Earlier and yesterday we looked at parts 1 and 2 of the evolution of the pterosaur tail, the reduction of the caudofemoralis, the rotation of the chevrons (attenuation of the entire tail), the development of the tail vane and the appearance of caudal rods.
This series was inspired by a guest post by Scott Persons at Pterosaur.net.blospot on Jan 2.
Today we’ll finish up by looking at Dorygnathus, its ancestors, its variety and descendants (Fig. 1). Previously unrecognized for its importance, Dorygnathus lies at the center of all later pterosaur evolution.
The story of Dorygnathus tails begins with a small Eudimorphodon specimen, Bsp 1994. (Fig. 1) that had a tail of unknown length with small chevrons and no caudal rods.
Sordes was derived from a sister to it and had a relatively short tail with short caudal rods.
The many species within the genus Dorygnathus were larger and had a longer tail supported by caudal rods, supporting our earlier hypothesis that size was a major factor, along with phylogeny, in the ossification of caudal rods.
One Dorygnathus specimen, SMNS 50164, gave rise to azhdarchids and their kin starting with TM10341. It was tiny, the tail was shorter and not stiffened by caudal rods or elongated chevrons. From this point on, elongated caudals and caudal rods no longer appeared in pterosaurs in this lineage.
Another Dorygnathus, R156, lies at the base of the much smaller short-tailed Ctenochasma and kin. From this point on, elongated caudals and caudal rods no longer appeared in pterosaurs in this lineage.
Jianchangnathus preserves only a short portion of its gracile tail, but this taxon gave rise to Scaphognathus. Two specimens demonstrate a reduction in overall size and a reduction in the tail, but n110 shows that elongated caudal “threads” were present stiffening the tail. This clade gave rise to all later pterosaurs, (Pterodactylus, Germanodactylus and their descendants and kin) all of which had a short weak tail. Following Scaphognathus, elongated caudals and caudal rods no longer appeared in pterosaurs in this lineage.
Jianchangnathus also gave rise to a dead-end clade, the darwinopterids, all of which had a medium-length stiff tail, except Pterorhynchus (Fig. 1), which had an elongated tail with segmented vanes running along most of its length. Quite unique, so far as we know, given the general lack of soft tissue preservation.
So the story of caudal rod development was not a simple one. Not all basal pterosaurs ossified them. They do appear primarily in larger forms, but Scaphognathus n110 is the exception on that matter. Caudal rods are not associated with flight, but are associated with vane development and size. They add bulk to the tail, so cannot be weight-saving devices, contra traditional opinion.
And tail vanes act like arrow vanes or weather vanes, keeping the tip of the tail close to the parasaggital plane and in line with the airstream passively. If a pterosaur wanted to turn it had to bank and to make a coordinated turn, that requires a rudder of spoilers in an airplane, a pterosaur could do all of that with just its wings, like a modern flying wing airplane.
Basal pterosaurs had such a thin-as-a-whisker tail that mass and balance were of little concern. Later large pterosaurs thickened the tail for their own romantic purposes. It may be no coincidence that head crests appeared about the time that long tails with vanes disappeared. That’s fashion for you. You’re either in or out.
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.”
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