Raeticodactylus: a pterosaur box kite

Raeticodactylus (Fig. 1) is a basal pterosaur of the Late Triassic. There is only one pterosaur that is more basal, MPUM 6009. A comparison between the two (Fig.1) should prove enlightening.

Figure 1. Click to enlarge. The two most basal pterosaurs, MPUM 6009 and Raeticodactylus. Note the antebrachium in Raeticodactylus has only lengthened without becoming relatively wider, creating the most gracile wing of all pterosaurs -- a pterosaur box kite. The depth of the anterior dentary in Raeticodactylus is a keel/crest. There's nothing like it in sister taxa. A restoration of the incomplete Raeticodactylus pes can be based on MPUM 6009.

Figure 1. Click to enlarge. The two most basal pterosaurs, MPUM 6009 and Raeticodactylus. To the right, they are to scale. Note the antebrachium in Raeticodactylus (red ellipse) has only lengthened without becoming relatively wider, creating the most gracile wing of all pterosaurs — a pterosaur box kite. The depth of the anterior dentary in Raeticodactylus is a keel/crest. There’s nothing like it in sister taxa. A restoration of the incomplete Raeticodactylus pes is based on MPUM 6009. That dorsal view with wings out is Raeticodactylus, as skinny as they get. The tail of Raeticodactylus is unknown, but it can’t get much thinner than it in MPUM6009.

The first thing you notice is
Raeticodactlylus (Fig. 1) is twice as tall as MPUM 6009. You might also notice that the humerus and antebrachium are relatively longer and the hind limbs are relatively shorter in Raeticodactylus. The third thing you might notice is the humerus and antebrachium are no thicker in Raeticodactylus than in the half-sized MPUM 6009. This makes Raeticodactylus the most gracile and fragile pterosaur of all time. Essentially Raeticodactylus was a pterosaurian box kite.

Raeticodactylus is also the first pterosaur capable of touching its hands to the substrate while walking upright. All other pterosaurs follow this pattern. It also had a longer neck, more than twice as long as in MPUM 6009.

Figure 2. The skull of MPUM 6009 with a soft tissue patch at the same place as the rostral horn/crest on Raeticodactylus.

Figure 2. The skull of MPUM 6009 with a soft tissue patch (pink arrow) at the same place as the rostral horn/crest on Raeticodactylus. This precursor soft tissue reaches its acme in Austriadactylus (Fig. 3).

The Raeticodactylus horn/crest deserves some attention
because it looks like it had a precursor in soft tissue in MPUM  6009 (Fig. 2). Th rostral crest also has a homolog in Austriadactylus (Fig. 3).

Raeticodactylus also had a dentary keel/crest (Fig. 1). And the best explanation I can come up with for the elongate articular at the rear of the jaw is for another sort of decoration by itself or as a post to hoist or open a throat decoration.

Figure 3. The two specimens attributed to Austriadactylus, sisters to Raeticodactylus.

Figure 3. Click to enlarge. The two specimens attributed to Austriadactylus, sisters to Raeticodactylus. The rostral crest is likely homologous. A larger sternal complex indicates Austriadactylus was a strong flyer, but probably an infrequent flapper. The smaller Austriadactylus (from Italy) is where dimorphodontids and eudimorphodontids parted ways.

Another skinny pterosaur
The skull of Austriadactylus might make it look tough, but like its sister, Raeticodactylus, it was just another hyper-gracile Triassic pterosaur. Even so, the caudals were more robust than in MPUM 6009, so something was happening there, at least as a counterbalance while bipedal.

Evidently greater size was important for Triassic pterosaurs. Greater strength, not so much.

So this brings up forelimb launch. 
In MPUM 6009, why would you give up using those nice long hind limbs to bend so low for a forelimb launch? It doesn’t make sense here.

In Raeticodactylus and Austriadactylus the fore limbs were longer, the hind limbs were shorter, but still, there is not great strength in those soda-straw-like bones. With larger skinnier wings, it appears as though these two were trying to maximize the lift/weight ratio. The forelimb launch has its problems, discussed here and here.

References
Dalla Vecchia FM 2009. Anatomy and systematics of the pterosaur Carniadactylus (gen. n.)rosenfeldi (Dalla Vecchia, 1995). Rivista Italiana de Paleontologia e Stratigrafia 115 (2): 159-188.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Stecher R 2008. A new Triassic pterosaur from Switzerland (Central Austroalpine, Grisons),Raeticodactylus filisurensis gen. et sp. nov.”. Swiss Journal of Geosciences 101: 185. doi:10.1007/s00015-008-1252-6. Online First
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.

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