Extinction Week: Triassic-Jurassic
Everything is about to change for you little guys. Also @apsaravis can I just say thanks for drawing so many pictures of basal Avemetatarsalians with protofloof? I wouldnât have any accurate drawings for this series otherwise. Source:Â http://apsaravis.tumblr.com/post/74736331120/back-to-the-triassic-period-redondasaurus-withÂ
Time: 201.3 million years ago, in the Rhaetian age of the Late Triassic of the Mesozoic of the PhanerozoicÂ
Analogy: Did you ever play on a team in High School? Even like an academic one? I was on my schoolâs Science Olympiad team (always won a medal in fossils, thank you very much. I also was in charge of organizing bio events, no big deal). Anyway do you remember when, like, the upperclassmen graduated, and suddenly you had a lot of responsibility and you were an important member of whatever club or team you were on? Like, they disappeared, and you got to take their place and be the big guys? Yeah thatâs this, but for dinosaurs.Â
This thing was one of the many, many, many weird non-dinosaurs of the Triassic. Art from http://markwitton-com.blogspot.com/Â
Causes: Where did we leave off? Oh right nearly everything died. My favorite! Well life eventually did recover after the Permian Extinction - obviously, as weâre all here today. Archosauromorphs were really taking over the scene, and there we many different kinds everywhere - dinosaurs were a relative minority compared to all the other different types of reptiles, and pterosaurs were extremely rare as well. Nothosaurs, Pachypleurosaurs, Placodonts, and the first Plesiosaurs evolved; there were Thalattosaurs, and Ichthyosaurs were very successful in the Triassic. There were many amphibians, Rhynchosaurs, Phytosaurs, Aetosaurs, Rauisuchians, the first Crocodylians, Prolacertiformes, the earliest turtles, and the cynodonts all evolved and diversified, with cynodonts getting closer to being mammals. In fact, looking at the assemblage of the Triassic, one could hardly guess that it would be dinosaurs that would become the dominant megafauna of the Mesozoic period - various Pseudosuchians (crocodile-line Archosaurs) seemed to be taking that role.Â
Nope, not a dinosaur. Art from http://www.arcadiastreet.com/cgvistas/earth/03_mesozoic/earth_03_mesozoic_0900.htmÂ
So what happened then? After all, this is pretty quick after the last death machine - only about fifty million years. Well, the causes of this extinction are actually⌠not very clear. Once again people have tried to pin it on an asteroid - which would make some sense, since it was the large non-dinosaurian archosaurs that were hit the hardest - but no such crater has been found, and all possible candidates have been ruled out. There was some climate change and sea level fluctuations, but they werenât sudden enough to explain the extinctions in the ocean - this was a very sudden pulse of death, more so than what you would expect from just gradual environmental fluctuations.Â
There may have been some massive volcanic eruptions again - yay! more lava! - which seems to be the best bet.  The flood basalts of the Central Atlantic Magmatic Province would have released carbon dioxide, sulfur dioxide, and aerosols, causing - once again - pretty terrible global warming and cooling. There seems to be evidence for a major CO2 greenhouse crisis in ocean extinction deposits, and evidence that both the marine and terrestrial extinctions happened at once, which may indicate that a sudden influx of carbon dioxide and accompanying warming caused the extinction. However, thereâs a chance that the eruption of the CAMP was too old to have caused the extinction.Â
More⌠lava? http://palaeo.gly.bris.ac.uk/palaeofiles/triassic/triextict.htmÂ
If it was the volcanic activity, perhaps exacerbated by already occurring climate change, then the increase in global temperatures would have had another terrible effect on life on the planet - which had just recovered, and thus would be fairly vulnerable.Â
Extinction Rate: This was not as bad of an extinction pulse as the last one, so if it was volcanic activity, it was the kind not in the worst possible place ever. Hooray! 23% of all families, 48% of all genera, and 70 to 75% of all species went extinct.
Effects: This was actually more acutely felt in the terrestrial environments than in the marine ones, though they were also affected. A large portion of the non-dinosaurian Archosauromorph groups went extinct, which emptied ecological niches on the land extensively. In the ocean, there seemed to just be less diversification - a decrease in speciation events rather than an extinction of already existing taxa. Phytosaurs and Aetosaurs were completely wiped out, as well as others.Â
Which means you got to evolve, buddy. Art from http://www.smithsonianmag.com/ist/?next=/science-nature/dilophosaurus-an-early-jurassic-icon-57431583/Â
If we can call any time periods much of anything, then, the Triassic was truly the age of reptiles, with a wide variety of forms and diversity across different major clades. One could argue, then that this age ended after the Triassic extinction - though many reptiles remained very common, it was really more specifically the Age of Dinosaurs after this point. Dinosaurs evolved to fill the vacant niches and finally started growing to the sizes to which we are accustomed with them - sauropods started evolving (at last, from stinking prosauropods), large theropods began to grow more and more common, and Ornithischians also finally had a chance to diverge.Â
In fact, the rapid pulse of diversification of dinosaurs after the end-Triassic extinction lead to a large morphological diversity of dinosaurs, and at some point between the middle and late Jurassic epochs, the earliest members of Avialae evolved - the group that would eventually lead to birds.Â
Birds. Birds everywhere. Empty niches everywhere means more dinosaur diversity which means trying out new and weird things which means birds. Art from http://emilywilloughby.com/gallery/paleoart/anchiornisÂ
Mammals also evolved in the Early Jurassic, probably due to pressures following this extinction event. They remained small predators, living in regions where there was still many places to hide, though they did diversify somewhat, and more so than previous paleontologists believed.Â
So, though a relatively minor extinction, it once again directly lead to the world we know and love today - mammals became a thing, dinosaurs grew into what we really know and love about them (widely diverse and weird, because birds are weird you guys,) and other reptiles began to dominate the sea and air (hellooooo pterosaurs, pterosaurs everywhere). In fact, dinosaurs were so versatile and widely morphologically diverse following the Triassic extinction, thanks to all those sweet, sweet available niches, that they were very well adapted to their world.Â
Of course, now we have to deal with sauropods, but whatever. http://haghani.deviantart.com/art/Some-of-the-Jurassic-period-dinosaurs-and-plants-385694737
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