Med sammendrag af årtiers arbejde, rapporterer en ny undersøgelse offentliggjort i denne uge i tidsskriftet Communications Biology om en tidligere udokumenteret udryddelsesbegivenhed, der fulgte overgangen mellem de geologiske perioder kaldet Eocæn og Oligocæn.
Denne periode var præget af dramatiske klimaforandringer. I et omvendt billede af, hvad der sker i dag, blev jorden køligere, iskapper udvidet, havniveauet faldt, skovene begyndte at skifte til græsarealer, og kuldioxid blev knappe. Næsten to tredjedele af de arter, der var kendt i Europa og Asien på det tidspunkt, uddøde.
Kommentar: Delvist oversat af Sott.net fra The unknown Eocene-Oligocene boundary mass extinction event was accompanied by climate change and super eruptions, followed by an explosion of life
Se også denne artikel af Jim Steele: The Antarctic Refrigerator Effect. Heri forklares klimatiske bidrag fra den ændrede placering af kontinenterne i forhold til hinanden gennem de sidste 50 millioner år.
Hvordan Antarktis forårsagede global afkøling, eller hvorfor jorden forbliver i istidstilstand i de næste 200 millioner år
Havene er et enormt reservoir af varme. De øvre tre meter havvand rummer mere varme end hele jordens atmosfære.
På grund af dannelsen af den antarktiske cirkumpolære strøm, aktiveret af pladetektonik, blev Antarktis termisk isoleret fra resten af jorden og startede 1) en permanent iskappe, 2) omfattende havis, der ekstruderede kold saltlage, der fyldte havets bundvand, og 3) Intensiveret opstigning, der øgede produktiviteten, der trak atmosfærisk CO2 ned til det nuværende niveau.
Som et resultat af stigende koldt antarktisk bundvand ventilerede havene sin forskudte gamle bundvandsvarme. Som et resultat af en nedkølingstendens på 50 millioner år er jorden nu låst fast i en svingende balance mellem kolde istider og varme mellemistider
Jim Steele er emeritusdirektør for San Francisco State Universitys Sierra Nevada Field Campus, har forfattet Landscapes and Cycles: An Environmentalist's Journey to Climate Skepticism og stolt medlem af CO2 -koalitionen.
African mammals were thought to have possibly escaped unscathed. Africa's mild climate and proximity to the Equator could have been a buffer from the worst of that period's cooling trend.
Now, thanks in great part to a large collection of fossils housed at the Duke Lemur Center Division of Fossil Primates (DLCDFP), researchers have shown that despite their relatively balmy environment, African mammals were just as affected as those from Europe and Asia. The collection was the life's work of the late Elwyn Simons of Duke, who scoured Egyptian deserts for fossils for decades.
Comment: If their environment remained relatively mild, isn't it likely that there were other drivers behind these extinctions that weren't solely climatic? Last mammoths plagued by genetic defects
The team, comprising researchers from the United States, England, and Egypt, looked at fossils of five mammal groups: A group of extinct carnivores called hyaenodonts; two rodent groups, the anomalures (scaly-tail squirrels) and the hystricognaths (a group that includes porcupines and naked mole rats); and two primate groups, the strepsirrhines (lemurs and lorises), and our very own ancestors, the anthropoids (apes and monkeys).
By gathering data on hundreds of fossils from multiple sites in Africa, the team was able to build evolutionary trees for these groups, pinpointing when new lineages branched out and time-stamping each species' first and last known appearances.
Their results show that all five mammal groups suffered huge losses around the Eocene-Oligocene boundary.
"It was a real reset button," said Dorien de Vries, a postdoctoral researcher at the University of Salford and lead author of the paper.
After a few million years, these groups start popping up again in the fossil record, but with a new look. The fossil species that re-appear later in the Oligocene, after the big extinction event, are not the same as those that were found before.
The evidence is in these animals' teeth. Molar teeth can tell a lot about what a mammal eats, which in turns tells a lot about their environment.
The rodents and primates that reappeared after a few million years had different teeth. These were new species, who ate different things, and had different habitats.
"We see a huge loss in tooth diversity, and then a recovery period with new dental shapes and new adaptations," said de Vries.
"Extinction is interesting in that way," said Matt Borths, curator of Duke University's DLCDFP and coauthor of the paper. "It kills things, but it also opens up new ecological opportunities for the lineages that survive into this new world."
This decline in diversity followed by a recovery confirms that the Eocene-Oligocene boundary acted as an evolutionary bottleneck: Most lineages went extinct, but a few survived. Over the next several millions of years, these surviving lines diversified.
Comment: Numerous studies are showing that following extinction level events there isn't just 'diversification, there is an explosion in variations and of new life that cannot be explained by Darwinian evolution: In Cambrian Explosion Debate, Intelligent Design Wins by Default
"In our anthropoid ancestors, diversity bottoms out to almost nothing around 30 million years ago, leaving them with a single tooth type," said Erik R. Seiffert, Professor and Chair of the Department of Integrative Anatomical Sciences at the Keck School of Medicine of the University of Southern California, a former graduate student of Simons, and senior co-author of the paper. "That ancestral tooth shape determined what was possible in terms of later dietary diversification."
"There's an interesting story about the role of that bottleneck in our own early evolutionary history," said Seiffert. "We came pretty close to never existing, if our monkey-like ancestors had gone extinct 30 million years ago. Luckily they didn't."
A rapidly changing climate wasn't the only challenge facing these few surviving types of mammals. As temperatures dropped, East Africa was pummeled by a series of major geological events, such as volcanic super eruptions and flood basalts — enormous eruptions that covered vast expanses with molten rock. It was also at that time that the Arabian Peninsula separated from East Africa, opening the Red Sea and the Gulf of Aden.
Comment: It's likely that, as we've seen throughout history, climate change is but one part of a much more cataclysmic sequence of events: Of Flash Frozen Mammoths and Cosmic Catastrophes
"We lost a lot of diversity at the Eocene-Oligocene boundary," said Borths. "But the species that survived apparently had enough of a toolkit to persist through this fluctuating climate."
"Climate changes through geological time have shaped the evolutionary tree of life," said Hesham Sallam, founder of the Mansoura University Vertebrate Paleontology Center in Egypt and co-author of the paper. "Collecting evidence from the past is the easiest way to learn about how climate change will affect ecological systems."
More information: Dorien de Vries et al, Widespread loss of mammalian lineage and dietary diversity in the early Oligocene of Afro-Arabia, Communications Biology (2021). DOI: 10.1038/s42003-021-02707-9 Journal information: Communications Biology