (eds.). The Age of Dinosaurs in Russia and Mongolia. Cambridge: Cambridge University Press. pp. 140-159.
Gareth Dyke, Gary Kaiser, ed. 2011. Living Dinosaurs: The Evolutionary History of Modern Birds. John Wiley & Sons. p. 10.
Khanna, D.R. 2004. Biology Of Reptiles. Discovery Publishing Hourse. pp. 78ff.
Benton, M.J. 1999. Scleromochlus taylori and the origin of dinosaurs and pterosaurs. Philosophical Transactions of the Royal Society of London 354: 1423-1446.
Sereno, P.C. 1999. The evolution of dinosaurs. Science 284 (5423): 2137-2147.
Brusatte, Stephen L. Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs. 321 (5895): 1485-1488.
Langer, Max C.; Ezcurra, Martin D.; Bittencourt, Jonathas S.; Novas, Fernando E. 2010. The origin and early evolution of dinosaurs. Biological Reviews 85 (1): 65-66, 82.