Cope’s Rule and Pterosaurs

In one of his first papers on pterosaurs, Dr. David Hone teamed with mentor Dr. Mike Benton in 2006 to report, “…the pterosaurs, show increasing body size over 100 million years of the Late Jurassic and Cretaceous, and this seems to be a rare example of a driven trend to large size (Cope’s Rule).” They used a best fit regression line drawn perfectly straight to demonstrate size increase through time. While their results were true, that straight line and their caption (Fig. 1) missed all the real excitement of the journey itself.  Unfortunately they reported, “Wingspans measured from juveniles…were excluded.” 

So they missed all the tiny adult pterosaurs at the bases of several major clades.

 Copes Rule in Pterosaurs as plotted by Hone and Benton 2007

Figure 1. Copes Rule in Pterosaurs as plotted by Hone and Benton 2007. Color cloud added. Despite the overall rise from tiny basal pterosaurs to Quetzalcoatlus, note the several dips over time despite the complete exclusion of tiny transitional pterosaurs in several clades. If pterosaurs had died out any time before 130mya, the regression line would have been even to slightly down.

Straight: Not Great.
Not sure why a straight line was so important. It’s presence negated all possibility of seeing any sort of rise and fall of size on the graph over time — which, after all, was the whole point! The various rises and falls are colored above (Fig. 1). While it is certainly true that early pterosaurs were smaller than the largest pterosaurs of all time, basically the Hone/Benton (2006) strategy negated all possibility of any other result based on the details, a pattern these two followed later (Hone and Benton 2007/2008) in trying to negate the possibility of fenestrasaurs as pterosaurs by eliminating them.

Forget the Graph, What Does Their Data Tell Us?
Let’s skip the straight line for now and keep our eye on the data alone. Just looking at the dot/plots on the graph, some pterosaurs from 225 mya were larger than any others for 75 million years. Then there’s a sharp decrease at 132 mya followed by a large rise. Then another dip at 100 mya ending with Quetzalcoatlus at the end of the Cretaceous. With such a specimen at the end, and no small to medium pterosaurs in the Late Cretaceous, any sort of straight line had to extend up over time.

If We Plotted Dinosaurs, What Would the Graph Look Like?
Dinosaurs, like mammals and reptiles, started small and got big. So their regression line would also rise. Among dinosaurs, sauropods would have plotted highest, but they are all extinct now and none of the really big ones were present in the Late Cretaceous. The average of all modern birds (ostrich to hummingbird) would probably fall somewhere between sparrows and storks (I’m guessing). So, if the data began with tiny Marasuchus, the best-fit regression line might tilt up slightly over time. If so, that would “prove” Cope’s Rule within bird evolution. Whether the line rose or fell slightly from the Triassic to the Present is interesting and quaint, but not the point. The point is: once again we’d miss all the rises and falls (at the K-T boundary) within the lineage over time. It’s the journey, not the destination.

What If Those Purported Juveniles Were Adults?
Readers of The Pterosaur Heresies certainly know the answer to this one. It’s a constant theme. The base of every major pterosaur clade began with a series of ever smaller then gradually larger specimens, unmatchable to any known adults.  Beyond the dips in the graph noted earlier, the insertion of the tiniest of pterosaur adults would have deepened several of the dips, or created their own dips. It’s also clear from the graph that no effort was made to follow specific clades of pterosaurs. For instance, Hone and Benton (2006) had no idea that Quetzalcoatlus arose from tiny ancestors like No. 42 and No. 44, which were themselves tiny descendants of much larger Dorygnathus specimens.

Good for What it is, But Missed The Point
Cope’s Rule did operate within the Pterosauria, as it did in every other tetrapod clade, but in smaller doses separated by intervals of size reduction. These should not have been overlooked or ignored.

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
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
Lawson DA 1975. Pterosaur from the latest Cretaceous of West Texas: discovery of the largest flying creature. Science 187: 947-948.

wiki/Quetzalcoatlus

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