When sea levels rise as dramatically as they did in the Cretaceous, coral reefs will be buried under rising waters and the ideal position, for both photosynthesis and oxygenation, is lost, and reefs can die, like burying a tree’s roots. About 125 mya, reefs made by , which thrived on , began to displace reefs made by stony corals. They may have prevailed because they could tolerate hot and saline waters better than stony corals could. About 116 mya, an , probably caused by volcanism, which temporarily halted rudist domination. But rudists flourished until the late Cretaceous, when they went extinct, perhaps due to changing climate, although there is also evidence that the rudists . Carbon dioxide levels steadily fell from the early Cretaceous until today, temperatures fell during the Cretaceous, and hot-climate organisms gradually became extinct during the Cretaceous. Around 93 mya, , perhaps caused by underwater volcanism, which again seems to have largely been confined to marine biomes. It was much more devastating than the previous one, and rudists were hit hard, although it was a more regional event. That event seems to have , and a family of . On land, , some of which seem to have , also went extinct. There had been a decline in sauropod and ornithischian diversity before that 93 mya extinction, but it subsequently rebounded. In the oceans, biomes beyond 60 degrees latitude were barely impacted, while those closer to the equator were devastated, which suggests that oceanic cooling was related. shows rising oxygen and declining carbon dioxide in the late Cretaceous, which reflected a general cooling trend that began in the mid-Cretaceous. Among the numerous hypotheses posited, late Cretaceous climate changes have been invoked for slowly driving dinosaurs to extinction, in the “they went out with a whimper, not a bang” scenario. However, it seems that dinosaurs did go out with a bang. A big one. Ammonoids seem to have been brought to the brink with nearly marine mass extinctions during their tenure on Earth, and it was no different with that late-Cretaceous extinction. Ammonoids recovered once again, and their lived in the late Cretaceous, but the end-Cretaceous extinction marked their final appearance as they went the way of and other iconic animals.
Current evidence suggests that many of the major events in hominin evolution occurred in East Africa. Hence, over the past two decades, there has been intensive work undertaken to understand African palaeoclimate and tectonics in order to put together a coherent picture of how the environment of Africa has varied over the past 10 Myr. A new consensus is emerging that suggests the unusual geology and climate of East Africa created a complex, environmentally very variable setting. This new understanding of East African climate has led to the pulsed climate variability hypothesis that suggests the long-term drying trend in East Africa was punctuated by episodes of short alternating periods of extreme humidity and aridity which may have driven hominin speciation, encephalization and dispersals out of Africa. This hypothesis is unique as it provides a conceptual framework within which other evolutionary theories can be examined: first, at macro-scale comparing phylogenetic gradualism and punctuated equilibrium; second, at a more focused level of human evolution comparing allopatric speciation, aridity hypothesis, turnover pulse hypothesis, variability selection hypothesis, Red Queen hypothesis and sympatric speciation based on sexual selection. It is proposed that each one of these mechanisms may have been acting on hominins during these short periods of climate variability, which then produce a range of different traits that led to the emergence of new species. In the case of Homo erectus (sensu lato), it is not just brain size that changes but life history (shortened inter-birth intervals, delayed development), body size and dimorphism, shoulder morphology to allow thrown projectiles, adaptation to long-distance running, ecological flexibility and social behaviour. The future of evolutionary research should be to create evidence-based meta-narratives, which encompass multiple mechanisms that select for different traits leading ultimately to speciation.
A Medley of Potpourri: Punctuated equilibrium
for mass extinctions have been suggested. speculated that extinctions might have regular periodicity, and other scientists have . Around 30 million years is the average time between mass extinctions, which set scientists speculating whether galactic dynamics could be responsible. from supernovas have been proposed as one possible agent, as have , but the periodicity hypothesis has fallen out of favor. The periodic nature of mass extinctions could be because it takes millions of years for complex ecosystems to recover from the previous extinction events and build themselves into unstable states again, when new events cause the ecosystems to collapse.
Quote Mine Project: Assorted Quotes - TalkOrigins …
The issue came into sharper focus after Niles Eldredge and Stephen Jay Gould introduced the concept of "punctuated equilibrium" into the discussion of evolution.
Probability of pregnancy by age - Gene Expression
"The rarity with which directional evolution was observed in this study corroborates a key claim of punctuated equilibria...."
Antonis Rokas et al., "Animal Evolution and the Molecular Signature of Radiations Compressed in Time" , 10.1126/science.1116759, p 1933-1938 v 310, , 23 Dec 2006.
Section 25_3: The Pace of Speciation - PBworks
CSCW is one example; other fields and regions might be similarly reassessed. Punctuated equilibrium is found in nature. Pressure on tectonic plates builds steadily beneath the Earth’s crust, but instead of gradual change at the surface, earthquakes punctuate periods of equilibrium. It could help explain why so often organizations and entire industries are caught flat-footed by waves of technology change. Explorations of history can reveal the power of unseen forces to shape events. To influence where we go, we must understand the waves we surf.