The Origin of the Pterosaur Tail Vane

The pterosaur tail vane is found in only a few clades of long-tailed pterosaurs. Campylognathoides + Rhamphorhynchus have traditional vanes (Fig. 1). Other basal pterosaurs have fibers, I suppose similar to pycnofibres, emanating from the tail, often concentrated at the posterior tip, sometimes very similar to a vane. Pterorhynchus creates its own pattern with a series of vanes on the posterior tail.

Vane Usage
Traditional workers, like Marsh (1882) have suggested a steering usage for the tail vane. The heretical view was expressed by Peters (2002), “The tail vane on a pterosaur would have acted passively, like a weather vane, keeping the tail near the parasagittal plane in turns and during gusts of wind. It would have made a poor steering device compared to simple banking of the wings.” An hypothesis suggesting the vane’s use as a secondary sexual signal was blogged earlier.

Early and Recent Observations and Hypotheses on Tail Vanes
Marsh (1882) described the segments of the tail vane in Rhamphorynchus phyllurus as extensions of the neural spines and chevrons.

Dr. David Hone reported in his blog“Structurally, all vanes have transverse banding across them which is presumably some form of reinforcement, though where the vanes are composed entirely of skin and interstitial tissues or have perhaps cartilage or anything else involved in their composition is not known.” 

Tail Hairs Expand and Coalesce to Form a Vane
The evolution and first appearance of a tail vane has not been covered in the literature. Figure 1 portrays select fenestrasaur and pterosaur taxa with tail hairs and tail vanes. It’s clear that one evolved from the other. Hairs can become cornified. Pangolin scales and rhinoceros horns are examples of this. Below are descriptions of several taxa that demonstrate the evolution of tail vanes from tail hairs (pycnofibres).

pterosaur tail vane

Figure 1. Pterosaur tail vane evolution. Click to enlarge.

Tail Hairs and Tail Vanes in Select Fenestrasaurs (including Pterosaurs)
Cosesaurus – Slender hairs, first identified as proto-feather divisions by Ellenberger (1978, 1993), emerge from the entire length of the tail.
Longisquama – Difficult to determine, but possible hairs gathered at the tail tip form a primitive decoration convergent with the vane of later pterosaurs.
Anurognathus – Tail hairs restricted to posterior half of the tail vestige.
Batrachognathus – Only a few longer hairs tip the vestigial tail.
Campylognathoides – The tail hairs coalesce to become a vertical tail vane that acts as a secondary sexual signal and passively reorients the tail tip in line with the flight path like feathers on an arrow.
Rhamphorhynchus intermedius – Phylogenetic size reduction included tail vane reduction.
Rhamphorhynchus (Vienna specimen) – The vane assumes a diamond shape.
Sordes – An atypical expansion of the distal tail bones including just a few tail hairs forming a vane shape, but not a vane.
Dorygnathus (Donau specimen) – The tail hairs coalesce at the tip but do not form a vane.
Dorygnathus SMNS 50164 – More substantial tail hairs indicated.
Pterorhynchus – Several dozen mini-vanes extend down the posterior half of the tail. These were individually expanded tail hairs.
Scaphognathus (Maxburg specimen) – An ephemeral trinagular tail vane may have been present. Subsequent taxa appear to reduce this.

Summary
Tail vanes in pterosaurs were most prominent in the Campylognathoides and Rhamphorhynchus clade. The evolved from tail hairs that first appeared in Cosesaurus. Tail vanes were reduced and disappeared in pterosaurs with a reduced tail.

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
Ellenberger P 1978. L’Origine des Oiseaux. Historique et méthodes nouvelles. Les problémes des Archaeornithes. La venue au jour de Cosesaurus aviceps (Muschelkalk supérieur) in Aspects Modernes des Recherches sur l’Evolution. In Bons, J. (ed.) Compt Ren. Coll. Montpellier12-16 Sept. 1977. Vol. 1. Montpellier, Mém. Trav. Ecole Prat. Hautes Etudes, De l’Institut de Montpellier 4: 89-117.
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Marsh 0C 1882. The wings of pterodactyles: American Journal of Science 23: 251-256.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.

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