Pterosaurs were so incredibly good at flying, that some exceeded by a factor of ten or twenty times the maximum mass a bird can fly with (according to the laws of engineering and aerodynamics). How did pterosaurs do this? By using a number of tricks birds never found, starting with a powerful jumping four legged take off (birds lift off with only two legs).
The heaviest mammal ever, Paraceratherium, a sort of horn-less rhinoceros, was no more than 20 tons (he lived in a warmer climate than now, 20 million years ago). The heaviest dinosaurs known so far are the Titanosaurs who lived towards the end of the age of dinosaurs, and reached up to 70 metric tonnes.
The answer for the achievements of these extinct giants is fascinating: it has probably to do with MESOTHERMIA: temperatures in between cold blooded and warm blooded animals. Animals who were neither cold blooded, nor warm blooded got two advantages: they could become more massive (hence less susceptible to infections and various attacks of the environment), and they were more economical: a cold blooded animal needs 30 times less fuel (food) than a warm blooded one (Lavoisier, in the 18C, discovered that animal metabolism was a slow combustion). A parallel essay will develop the more philosophical side.
Endotherm, so-called warm-blooded animals maintain a constant body temperature independent of the external temperatures. Endotherms include birds and mammals; however, some insects, fish, and reptiles can also be endothermic where or when needed, by flexing muscles: a brooding Burmese python on his nest generates heat through muscle contractions. The swim muscle of a tuna reaches mammal-like temperatures, because muscle works better around 37 Celsius. Even some plants and flowers can reach equivalent temperatures in sensitive areas… Because the chemistry is much more active at 37C than at 15 C (the latter is the biosphere average temperature).
If heat loss exceeds heat generation, metabolism increases to make up for the loss or the animal shivers to raise its body temperature: a butterfly which wants to take off in cold weather will move its wings ever more to progressively generate heat which makes the fly muscle efficient enough for taking off.
If heat generation exceeds the heat loss, mechanisms such as panting or perspiring increase heat loss. Unlike ectotherms, endotherms can be active and survive at quite low external temperatures, but because they must produce heat continuously, they require high quantities of “fuel” (i.e., food).
However, heat loss to the environment can be decreased by the possession of larger body sizes that increase thermal inertia. This happens because surface increases like the square of linear dimension, and mass as the cube. This strategy is illustrated by leatherback turtles, Dermochelys coriacea, whose gigantic body size associated with relatively high rates of muscle metabolism allows for the maintenance of body temperature much warmer than the cold waters used for foraging.
Reciprocally, though, large animals are threatened with overheating: elephants have huge, heavily vascularized flapping ears to help cool down.
So too big makes too much heat… Except if, to start with, one’s cells have less heat generating capability… The definition of mesotherm animals: as for dinosaurs (certainly) and pterosaurs (probably)... Paradoxically, that mesothermy was made possible by the warm climate of the Jurassic.
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Mid-Cretaceous surface water temperatures were about 30 °C (86 °F) at the Equator year-round, but at the poles they were 14 °C (57 °F) in winter and 17 °C (63 °F) in summer. A temperature of 17 °C was possible for the ocean bottom during the Albian Age, but it may have declined to 10 °C (50 °F) by the Maastrichtian. These temperatures have been calculated from oxygen isotope measurements of the calcitic remains of marine organisms. The end of the Cretaceous was coolest, yet still much warmer than Earth is today.
This entire subject is full of unknowns: how do endotherms generate heat? Mitochondria is a big part of the answer. Eyes and brains of swordfishes, which enjoy mammal like temperatures are full of mitochondria. A team at the University of Paris 7 (one of three alma mater of mine), using new technology, published in 2017 the discovery that mitochondria would reach colossal temperatures, 50 C (this huge surprise has been sort of confirmed since).
What is clear is that the most massive animal who ever lived, our contemporary, the Blue Whale (up to 160 tons, and nearly hunted to extinction), swims in the cool ocean… So the Blue Rorqual is able to dissipate heat in said ocean (Blue Whales swim so fast, they were long out of reach to hunters).
A way to get rid of dinosaurs and their cousins was then just to lower the temperature of the climate. And sure enough, the climate got cooler, for many million years, towards the end of dinosaurs and after. To compensate, dinosaurs could get even bigger. Is it why Titanosaurs lived at the end of the Cretaceous? Reciprocally, if the climate cooled, or if there was a burst of nuclear winter, mammals and birds, both warm blooded with high metabolism even in colder temps, could survive and eat dinosaur eggs (as I argued; mammal species surviving the extinction Cretaceous-Paleocene were… omnivorous)…
Mesothermia may have enabled dinosaurs, pterosaurs and their cousins to spare themselves the cost and waste of endothermia… and thus have the evolutionary room to specialize in other ways… Maybe even bigger brains (at this point in paleontology, claims have been made of high intelligence for some dinosaurs, including T Rx).
All this evolutionary specialization, not to say evolutionary hubris, came to a crashing halt 66 million years ago. Brain size seem to have taken more than ten million years to recover…
So here we are, jacks of all trades, and masters of some we are busy inventing.
Evolutionary speaking we look good, but for that little detail:
PUTIN DELENDUS EST!
Patrice Ayme
Patrice Ayme's Thoughts 