A Tale of Two Extinctions: What the Triassic Tells us about Who Lives, Who Dies and Why it Matters Today
About 250 million years ago, the worst mass extinction Earth has ever seen left a gaping hole in ecosystems the world over. The organisms that filled that hole would come to define the world of the Mesozoic and leave fundamental changes in their ecology that still shape the living world today. On land, there was an explosive diversification of reptiles, including among its array of weird wonders the ancestors of modern lizards, turtles, and crocodiles. In the seas, an invasion of new predators and shell-crushers set off an arms race that produced the diverse community of fast-moving fish and heavily-armored invertebrates we see today. And under the feet of the new ruling reptiles, a last gasp of the Permian world would make one of the most significant morphological innovations in mammalian history (Benton et al, Dal Corso et al). Then, after only 50 million years, the world would be wiped clean again, ending a range of Triassic experiments and giving way to the more recognizable world of the Jurassic and Cretaceous. Among the beneficiaries of this new world were the dinosaurs, who started as a small part of an already crowded fauna, but, by the beginning of the Jurassic, had come to dominate a newly-emptied ecosystem, and, with very little serious competition, radiated out into the variety of forms that characterized the later Mesozoic (Dunne et al). But what caused this change, from thriving ecosystems full of evolutionary novelties to barren landscapes monopolized by a small handful of survivors, and what advantage did they have that allowed them to make it through and recolonize this barren world?
Historically, dinosaurs were believed to have been "competitively superior" to other animals in their environment, especially the synapsid holdovers from the Permian. For a variety of reasons, ranging from more efficient breathing to greater maneuverability to even endothermy, dinosaurs were thought to have ecologically crowded out large protomammals, forcing them into a "side- squeeze" where they developed small size, nocturnal lifestyles, insulation, and parental care in response to becoming prey animals. Research from Prof. Michael Benton (University of Bristol) suggests that this takeover may not have been due to ecological competition at all, but that the dinosaurs were beneficiaries of the same events that allowed mammals to expand into these new niches, and that their success- along with a number of decidedly more "modern-looking" organisms- was more closely intertwined than anyone expected. In a series of reviews (Benton, Benton et al, Dal Corso et al), he argued that the two greater lineages of archosaurs and synapsids were, from the end-Permian, engaged in an arms race for control of their ecosystems, an escalation of adaptations for more active lifestyles that culminated in dinosaurs on one hand, mammals on the other. As he put it in an interview with Phys.org, "who survived (the Permian-Triassic extinction) depended on intense competition in a tough world. Because a few of the survivors were already endothermic in a primitive way, all the others had to become endothermic to survive in the new fast-paced world" (Benton B).
Endothermy has a variety of advantages, including the ability to survive in cold environments and engage in sustained high-energy activity, but at the cost of massive energy requirements. In stable environments, ectotherms can be much more energy-efficient (Benton). The Early Triassic was not a stable environment. After massive volcanic eruptions pumped greenhouse gasses into the atmosphere, much of the world became unlivable for extended periods; acid rain killed forests, leading to widespread anoxia; erosion killed off calcium-producing marine life, perpetuating global warming and ocean acidification (Benton; Benton et al). This produced strong selection pressures for animals that could regulate their body temperature and breathe as efficiently as possible in the deoxygenated atmosphere.
It's long been noticed that dinosaurs had straighter legs, meaning more efficient locomotion, than early archosaurs (Cuff et al); it was thought that in the Early Triassic, all archosaurs sprawled like lizards. Studies of trackways by Benton and Kubo showed, as predicted, that average postures became more upright through the Triassic (Benton), as archosaurs and synapsids competed for ecological dominance through an array of increasingly active carnivores and herbivores of all sizes; as one got bigger and faster, the other got smaller and smarter. Benton told Phys.org, "animals on land and in the oceans were speeding up, using more energy, and moving faster (...) if the predator gets faster, the prey does too in order to escape" (Benton et al B) What was surprising was that the switch from sprawling to upright seemed to happen instantly at the Permian-Triassic boundary. Why the change?
David Carrier discovered that upright-walking animals have greater stamina because sprawlers are unable to breathe while moving; the same muscles do both, and deoxygenated air is passed between the lungs as the body moves from side to side (Benton; Cuff et al). Before the Triassic, synapsids had overcome this problem by reducing their ribcage, creating specialized diaphragm muscles that allowed for breathing in and out while the backbone moved up and down. With all the sprawlers dying off, archosaurs were forced to adapt quickly, either in response to these environmental changes or in direct competition with synapsids. The embryology of living crocodiles suggests they once had birdlike lungs and 4-chambered hearts, vital for taking in oxygen; they may have lowered their metabolism secondarily to stay underwater longer. Likewise, both synapsids and archosaurs show evidence of fast-growing bone, suggesting high metabolic rates; high O-16/ O-18 ratios (the heavier O-18 condenses in colder environments), suggesting consistently warm internal temperatures; and bones filled with small, densely packed canals for transmitting oxygen through blood, vital for high-capacity activity and tolerating hypoxia (Benton, and sources therein)
In fact, synapsids seem to have been ahead of archosaurs in a number of features, including pits for sensory hairs, suggesting insulation from the extremes of hot and cold of the post-Permian climate; differentiated teeth, allowing for more efficient processing of food; a palate between their mouth and nose, allowing them to breathe while eating; and nasal turbinals, which allow for cooling and moisture retention in the hot, dry climate. Even the classic mammalian feature of parental care seems to have evolved early: Early Triassic cynodonts have been found in large groups in burrows, likely dug for insulation of the young against extremes of temperature (Benton, and sources therein). If they had such an early advantage, why was the post-Triassic world dominated by dinosaurs instead?
The beginning of the Late Triassic saw another faunal turnover, spurred by another eruption. The Carnian Pluvial Event was a worldwide change for the wetter, when swamp forests replaced the earlier desert vegetation, the new carbon burial led to flourishing calcifying reefs, and the global climate stabilized. Once the eruptions ended, the previously massively abundant large herbivores had gone extinct, along with the animals that preyed on them, and more "modern" animals, including both mammals and dinosaurs, radiated out to fill the ecological space they left behind. Dinosaurs, in short, didn't outcompete large synapsids; they replaced them (Benton, Benton et al, Dal Corso et al)
The Carnian saw the first phase of the dinosaur takeover, but they were still one part of their fauna among many, relegated to temperate high latitudes at a time when CO2 levels were up to five times today's levels, and huge swaths of tropical forest, dominated by croc-line archosaurs, covered much of the world. The world was just as hot during most of the Jurassic when dinosaurs spread around the world, so it was assumed they were kept out of this climatic belt by competition, only radiating out once other large animals were gone (Dunne et al B; Olsen et al). Two recent studies have called this narrative into question. A "climatic niche" model, made by a team from the University of Birmingham, suggests that dinosaur distribution was restricted climatically until the Jurassic, when the world got uniformly cooler, killing off big heat-adapted archosaurs. When it heated up again, they "shifted to a warmer niche," adapting to their new environments (Dunne et al). An excavation of lake bottoms in China's Junggar Basin, which would have sat about 71° north, provides the first direct evidence for this global freeze. Researchers described dinosaur footprints along lake shorelines, alongside abnormally large pebbles, which they suggest were formed as ice-rafted debris left behind by the frozen lake picking up sediment from the shoreline. This suggests dinosaurs were already living in wintery conditions, that spread into the tropics when the end-Triassic eruptions spit sulfur into the atmosphere (Olsen et al). Dinosaurs, already adapted to the cold, survived, while tropical animals died.
The Triassic, bounded by two of the biggest extinctions, was one of the most significant times in the history of life. After the destruction wrought by the Permian, life bounced back more diverse and energetic than ever before, producing the two disaster-proof lineages that would dominate the world from that point on. As Michael Benton put it (Benton et al B):
"These are not new ideas. What is new is that we are now finding that they were all apparently happening about the same time (...) This emphasizes a kind of positive aspect of mass extinctions (...) the mass clear-out of ecosystems in this case gave huge numbers of opportunities for the biosphere to rebuild itself, and it did so at higher octane than before the crisis."
Works Cited
Michael J. Benton. The origin of endothermy in synapsids and archosaurs and arms races in the Triassic, Gondwana Research (2020). DOI: 10.1016/j.gr.2020.08.003. Accessed from https://phys.org/news/2020-10-world-greatest-mass-extinction-triggered.html.
Dal Corso, J., Bernardi, M., Sun, Y., Song, H., Seyfullah, L. J., Preto, N., Gianolla, P., Ruffell, A., Kustatscher, E., Roghi, G., Merico, A., Hohn, S., Schmidt, A. R., Marzoli, A., Newton, R. J., Wignall, P. B., & Benton, M. J. (2020). Extinction and dawn of the modern world in the Carnian (late triassic). Science Advances, 6(38). https://www.science.org/doi/10.1126/sciadv.aba0099.
Michael J. Benton et al, Triassic Revolution, Frontiers in Earth Science (2022). DOI: 10.3389/feart.2022.899541. Accessed from https://phys.org/news/2022-06-triassic-revolution-animals-grew-faster.html.
Paul Olsen et al, Arctic ice and the ecological rise of the dinosaurs, Science Advances (2022). DOI: 10.1126/sciadv.abo6342. Accessed from phys.org/news/2022-07-dinosaurs-ice-warmth-ancient-mass
Emma M. Dunne, Alexander Farnsworth, Roger B.J. Benson, Pedro L. Godoy, Sarah E. Greene, Paul J. Valdes, Daniel J. Lunt, Richard J. Butler. Climatic controls on the ecological ascendancy of dinosaurs. Current Biology, 2022; DOI: 10.1016/j.cub.2022.11.064. Accessed from sciencedaily.com/releases/2022/12/221216112905.htm.
Cuff, A. R., Demuth, O. E., Michel, K., Otero, A., Pintore, R., Polet, D. T., Wiseman, A. L., & Hutchinson, J. R. (2022). Walking—and running and jumping—with dinosaurs and their cousins, viewed through the lens of evolutionary biomechanics. Integrative and Comparative Biology, 62(5), 1281–1305. DOI: 10.1093/icb/icac049. researchgate.net/publication/360763218
