Quand les mammifères sont devenus diurnes
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Paleontologists believe that the first mammals to evolve on Earth were small nocturnal creatures that used a keen sense of smell and hearing to operate in the dark, which was a good place to be in the age of the dinosaurs. These days, many mammal species spend most of their time operating during the daytime, and many other species are crepuscular, which means they do most of their hunting, mating and interacting in the early morning and twilight hours.
But when did mammals make the switch from the night life to day life? Now, reports Gretchen Vogel at Science, a new study pinpoints the time in the distant past when mammals came out of the dark. And it turns out, it’s immediately after the demise of the dinosaurs.
Premiers primates
Shortly after the extinction of the dinosaurs, the earliest known archaic primates, such as the newly described species Purgatorius mckeeveri shown in the foreground, quickly set themselves apart from their competition—like the archaic ungulate mammal on the forest floor—by specializing in an omnivorous diet including fruit found up in the trees.
Une journée dévonienne
Les poissons du Dévonien vivaient des journées plutôt courtes. On sait en effet que les années dévoniennes comptaient 400 jours de près de 22 heures chacun. Décidément, rien n’est constant sur cette Terre! En plus des continents qui se déplacent, voilà que même la vitesse de rotation de notre planète varie.
Comment est-ce possible? C’est en quelque sorte la faute de la Lune, notre compagnon céleste. Depuis sa formation, alors que la Terre n’avait que 4,2 milliards d’années (Ga), la Lune provoque des marées sur notre planète. À cause de la gravitation, les grandes masses d’eau sont attirées vers la Lune lorsqu’elles passent vis-à-vis d’elle. Comme la Terre tourne vers l’est et que les marées se déplacent vers l’ouest, ce phénomène freine imperceptiblement la rotation de notre planète: 0.0016 seconde à tous les siècles!
Des fossiles plus anciens que ceux de Miguasha ont été découverts à Gaspé
Fossile d'un poisson acanthodien datant de 400 millions d'années.
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Environ 80 fossiles de vertébrés ont été découverts dans la région de Gaspé, en deux semaines seulement. Une étudiante de l'Université du Québec à Rimouski, Marion Chevrinais, a repéré des sites fossilifères le long de la falaise de Cap-aux-Os ainsi qu'à l'Anse-à-Brillant.
Datant de 400 millions d'années, les fossiles retrouvés à Gaspé sont même plus anciens que ceux du parc national de Miguasha, qui eux, datent de 380 millions d'années.
Microconodon
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Dromatherium sylvestre and Microconodon tenuirostris were tiny rat or shrew-like animals known from a few jaws and teeth from the Latest Carnian (Adamanian stage) of the Newark Supergroup of Eastern North America (North Carolina, Virginia, and Pennsylvania). Originally (during the 19th century) considered among the earliest and most primitive mammals, they were reinterpreted in the 1920s by G.G. Simpson. Their several supposedly unique mammalian features are shared by tritheledontids and other advanced cynodonts, and they are now best regarded as advanced or derived eucynodonts of uncertain affinities. A dromatherid has been reported from the Tiki Formation, India, and several problematic taxa known only from isolated teeth from the Late Triassic and Early Jurassic of Europe are also usually placed in the Dromatheridae.
Photo trouvée ici.
Dessin trouvé ici.
Trouvé ici.
Membre de la famille, trouvé ici.
Évolution des salmonidés
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Based on the most current evidence, salmonids diverged from the rest of teleost fish no later than 88 million years ago, during the late Cretaceous. This divergence was marked by a whole-genome duplication event in the ancestral salmonid, where the diploid ancestor became tetraploid. This duplication is the fourth of its kind to happen in the evolutionary lineage of the salmonids, with two having occurred commonly to all bony vertebrates, and another specifically in the teleost fishes.
(...) The first fossil species representing a true salmonid fish (E. driftwoodensis) does not appear until the middle Eocene. This fossil already displays traits associated with extant salmonids, but as the genome of E. driftwoodensis cannot be sequenced, it cannot be confirmed if polyploidy was present in this animal at this point in time. This fossil is also significantly younger than the proposed salmonid divergence from the rest of the teleost fishes, and is the earliest confirmed salmonid currently known. This means that the salmonids have a ghost lineage of approximately 33 million years.
Les études pertinentes sont ici et ici.
Intense réchauffement il y a 55 millions d'années
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About 55 million years ago global temperatures spiked. Then, as now, sea levels rose, the oceans became more acidic, and species disappeared forever.
Little wonder, then, that researchers view this ancient event – known as the "Palaeocene-Eocene Thermal Maximum" or PETM – as a potential goldmine of useful information for understanding modern climate change.
We now know that the PETM was one of the most rapid and dramatic instances of climate change in Earth's history. Its causes are still up for debate, but there seem to be eerie parallels with the causes of modern climate change. What is absolutely clear is that the PETM's effects were far-reaching. It may have altered the course of life on Earth.
(...) the PETM seems to have been caused by greenhouse gases just like modern-day climate change.
(...) Researchers examined another set of muds that formed at the bottom of the ocean 55 million years ago, this time in the north-west Atlantic. They found banding in the muds that they argued was formed by annual cycles.
When they traced the oxygen and carbon isotope blips associated with the PETM, they found that they were contained in just 13 bands. This means, they said, that the PETM temperature surge came in just 13 years.
This does not imply that the PETM came and went in little more than a decade. All researchers agree that the unusually warm conditions, with global temperatures at least 5 °C above average, lasted about 170,000 years.
What it would imply is that global temperatures ramped up to that 5 °C figure in just 13 years. Today, in contrast, global temperatures have risen about 1 °C since the late 19th century.
If PETM climate change really were so rapid, there would be implications for the event that triggered the warming. To create such a rapid rise in global temperature, the atmosphere would have had to be flooded with greenhouse gases almost literally overnight.
Perhaps the release of gases from the melting of a huge carbon-rich comet that flew too close to the Earth would do the trick.
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