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Pterosaur Tails, woohoo!

January 10, 2013
Boreopterus/Zhenyuanopterus by Mark Witton. Note that the animal's tail is almost as long as it's torso; quite a lot for a so called "short tailed pterosaur".

Boreopterus/Zhenyuanopterus by Mark Witton. Note that the animal’s tail is almost as long as it’s torso; quite a lot for a so called “short tailed pterosaur”.

Classically, pterosaurs have been classified due to their tails, as the long tailed “rhamphorhynchoids” (which are not a natural clade, but a paraphyletic assemblage), and the short tailed pterodactyloids. It has been assumed that, as with birds, pterosaurs reduced their tails in order to become better flyers, and that the pterodactyloids, with their small tails, represented an unavoidable evolutiopnary conclusion.

However, over the years, the dicovery of several pterosaur taxa has shown that this dichotomy is not very accurate, and that tail reduction is not necessarily required for flying vertebrates.

What we have learned over the years

With the discovery of Anurognathidae in 1923, we’ve learn that pterosaurs outside of Pterodactyloidea had short tails. Most interesting, however, is that anurognathids are quite possibly the most basal pterosaurs known. This could mean that the pterosaur short tail is a basal condition, and that longer tails developed secondarily.

Less controversial is the discovery of long tailed pterodactyloids. Within Ornithocheiroidea, and the clades Ornithocheiridae/Anhangueridae/whatever, Boreopteridae and Pteranodontidae, remarkably long tails are not uncommon. While certainly nowhere as long as rhamphorhynchid tails, the tails of creatures like Boreopterus and Anhanguera are almost if not as long as the animals’ torsos. Certainly not vestigial, as previously thought. Furthermore, these were the most aerial of all pterodactyloids, with the terrestrial foraging azhdarchoids, ctenochasmatoids and dsungaripteroids being the ones with short tails. Therefore, if the tail was detrimental to flight, the inverse should occur.

Finally, compared to birds, pterosaurs were rather conservative in regards to their tails. Even if anurognathids were not the most basal pterosaurs and their short tails not the ancestral condition, it would represent a total of two times among the entire history of Pterosauria in which the tail was reduced, with a wide interval between the two events (anurognathids having evolved in the Triassic according to phylogenetic brackening, and pterodactyloids having evolved in the late Jurassic). Compare to birds, which reduced their tail very early on, on possibly up to four seperate events.

Under these conclusions, therefore, to pterosaurs, the tail was not very detrimental to flight, and might in fact had been advantageous, as even if anurognathids did not represent the basal condition, the very aerial ornithocheiroids still had much longer tails than other pterodactyloids.

Tail Morphology and it’s relevance to aerial control

Rhamphorhynchus' tail musculature, discussed previously as very deinonychosaur-like

Rhamphorhynchus’ tail musculature, discussed previously as very deinonychosaur-like

Pterosaur tails, in addition to being generally long, were also rather complex, with stiffening chevron rods and well developed associated musculature. Likewise, as well known, many “rhamphorhynchoid” species had notable diamond-like discs at the end of their tails.

According to german Rhamphorhynchus specimens, these tail discs must had been quite relevant for steering; across the animal’s lifetime (and, therefore, alongside it’s ecological niches and flight style), these discs changed, starting as small ovals, then adquiring a larger, more diamond shape and finally developing large, triangular lobes. This seems to coincide with more advanced flight capacities, as juveniles started as small, possibly terrestrial insectivores while the adults were aerial fish hunters.

Thus, the tail must had been fundamentally important for aerial steering, and therefore species adapted to spend most of their lives on the air conserved reasonably long tails. Aerodynamic studies show that shorter tails = more manouverability – which is probably why the small aerial insectivores like anurognathids didn’t had a long tail -, but longer tails help to steer without requiring as much neural power, as the caudal rudder doesn’t require nearly as many motions to operate (think aeroplane tail), and for animals spending most of their lives on the air, wasting energy could be fatal.

This also leads to another debate, that of whereas the uropatagium was connected to the tail or simply united the legs. In order for the tail to act as an independent rudder, this probably means that it wasn’t connected to the uropatagium – which would therefore be rendered a cruropatagium, or “ankle membrane” -, or that if it was connected it likely did not affect the uropatagium’s size significantly (i.e. it was connected only to the tail’s base).

Why bird tails are different, and deinonichosaur ones aren’t

Microraptor reconstruction. Microraptor and similar deinonychosaurs have been noted as having tails very similar to pterosaurian ones, and as you can see, much like "rhamphorhynchoids", they had two different steering organs.

Microraptor reconstruction. Microraptor and similar deinonychosaurs have been noted as having tails very similar to pterosaurian ones, and as you can see, much like “rhamphorhynchoids”, they had two different steering organs.

Birds, unlike pterosaurs, lost their tails far more frequently in their evolution, as well as very early. This is presumably due to the fact that birds were far more pressured by their detrimental extra weight than pterosaurs were, and that, unlike the other flying sauropsids, their tail feathers allowed a rather efficient rudder whilst allowing them to reduce their tail. Still, only ornithurines and longipterids has proper tail feathers; most other birds had to do with featherless tails, relying on their wings for flight control, meaning that weight-saving was both a more important factor and that they were far less capable at steering than pterosaurs.

In flying dromaeosaurs, however, the tail was kept long. More importantly, volant dromaeosaurs developed the same unique tail morphology as “rhamphorhynchoid” pterosaurs, with the chevron rods and the same musculature. All dromaeosaurs with preserved tail feathers also show a fan at the tip of the tail that is remarkably similar to the “rhamphorhynchoid” tail disc, and that most likely played the same role in steering.

The reason for these rather pterosaurian adaptations is likely derived from the fact that, much like pterosaurs, dromaeosaurs were less pressured to loose weight. Dromaeosaurs, after all, had those spectactular hindwings, which probably served the same function as pterosaurian cruropatagia, and that likely allowed them to waste less energy in powered flight.

The more we understand how animals flew, the more we learn that the avian bodyplan is not the ideal one, but just a possibility amidst many more.

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