The K/T Extinction Event
Everyone knows that pterosaurs, dinosaurs and a host of other prehistoric reptiles died out at the K/T (Cretaceous/Tertiary) boundary ~65 mya. But SOME birds, lizards, turtles, crocs and mammals survived. So, why did ALL pterosaurs die out?
As in dinosaurs, the pterosaurs we know from the latest Cretaceous were not the same pterosaurs living in the Triassic, Jurassic or Early Cretaceous. All of these earlier pterosaurs became extinct, but a few genetic lines survived by evolving into the Late Cretaceous forms we know and love. Phylogenetic analysis indicates that certain lucky Middle Jurassic Dorygnathus specimens ultimately evolved (via several transitional taxa) into Quetzalcoatlus, Pteranodon, Nyctosaurus, Tupuxuara and any other Late Cretaceous pterosaurs I’m forgetting (the current list is not much longer than this).
The Example of Dorygnathus
Analysis illustrates how the descendants of Dorygnathus changed in size and shape as they evolved into the above Late Cretaceous taxa. Therein, l think, lies the answer to why pterosaurs were not able to continue evolving into the modern day.
If we were to follow the lineage of Dorygnathus through Quetzalcoatlus (Fig. 1) we would meet the following taxa in order: Dorygnathus (SMNS 50164), Pterodactylus? spectabilis (TM 10134), Beipiaopterus, No. 44, No. 42, Jidapterus, Chaoyangopterus, Zhejiangopterus and finally the two species of Quetzalcoatlus. Setting aside the huge size differences between the two Qs and their phylogenetic predecessor, Zhejiangopterus, note that tiny TM 10134 and two other tiny pteros, No. 42 and No. 44, are in this line-up.
In the Late Jurassic the genetic lineage of Dorygnathus, of the Middle Jurassic, was represented by a tiny version of itself, TM 10134. There were no other full-size Dorygnathus present in the Late Jurassic. Something killed every other one over a certain size. Only tiny dory descendants somehow survived. Was it because of their size?
Major Morphological Changes in Tiny Taxa
As mentioned above (Fig. 1) other Late Jurassic tiny dorygnathids also include No. 42 and No. 44, both of which evolved a slender elongated neck, a low trostrum, smaller teeth and longer more gracile limbs. These traits were retained in all later and larger azhdarchids and huanhepterids (Fig. 1). (Pterorhynchids, scaphognathids and ctenochasmatids were also Dorygnathus descendants you can read about here, here and here).
When the threat of extinction did not loom over pterosaurs, they tended to become bigger. Evidently this was especially true during the latest Cretaceous because pterosaurs reached their greatest sizes right at 65 million years ago.
Not Being Small Is What Killed Late Cretaceous Pterosaurs
Just as being small saved many pterosaur lines earlier, being small saved many other vertebrates following the K/T mass extinction event. Big vertebrates did not survive. Unfortunately the giant pterosaurs of the latest Cretaceous could not breed small enough to save themselves, as their ancestors had done. We don’t find any pterosaurs smaller than Nyctosaurus in the Late Cretaceous.
Serial Size Reduction and How It Happens
ln pterosaurs phylogenetic size reduction from Dorygnathus to TM 10134 was made possible by reaching sexual maturity at half their final size (Chinsamy et al. 2008). Smaller pelves would have passed smaller eggs, smaller hatchlings and an even smaller second generation in serial fashion. Smaller vertebrates typically have a relatively faster maturation process, creating more tiny hatchlings earlier and at a faster clip. This increase in reproductive rates raised the odds that whatever was killing the larger, slower-to-breed individuals could be overcome by an acceleration in breeding, producing an acceleration in genetic variation and mutation. Such a serial size reduction pattern occurred at the base of nearly every major clade within the Pterosauria. When the same process is observed about a dozen times that verifies its veracity.
If only some tiny pteros existed at the Late Cretaceous, we might have some “thunderbirds” flying around today (Fig. 2).
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.