The rise and fall of the pterosaur tail – part 1

A recent post at Pterosaur.Net (a guest post by Scott Persons) compared the tail of basal pterosaurs to velociraptors and it piqued my interest. Let’s take a look at the situation in phylogenetic terms and touch on topics overlooked earlier. This will be #1 in a short series of three or four.

The evolution of the pterosaur tail beginning with a basal lizard, Huehuecuetzpalli.

Figure 1. The evolution of the pterosaur tail beginning with a basal lizard, Huehuecuetzpalli. Already small in Huehuecuetzpalli, the caudofemoralis is further reduced in derived fenestrasaurs as the anterior ilium develops to anchor larger anterior thigh muscles. BTW, you won’t find a better gradual accumulation of pterosaurian traits among archosaurs, no matter how many millions of years you look. Why this hasn’t been embraced can only be due to politics or religion.

Persons noted, “The caudal skeletons of these long-tailed pterosaurs (with the exceptions of the dimorphodontids and very primitive forms) are strikingly similar to that of Deinonychus. In the case of long-tailed pterosaurs, the function of the caudal rods has always seemed obvious. As flying animals, increased rigidity would have helped a tail to serve as a stabilizer or as a rudder.” Not sure I can agree with this.

No pterosaur tail vane acted as a rudder. This is an old paradigm that refuses to die. The rudder was a secondary sexual trait that developed only in pterosaurs with a really robust tail (we’ll see those in the coming days with hints below). Aerodynamically the rudder acted like an arrow vane, keeping the back end of the tail in line with the front end, rather than flopping to one side or the other causing imbalance problems. Moreover, vanes were not universal on basal pterosaurs.

Persons noted a different need for tail stiffening: against ventral bending along its length due to gravity — and all the better if just bone did the work. He remarked, “We can be certain (or about as certain as the fossil record ever permits) that, when the caudal rods of pterosaurs evolved, it was in the context of an aerial lifestyle.” Unfortunately, without a phylogenetic context Persons was speaking beyond his ken. The weight of the tail was already greatly reduced from Huehuecuetzpalli to Cosesaurus and already stiff, judging by the extreme rigidity and length of the in situ specimens of Huehuecuetzpalli Reynoso (1998). It only became stiffer and thinner by Sharovipteryx, a biped, where this attenuated structure balanced the forequarters, as in dinosaurs, but without the great weight, only great length.

Caudal rods actually developed in derived pterosaurs and in every case the tail developed into a more robust, and therfore weighty, structure than in their forebearers, who were already flying. So, caudal rods developed along with tail vanes to be sexy. Dozens of pterosaurs without caudal rods demonstrate their superfluous use in flying.

Not all long-tailed pterosaurs had a tail vane. It developed from stiff tail hairs bunched at the tip, slowly evolving to the vertical plane.

More to the point of this series, only a few long-tailed pterosaurs had intertwining caudal rods and none achieved the acme seen in Campylognathoides and Rhamphorhynchus (itself derived from Campylognathoides). We don’t see this intertwining in dimorphodontids, Sordes, Eudimorphodon and basal dorygnathids (which we’ll look at later). Derived Dorygnathus did have intertwined rods by convergence.

The extremely long tail of MPUM6009 was extremely straight without ossified caudal rods extending more than a centrum in length.

Persons made the point that several tails of dromaeosaurids were actually quite bendable, based on their in situ  positions. Other dromaeosauruid tails were not flexible, again based on their in situ preservation. Persons did not present any pterosaur tails preserved sinuously, but didn’t mention the reason: They don’t exist.

Update: Notably, the metronome hypothesis places pterosaurs on the ground when they do “their thang” with their highly ossified tails. Notably, dromaeosaurids were grounded Archaeopteryx descendants. So, caudal supports in both cases were NOT for aerodynamics but terra-dynamics. (Contra Persons and Currie (2012) who reported, “the unique caudal morphologies of dromaeosaurids and rhamphorhynchids were both adaptations for an aerial lifestyle.”

More on this topic tomorrow.

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 (published after posted)
Persons WS IV and Currie PJ 2012. Dragon Tails: Convergent Caudal Morphology in Winged Archosaurs. Acta Geologica Sinica – English Edition 86 (6): 1402–1412. DOI: 10.1111/1755-6724.12009. http://onlinelibrary.wiley.com/doi/10.1111/1755-6724.12009/abstract

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