The Strange Fingers of Istiodactylus sinensis

Drs. Brian Andres and Ji Qiang (2006) reported on the second specimen ever discovered of the lower Cretaceous ornithocheirid pterosaur, Istiodactylus. The holotype came from the Isle of Wight in England. The smaller Andres and Qiang (2006) specimen was discovered in China. Among the smaller hand elements, only distal metacarpal 4 is known from the English holotype. Thankfully the Chinese specimen preserves two virtually complete sets of metacarpals and fingers.

Fingers of Istiodactylus sinensis.

Figure 1. Fingers of Istiodactylus sinensis. Above: As originally interpreted. Middle: Re-identified elements. Below in gray: Reconstructed fingers. DGS (digital graphic segregation) enabled the shifting of elements. This is the only pterosaur known in which the phalangeal formula is 2-2-2 for digits 1-3.

Original Report
Andres and Qiang (2006) reported that only metacarpals 1 and 4 articulated with the carpus. In their view metacarpals 2 and 3 tapered to small points less than halfway toward the carpus. Andres and Qiang (2006) were able to identify the 6 unguals, but only 5 penultimate phalanges. Among the six proximal phalanges typically found in digits 2 and 3 only a single m2.1 was identified.

Metacarpals Re-identified
Like all tetrapods, in pterosaurs metacarpals 1-4 were aligned side by side. The only difference is metacarpal 4 rotated posteriorly for wing folding (Peters 2002). Thus metacarpal 3 is between mc2 and mc4. Whenever pterosaur metacarpals do not all reach the carpus (as in Pteranodon), metacarpal 3 is always the longest. The other two metacarpals stack anteriorly 4 > 3 > 2 > 1. Thus the only slender metacarpal that reached the carpus in Istiodactylus sinensis was mc3. It was misidentified as mc1 by Andres and Qiang (2006) probably because associated digits had only one non-ungual phalanx (see below), which is VERY unusual!

Both metacarpals 1 were preserved at mid-metacarpus, but no sister taxa share this trait.  All other ornithocheirds extend metacarpals 1-3 to the carpus where these elements are known. Thus the displacements were due to taphonomy.

No Other Pterosaur Demonstrates Manual Phalanx Fusion
Because only one non-ungual phalanx can be identified in digits 2 and 3, evidently the three non-ungual phalanges of digit 3 had fused together. The two non-ungual phalanges of digit 2 had also fused together. The fragment previously identified as m2.1 was actually part of the otherwise missing phalanx of digit 3. Since the non-ungual phalanges had become fused in digits 2 and 3 and thus had become nearly identical to digit 1, matching of the distinct ungual shapes made possible the identification of the digits. Manual 1.1 was also more gracile than the others. This is the only pterosaur known in which the phalangeal formula is 2-2-2 for the first three fingers.

Reconstruction 
A reconstruction of the Istiodactylus sinensis manus (Fig. 1) shifts all metacarpals to terminate in a distal line. Metacarpal 3 reaches the carpus. Metacarpals 1 and 2 stack anteriorly. The fingers, each composed of two phalanges (including the ungual), were subequal in length so the PILs (parallel interphalangeal lines, Peters 2000) were simple and aligned.

Digit 5
There’s even a disarticulated vestigial digit 5 here (Fig. 2). More examples of the vestige here. It’s easily overlooked, especially when disarticulated.

manual digit 5 in a pterosaur, Istiodactylus

Figure 2. Vestigial digit 5 in Istiodactylus sinensis. Red = ungual. Green = two phalanges. Blue = metacarpal 5. yellow = Carpal 5. Click to see more examples of this vestige.

Comparison
Most ornithocheirids have digits 2 and 3 subequal with digit 1 shorter. In most ornithocheirids all five metacarpals reach the carpus. No other ornithocheirids fuse the proximal phalanges in digits 2 and 3. Only in Coloborhynchus, a sister to Istiodactylus, does digit 1 equal digit 2 and digit 3 in length. Nurhachius is a closer sister that does not have a subequal digit 1 and all four metacarpals presumeably reach the carpus because metacarpal 1 reaches the carpus while 2 and 3 are largely hidden beneath 4. Thus the misidentification of elements by Andres and Ji (2006) was reasonable, but stands corrected here.

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
Andres B and Ji Q 2006. A new species of Istiodactylus (Pterosauria, Pterodactyloidea) from the Lower Cretaceous of Liaoning, China. Journal of Vertebrate Paleontology, 26: 70-78.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
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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