My hypothesis is that smaller pterosaurs carried diseases which killed the larger pterosaurs. This is normal in closely related groups of animals. In the Cretaceous, the birds competed with the smaller pterosaurs and drove them into extinction. With the disease carrying smaller pterosaurs out of the way, the larger pterosaurs were able to grow larger. So the pterosaurs evolved larger and larger species as the smaller species went extinct, until one of the last of them, quetzalcoatlus reached enormous size. Their wingspans have been estimated at between 12 and 18 meters or 40 to 59 feet. It is widely believed that Quetzalcoatlus was the largest organism ever to fly.
The birds could not compete with the pterosaurs at the 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 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, which is one reason that the 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 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.
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 arowanas.
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 sub-directory.
It has been argued that Quetzalcoatlus was rather 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. 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.
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 it 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.
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 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 they are safe. This is more extensively discussed on my web page on large predators and people.
There are many strategies that might allow a large flying animal to deal with predation. One can imagine that a giant flying, gliding, soaring bird might have a very effective survival strategy. Why then do we not have them. Presumably, because the communicable diseases of the smaller birds prevent it.
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.
Currently, I am a 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 August 30, 2016