My hypothesis is that smaller pterosaurs carried diseases which killed the larger pterosaurs. This is normal in closely related groups of animals. For example, humans and chimps share all or at least almost all of their communicable diseases, and the other great apes share most of the same communicable diseases with humans and chimps.
In the Cretaceous, the birds competed with the smaller pterosaurs and no doubt drove some species to extinction. With the disease carrying smaller pterosaurs extinct or at least more rare, the larger pterosaurs were able to grow larger still. So the pterosaurs evolved larger and larger species as the smaller species went extinct or became rare, until several of the last of pterosaurs from the family Azhdarchidae, including Quetzalcoatlus, reached enormous size. Quetzalcoatlus' wingspans have been estimated at between 12 and 18 meters or 40 to 59 feet. It is widely believed that Quetzalcoatlus and related species from the late cretaceous were the largest organisms ever to fly.
The birds could not compete with the pterosaurs at those very large sizes. This was not necessarily because the physiology of birds prevented them from becoming large. Rather it was probably because very large birds would have have been killed by the communicable diseases carried by the smaller birds.
If you live in English speaking Europe remember we Americans use the word moose for what you call an elk. What we call elk are close relatives to your red deer.
Particularly for Americans, the case of the white-tailed deer is close to home, but our closest relatives, the great apes, provide another example that is close to home for everyone. The great apes are suffering from our diseases and in danger of extinction. We are not necessarily smaller than they, but we are very common, which makes us a prime target for parasites. Once the parasites have adapted to us that also makes them well adapted to our close relatives. In fact, chimps and humans share almost all and perhaps exactly the same communicable diseases. The communicable diseases carried by humans are a major reason that the nonhuman great apes are in danger of extinction.
What the above examples illustrate is that this is not an obscure phenomenon. It is a powerful phenomenon that profoundly shapes the biological world, particularly the larger organisms.
The second advantage of the small is that they are more numerous, and each individual is another chance to have a favorable mutation, including a mutation that gives it the ability to successfully live with the disease.
The giant redwood tree and the Sequoia are living fossils, their closest smaller relative is the dawn redwood that lives in China, safely removed from California where the giant redwoods and Sequoias live. While both the giant redwood and the Sequoia live in California, their natural ranges do not overlap. So both the giant redwoods and the Sequoias have no close relatives in their range to infect them with diseases.
Similarly, the largest freshwater fish in North America is the Alligator Gar another living fossil. The gars as a group are large predatory living fossils.
In South America, we find much the same story. The largest freshwater fish is the Arapaima, a large predatory living fossil. The Arapaima has no real close relatives, but its closest relatives in South America are also large predatory living fossils, the South American arowana.
The sturgeon is similar, it is not predatory, but sturgeons are large to very large living fossils, and among the largest freshwater fish.
In the ocean, the large and predatory roles for fish are often held by another group of living fossils, the sharks.
One could give many more examples, and many other examples are given in the other essays of this biology sub-directory.
It has been argued that Quetzalcoatlus was a glider that kept itself aloft on rising air currents. This same strategy is used by large birds today, particularly predators and scavengers.
I have read theories that said that Quetzalcoatlus needed a cliff or at least a downward slope to gain the airspeed necessary to become airborne. But are these the only possiblilities? Wind should also lift them into the sky. They could spread their wings and catch the breeze.
But relying on wind would be risky if a large terrestrial predator suddenly appeared when there was no wind. Still if the Quetzalcoatlus was hungary and desperate, or the wind was strong and steady it might be worth the risk even if there was no cliff or downward slope available.
First, Quetzalcoatlus soared above the landscape. So after spotting a carcass it could look around for large predators. If there were no predators large enough to eat it close by then the quetzalcoatlus could descend on the carcass.
If there were several Quetzalcoatlus and they were cooperating one Quetzalcoatlus could remain aloft watching for predators, and giving warning long before the predator showed up. This would give those on the ground quite a bit of time to catch a breeze, climb a tree, rock, or cliff, or perhaps hide.
Perhaps the Quetzalcoatlus on lookout duty was younger and smaller. The large full grown Quetzalcoatlus could drive it off the carcass and keep it on lookout duty. But perhaps it was also small enough to fly even if there was no wind. If so, it could give a warning cry and as the large Quetzalcoatlus took to the air the smaller one could have its turn at the carcass. When the predator showed up the Quetzalcoatlus that was small enough to fly could flap its wings, take off and escape.
If the Quetzalcoatlus were not so cooperative, one or more might prevent another Quetzalcoatlus from landing on the carcass. When the flying Quetzalcoatlus saw a predator coming it might start to drift away looking for another carcass. This could warn the Quetzalcoatlus on the ground that a predator was approaching. Similarly, the Quetzalcoatlus might drive off birds that were trying to scavenge the carcass. If those birds saw a predator approaching they might start to drift off looking for another carcass thereby inadvertently alerting the Quetzalcoatlus.
The Quetzalcoatlus might be able to climb either trees or rocks. The ability of predatory dinosaurs to do either was likely to be limited. The Quetzalcoatlus might choose carcasses near trees or rocks that they could climb and avoid a carcass with no close by escape route.
Another possibility is that the Quetzalcoatlus alerted that the predator was approaching might hide. The predator attracted by the smell of the carcass would not be looking for the Quetzalcoatlus. Hiding might buy the Quetzalcoatlus time to catch a breeze to carry it into the sky.
If this and possibly other strategies failed the Quetzalcoatlus might simply back off the carcass. If there was plenty of meat left on the carcass the predator might choose to eat that rather than risking an attack on the Quetzalcoatlus.
Note, that predators practically never eat people. I suspect that there are more than fifty thousand coyotes living in American cities, they have probably eaten millions of our cats and dogs, and yet they have only killed one child. There are about a million mammal predators in the United States and Canada that when full grown are large enough to eat a person, yet only three or four of the more than a third of a billion people living in the United States and Canada are eaten every year. We swim in the ocean and the sharks and other predators simply leave us alone. Clearly, predators sometimes leave potential prey alone. Predators maybe like the rest of us, they avoid eating things when they do not know the food is safe. This is more extensively discussed on my web page on large predators and people. So perhaps predators also did not eat pterosaurs.
Given all these strategies it is easy to imagine that a giant flying, gliding, soaring bird might have a very effective survival strategy even if it could not takeoff on flat ground when there was no wind.
Why then do we not see these giant birds. Presumably, because the communicable diseases of the smaller birds prevent it. As a more advanced clade takes over the traditional niches of the declining clade driving many species of the declining clade into extinction new opportunities open to exploit previously unexploted niches, often involving large size.
Quetzalcoatlus is mystery to scientists. How could an animal as tall as a giraffee, with the wing span of a plane, get off the ground, how could it survive. The above just so story makes it a lot less mysterious.
But just before they went extinct they evolved what are generally thought to be the biggest species that ever flew. This essay claims that was not coincidence. As the clade approached extinction there were releatively few small organisms in the clade that could give the giants diseases. This allowed them to reach great size.
Several of the biology professors at the University of California at Davis said I should try to publish the idea that is at the center of the above. They even suggested a particular academic journal.
I am a retired substitute teacher and staff member for the local public schools. Formerly, I was a full-time economics instructor for seven semesters in New York City at St. John's University. I have a B.A., M.A. and Ph.C. in Economics from the University of California. A Ph.C. means I was advanced to candidacy on a Ph.D., in more common terms all but dissertation, A.B.D. You can read more about my qualifications on my biology index page.
If you are not a biologist, well you may be able to experience the thrill of knowing that you knew about a major advance in biology before the biologists. Link here and contribute to a major breakthrough.
You can check out my other speculative essays on biology on my Biology Index Page. I have essays based on similar reasoning concerning whales and many animals of the Mesozoic, including dinosaurs, archosaurs, therapsids, and Mesozoic marine reptiles.
Last Updated September 24, 2018