Waning Dinosaurs, Victim Of Their Success In A Warm Climate?

The question of the disappearance of the dinosaurs is closely entangled with the deepest philosophical questions (and already were so at the time of the Ancient Greece or China, where as recently as a few years ago dinosaur bones were mistaken for dragon remains, and finished as soup…) Do smarts have to do with it?

An old, all too old, idea is that, if dinosaurs had no gone extinct, mammals in general and homo in particular would not have arisen… Dinosaurs would have kept on eating mammals. Instead highest intelligence would have arisen from dinosaurs. Some scientists have claimed that, based on the size of T Rex’s brain, it was as smart as a chimp. But that dinosaurs prevented the rise of primates, let alone mammals, is a complete absurdity, as it blatantly contradicts logic and facts.

Instead, one can turn the argument on its head: maybe mammals caused, ultimately the disappearance of dinosaurs… With the little help from three well-known catastrophes…

Indeed here is the general scheme I propose: mammalian reptiles are arguably older than dinosaurs… They may have arisen directly from the previous mass extinction 250 million years ago. Then, as the climate became balmier for around 150 million years, dinosaurs could evolve… in a less demanding way. Dinosaurs were mesotherms: they generated some internal heat, and they often had some sorts of feathers for insulation. Still they were not fully generating their own heat in 36 C-96 F to 40 C, 100F range as mammals and birds do.

An analogy is aircraft carriers: the latest class of US supercarriers is on the verge of service. USS Gerald Ford (WW2 naval aviator who became president) is undergoing its last trials, which consist in giant submarine explosions next to the ship to check for shock resistance. The Ford is the same size and mass as the earlier carriers the last of which was built 35 years ago. However, Ford carries three times the electric power of the preceding Nimitz class. Ford is more energized than Nimitz. That enables Ford to do things Nimitz could not have done.

By being higher temperature constantly, thus much more energized, constantly, mammals and birds were constantly dangerous to dinosaurs.

Imagine this: the planet cooling, a whiff of a cold blast. What would dinosaurs do? Go sluggish, we know they were expert at that: some were living next to the poles and were probably used to semi-hibernation (as many bear species in cold climes do today). One can imagine the dinosaurs’ nests raided, during a cold worldwide “nuclear winter” by enterprising mammals and birds. Well before the mass extinction event of 66 million years ago, there were primates, and they survived the mass extinction (some fossils are from 100,000 years afterwards). Several species of the genus Purgatorius are known (see below).

The advantage and characteristic of the most powerful philosophy is to love, discern and use the biggest, strongest arguments. For example why two different inflation mechanisms in cosmic evolution… One of them all too much an apparent copy of the Bible… Yes, really! Do away with that latter Judaic inspiration, and simplify matters with an expanding but much older universe, using the only cosmic expansion we can see instead of a superluminal expansion ad hoc to save the Bible? That (philosophical) argument is so simple, I have no doubt it will impose itself, and everybody will forget that I originated it… Once the philosophical argument has been found, an educated guess, the scientific method can move in and check the details.

So back to dinosaurs, plesiosaurs, and pterosaurs. The pterosaurs were highly evolved, and much better fliers than the birds: whereas birds appeared around 150 million years ago, pterosaurs appeared 80 million years prior (230 million years ago), and birds probably hid in the forests, flying between the trees, thanks to their feathers, for as long as it took (namely 84 million years) to not become pterosaurs’ dinners (or then they live in climes too cold for pterosaurs… we have evidence of this). Still, then, in spite of their tremendous aerial mastery, and their four limb capability to launch their enormous masses in the air, the enormous flying dragons disappeared with the dinosaurs…. Probably victims of that same evolutionary trap, the balmy Jurassic-Cretaceous climate. Sluggish cold pterosaurs’ eggs eaten by rats or the like? That’s my (gory) vision of what happened. Here is, from the prestigious scientific journal Nature, June 29, 2021:

Dinosaur biodiversity declined well before the asteroid impact, influenced by ecological and environmental pressures

The question why non-avian dinosaurs went extinct 66 million years ago (Ma) remains unresolved because of the coarseness of the fossil record. A sudden extinction caused by an asteroid is the most accepted hypothesis but it is debated whether dinosaurs were in decline or not before the impact. We analyse the speciation-extinction dynamics for six key dinosaur families, and find a decline across dinosaurs, where diversification shifted to a declining-diversity pattern ~76 Ma. We investigate the influence of ecological and physical factors, and find that the decline of dinosaurs was likely driven by global climate cooling and herbivorous diversity drop. The latter is likely due to hadrosaurs outcompeting other herbivores. We also estimate that extinction risk is related to species age during the decline, suggesting a lack of evolutionary novelty or adaptation to changing environments. These results support an environmentally driven decline of non-avian dinosaurs well before the asteroid impact.

Dinosaurs were dying, and had been dying out for millions of years, out-competed in a cooling climate by hot, conniving primates, warm energizer rabbits and hot, angry birds:

If I added the planetary temperature graph, it would look a bit like the collapsing diversification rate on the right…

The most famous mass extinction was the disappearance of non-avian dinosaurs at the end of the Cretaceous, 66 million years ago (Mya), after ruling the Earth for 170 million years1,2,3. The best-supported extinction model is the impact of a large asteroid in the Yucatán Peninsula (Chicxulub, Mexico), which set off a global cataclysm and environmental upheaval4,5. Although evidence for an end-Cretaceous impact is indisputable6, most scientific debate has focused on whether the extinction was geologically abrupt or gradual7,8,9,10,11, whether it was caused by factors intrinsic to dinosaurs12,13,14,15 or by extrinsic physical drivers16,17,18 or both acting in concert19,20. If extrinsic events had a role, the question is whether this driver was terrestrial or extra-terrestrial21,22,23. It has proved harder to posit a convincing killing model that explains exactly how the dinosaurs, as well as many other groups24,25, vanished. And yet other groups of animals and plants survived through this singular, short-term crisis26,27. Could some groups have been teetering on the brink already? Furthermore, the extinctions coincide with a period of long-term environmental changes that resulted in remarkably high sea levels, cooling climates and the spread of new habitat types on land, as well as massive volcanic activity at the end of the Cretaceous16,19,20.

That there was an impact at the time, there is no doubt. That there was massive Large Igneous Province (LIP) at the time, there is no doubt. A LIP, the Siberian Traps, caused the even more massive mass extinction, 250 million years ago. Remarkably the bolide (I suspect a comet) impacted just when the Dekkan Traps were at their eruptive maximum (or so it seems at this point, 2021). It always seemed to me that one impactor the size of the one on Yucatan, was not enough. However, if the bolide was a comet, it may well have disintegrated in several fragments prior to impact, as happened with Jupiter and Shoemaker-Levy comet (which broke up in seven main fragments).

But there is an even bigger factor which seems indubitable: the biosphere was cooling. And we are talking drastic cooling, as much as seven (7) degrees centigrade over the Atlantic… for comparison, the Atlantic was only five degree C lower in temperature during the last glacial maximum, when monster glaciers kilometers high crushed New York…   

So I propose (and have long proposed, see Trapped By Super Traps) three layers in the dinosaur extinction:

1) The surface of the planet was cooling, and had been for dozens of millions of years. (Such large cooling, over dozens of millions of years are probably related to plate tectonic preventing warm water to get to the poles; this is why Antarctica got covered by ice, from the closing by the Indonesia island arc, simultaneous with the closing of Mesoamerica; once the poles ice up, the albedo of the planet shoots up, as solar radiation is sent back to space by ice and snow

2) Dekkan Traps had been erupting, for millions of years, causing all sorts of mayhem, from poisonous gases, sometimes heating the atmosphere, sometimes cooling it, acidifying the seas.

3) … with the impact of bolide(s) being the coup de grâce, driven by warm blooded hunger (the latest my idea).

From the Nature article:

The Late Cretaceous was a greenhouse world, characterized by high temperatures and reduced latitudinal temperature gradients72,73, which likely contributed to a flat latitudinal diversity gradient in dinosaurs74. Peak warmth was attained in the Cenomanian-Turonian (100–90 Ma) with sea-surface temperatures reaching ≥30 °C in the tropics and the southern latitudes72,73,75. After this interval, sea-surface and deep-water temperatures decreased to the cooler greenhouse of the Campanian–Maastrichtian (80–66 Ma)72,73,76. This important cooling (~7 °C in North Atlantic77, and ~10 °C in southern latitudes76), which intensified during the Maastrichtian until the K/Pg event, could have impacted dinosaur diversity. However, other factors, such as Cretaceous geological changes17,41 or floristic turnover78,79 should not be discarded.

These results imply that warm periods favoured dinosaur diversification whereas cooler periods led to enhanced extinctions, as observed in the latest Late Cretaceous77. This result is particularly in agreement with a recent analysis of multiple tetrapod phylogenies showing the significant effect of Cenozoic cooling on diversification slowdowns, which is linked to the metabolic theory of biodiversity63. As dinosaurs were probably mesothermic organisms85 with varying thermoregulation abilities in different groups86, their activities were probably partially constrained by environmental temperatures. This is particularly true of larger dinosaurs, which almost certainly relied substantially on mass homeothermy to maintain constant body temperatures. It is likely that climatic deterioration would have made such a thermoregulatory strategy more difficult, and that global climate cooling was an important driver of the dinosaur diversity decline as indicated by the MBD diversification model. A physiological explanation for the cooling-driven extinction could be the hypothesis that if sex determination in dinosaurs was temperature dependent, as in crocodiles and turtles, sex switching of embryos could have contributed to diversity loss with a cooling global climate at the end of the Cretaceous87.

So basically the essay is saying in a thorough way what I also said… With, in my case, the twist that predation by fast, hot species may have been the ultimate factor. 

A few years ago I was talking with a friend of mine, a top paleontologist (he is the top editor of the subject at Science, chair of a paleontology department in a most prestigious university, and head of a museum…). And also an extinction-by-bolide advocate. When I exposed the preceding to him, he said: “Oh, but then you are saying that the bolide gave the coup de grâce… So the bolide caused the extinction…” No, not quite. The house was on fire (that’s the 25 million years cooling), then got full of lethal smoke (that’s the Deccan Traps), then a beam fell on the unfortunate half passed out dinosaurs (that’s the bolide), trapping its leg, but even then the coup de grâce happened when the rats ate alive the trapped beast…

Causation is often a complicated matter, more of a causal web than a causal chain.

As long as the planetary climate was cooling the mesothermic (middle-temperature) creatures were doomed, from competition from warm mammals and hot avian, high metabolism dinosaurs (the birds). Paradoxically the very success of say, Pterosaurs, caused their doom. Pterosaurs were such efficient fliers that they did not need the highest metabolism, high temperatures of avian dinosaurs: Pterosaurs fell victim of a sort of evolutionary hubris. 

But wait a minute, some may say, what about dinosaurs in Alaska? Yes, certainly. Polar dinosaurs are an archeological fact.

Dinosaurs certainly could not migrate out of the way of Antarctic or Alaskan winter, the distances are immense, no non-flying animal could cover them. Evidence of adaptation to Arctic conditions, such as smaller size, are clear. Predators, such as in Troodon, a feathery, eight-foot-long dinosaur genus with large eyes, to see in the polar night, showed that they spent the winter there. So then what of brutal winter caused by an impacting comet or massive eruption, why couldn’t dinosaurs resist to it? If they could resist to winter? As I said, a sudden drop of say twenty centigrade all over, would have made life very difficult. We know, from fossil evidence that dinosaurs became sluggish in winter, they stopped growing. So a “nuclear winter” would have made them go comatose big time, and the frantic hot blooded critters would have viewed dinosaurs as living refrigerators, full of ready to eat meals… Actually feathered dinosaurs found in Australia, at that time, more than 100 million years ago, by the South Pole had dark feathers (in summer)… and shared their ecology with flight capable birds… not pterosaurs… This observation, if it holds, shows that pterosaurs, which are suspected to have been warm blooded, and furry (with sort-of hair), were not as warm blooded as birds. In any case the K-T extinction was massive, and many bird and mammals species went extinct. The slight difference in sluggishness may be all one needs to get an explanation for the disappearance of dinosaurs and pterosaurs, etc.. When the going gets tough, the toughs get going…

Here is one of our potential ancestors, Purgatorius, below, an omnivorous primate, getting smarter, hunting meat, was around while dinosaurs roamed. Purgatorius is highly evolved, so it is likely that primates got started well before 100 million years ago. 

As the long sharp teeth above show, our ancestors were omnivorous… And that means that, if dinosaurs were too sluggish, they, or their young or eggs, were dinner. So it is entirely possible that, in the end, we, or at least our fellow warm blooded relatives, are the ultimate cause of the dinosaurs’ disappearance…

Patrice Ayme

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