Rhamphorhynchus fingers

There are two issues revolving around pterosaur fingers. 

1. How many are they? Others say 4. I say 5, and #5 is a vestige.

2. What was their orientation? Others say palmar side anterior in flight. I say palmar side ventral in flight.

Figure 1. The famous Rhamphorhynchus entangled with the ganoid fish Aspidorhynchus. Here the fingers are color coded and rotated to their invivo orientation.  Digit 5 is shown.

Figure 1. Click to enlarge. The famous Rhamphorhynchus entangled with the ganoid fish Aspidorhynchus (Frey and Tischlinger 2012, WDC CSG 255). Here the fingers are color coded and rotated from their in situ positions (above) to their in vivo orientation (below). The “right ventral” image includes metacarpal 4 in anteroventral view. Digit 5 is shown in orange. The quadrant arrow shows how fingers 1-3 rise like a drawbridge during taphonomy when the large extensor tendon for the wing finger is gone.

Manual digit 5
This specimen, WDC CSG 255, is yet one more example of many demonstrating the existence of a vestigial manual digit 5 on the dorsolateral corner of metacarpal 4. This configuration reflects the primitive lateral placement of manual digit 5 plus the enlargement and axial rotation of metacarpal 4. On the left hand manual digit 5 is buried beneath metacarpal 4.

Digits 1-3 Orientation
Earlier we discussed the drawbridge effect on manual digits 1-3. Each metacarpal is bound to its partner laterally and metacarpal 3 is bound to metacarpal 4 laterally. Since metacarpal 4 is enlarged and axially rotated this 3-4 connection is low, on the medial (now ventral) side of metacarpal 4. This observation is confirmed again in WDC CSG 255, in which right metacarpals 1-3 rotate up against right metacarpal 4, but note they are not bound to it. The 3-4 hinge is broken yet metacarpal 3 is partly ventral to metacarpal 4. The left hand is more natural in configuration with metacarpals 1-3 exposed dorsally and in the plane of the wing while metacarpal 4 exposed laterally (axially rotated to dorsally). Note that the base of metacarpal 3 is largely tucked under (ventral to) metacarpal 4 as in the left metacarpus. Metacarpals 1-3 are not bound dorsally to metacarpal 4 in either case. Rather metacarpal 3 is between 2 and 4, as in other tetrapods.

References
Frey E, Tischlinger H 2012. The Late Jurassic Pterosaur Rhamphorhynchus, a Frequent Victim of the Ganoid Fish Aspidorhynchus? PLoS ONE 7(3): e31945. doi:10.1371/journal.pone.0031945

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