The Triassic furnishes an invaluable opportunity to study the ability of various forms of Life to rebound after a devastating event: in this case, the Permian Extinction. The Triassic is the first period in the new Mesozoic Era and so we see in all flora and fauna a mix of the old, the new, and the transition between the two. The Paleozoic life forms didn’t die precisely at the Permo-Triassic boundary. They continued to live on, adapting, diversifying, becoming new versions of the old forms as they faced new challenges in the Triassic. Eventually the old Paleozoic forms succumbed while the newer and modified versions thrived – a story repeated over and over again in the history of Life on Earth.
In general the animals of the Triassic (both marine and terrestrial) were smaller than those of the Permian as oxygen levels continued to fall. In such low oxygen conditions it is thought that the fauna were forced to quickly innovate to produce new physical adaptations capable of coping with low oxygen, high carbon dioxide and extremes of heat and aridity.1 Peter Ward from the University of Washington states the following:
But it is on land that the most sweeping changes in terms of body plan replacements – and body plan experimentation – took place. Never before and never since has the world seen such a diverse group of different anatomies on land...From mammal-like reptiles to lizards, earliest mammals to true dinosaurs, the Triassic was a huge experiment in animal design.2
Dr. Ward goes on to say that that these Triassic terrestrial body plans were as diverse as the marine body plans that occurred during the Cambrian Explosion.3 That’s quite a statement! If you look at the world today, there seems to be an endless variety of creatures. Just think. All this variety that we see is minor when compared to the variety that existed in the Triassic.
As in the Late Permian, the animals of the Early Triassic had a slow gait and reptile waddle. Both the low-oxygen conditions and their body structure continued to prevent them from moving rapidly or effortlessly. By the Middle Triassic a revolution in locomotion took place as one special group of animals, the dinosaurs, outpaced their competitors becoming bipedal predators poised for supremacy.
There is one benefit to surviving a mass extinction: the available space is incredible and the rents are low. Because the Permian Extinction had eliminated species in many different environmental niches, there were plenty of available habitats free of competitors. Many of the species in the Early Triassic (such as Lystrosaurus) quickly took over regions, rapidly multiplying and diversifying, populating the landscape and filling a variety of ecological roles. As circumstances improved, however, these same species found it difficult to tolerate the changing conditions or compete against other fauna that were beginning to thrive. The animal species that made it through the Permian Extinction, and seemed at first to flourish in the Early Triassic, were similar to the “weedy opportunists” in the plant kingdom: thriving in disturbed circumstances but giving way when competition increased.
The availability of habitat, low competition and a harsh environment were all factors that made the Triassic a perfect petri dish for a grand experiment to create completely new body plans, new genera and new species. By the Middle Triassic, the old Paleozoic predecessors were mostly gone and new body plans emerged in the form of mammals and dinosaurs.
The Permian Mass Extinction eliminated most of the ammonite genera with only a few exceptions: the Prolecanitida and the Ceratites. It is amazing that these two groups were able to endure (albeit in small numbers) the Permian Mass Extinction and the extremely difficult conditions in the post-extinction world of the Early Triassic; but not only did they survive, the Ceratites thrived. What's more, a new order of ammonites appeared in the Early Triassic: the Phylloceratida.
During the Early Triassic there were freqent mass extinctions as conditions cycled through good and bad episodes. These extinction weren't on the scale of the major mass extinctions, but many of the ocean's inhabitants were profoundly affected. These difficulties took their eventual toll on the Prolecanitida and this order disappears by the end of the Early Triassic.
The Ceratida also have a unique story. These hardy survivors made it through the worst mass extinction at the end of the Permian period, endured terrible conditions throughout the Early Triassic, but finally succumbed to the extinction that occurred at the end of the Triassic. Such stories begs innumerable questions which have yet to be answered.
1. Ward, Out of Thin Air, p. 159ff.
2. Ibid., p. 160.
3. Ibid., p. 161.