The Evolution of Gigantism in Pterosaurs: The Ancestry of Quetzalcoatlus

Earlier we followed up on a National Geographic article on how many generations it takes to create a blue whale and an elephant. Today we’ll look at the evolution of the giant pterosaur Quetzalcoatlus (Fig.1) from the Latest Cretaceous (65 mya) from a much smaller ancestor of the Late Jurassic (150 mya), a span of 85 million years.

Others consider azhdarchids, like Quetzalcoatlus, to be related to tapejarids based on the shared trait of an antorbital fenestra taller than the orbit. The large reptile tree did not recover that relationship, but found that antorbital trait to be a convergence.

The beauty of the large reptile tree and the large pterosaur tree is the ability to trace the ancestry of any listed taxon back to the basal tetrapod, Ichthyostega. Today we won’t go that far back. Rather we’ll start with the first Late Jurassic pterosaur in the lineage of Quetzalcoatlus that actually has the approximate proportions of Quetzalcoatlus. That pterosaur is tiny specimen inaccurately referred to Pterodactylus? elegans? BSPG 1911 I 31 (no. 42 in the Wellnhofer 1970 catalog, Fig. 1).

Quetzalcoatlus and its ancestor, no 42, note scale bars.

Fig. 1. Quetzalcoatlus and its ancestor, no 42, note scale bars. Also note the flat rostrum in both taxa, distinct from the pointed tapejarid rostrum derived from Germanodactylus.

Similar but different, especially in size.
We’re often taught that as organisms grow larger they also become more robust, with stronger, thicker bones to withstand the effects of their greater mass and weight. Here, not so much. Giant Quetzalcoatlus was just as gracile as the VERY much smaller, no. 42. Sure the humerus is more robust. So is the pectoral girdle. Otherwise it’s hard to find thicker bones and the neck is definitely more gracile in Quetzalcoatlus. That’s engineering!!

Tiny no 42 is dwarfed by the specimen referred to Quetzalcoatlus, which is dwarfed by the hypothetical skull of Q. northropi, based on wing elements.

Figure 2. Tiny no 42 is dwarfed by the specimen referred to Quetzalcoatlus, which is dwarfed by the hypothetical skull of Q. northropi, based on wing elements. Click to learn more.

What can we learn here?
Between no. 42, which was reduced from early Dorygnathus specimens, and Quetzalocoatlus, one of the largest known azhdarchids, the proportions are not much different overall. This is likely due to the restrictions imposed by flight. Only certain balances between power and weight can fly. Over time and millions of generations the lineage of Quetzalcoatlus gradually grew and reengineered itself to withstand the increasing stresses imposed by that growth.

Forelimb Take-Off
Dr. Mike Habib (2008) has noted the greater size of the humerus vs femur in Quetzalcoatlus and other large pterosaurs. He didn’t mention no. 42, which does not have a humerus greater in diameter than the femur. Habib considered the more robust humerus a sign that pterosaurs used a vampire bat-like forelimb launch sequence demonstrated here, rather than a bird-like hind limb launch demonstrated here. Unfortunately, its all math at present. We know of no pterosaur take-off tracks, nor any that document the implantation of the wing metacarpal into the substrate. Rather only the fragile first three digits make any impression. Perhaps the increased size of the humerus is a sign that the pectoral engine for wing flapping is much larger to drive the larger wings. After all, traditional physics has to enter into pterosaur evolution someplace!

Tomorrow we’ll look at the similar situation in the ancestry of Arthurdactylus and Anhanguera, two giant ornithocheirids.

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.

References
Habib MB 2008. Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana B28:159-166.
Hone and Benton 2006. Cope’s Rule in the Pterosauria, and differing perceptions of Cope’s Rule at different taxonomic levels. Journal of Evolutionary Biology 20(3): 1164–1170. doi: 10.1111/j.1420-9101.2006.01284.x
Kellner AWA and Langston W 1996. Cranial remains of Quetzalcoatlus (Pterosauria, Azhdarchidae) from late Cretaceous sediments of Big Bend National Park, Texas. – Journal of Vertebrate Paleontology 16: 222–231.
Lawson DA 1975. Pterosaur from the latest Cretaceous of West Texas: discovery of the largest flying creature. Science 187: 947-948.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

wiki/Quetzalcoatlus

2 thoughts on “The Evolution of Gigantism in Pterosaurs: The Ancestry of Quetzalcoatlus

  1. Kinda off topic but the Quetzalcoatl/T-Rex scene you painted long ago remains one of my fave paleoart pieces :D Weird creatures, azhdarchids…

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