Enigma pterosaur wing bone from Late Campanian Utah

Farke 2021
brings us a large ‘pterosaur limb bone’ (Fig. 1) from the Late Cretaceous of Utah. The author guessed the bone was an ulna, but could not determine which end was proximal.

Figure 1. Radius RMA 22574 from Farke 2021.

Figure 1. Ulna RMA 22574 from Farke 2021. Distal is at bottom.

From the abstract
“A large pterosaur bone from the Kaiparowits Formation (late Campanian, ~76–74 Ma) of southern Utah, USA, is tentatively identified as an ulna, although its phylogenetic placement cannot be precisely constrained beyond Pterosauria. The element measures over 36 cm in preserved maximum length, indicating a comparatively large individual with an estimated wingspan between 4.3 and 5.9 m, the largest pterosaur yet reported from the Kaiparowits Formation.”

That tentative identity as an ulna is confirmed here.  
Other than its more robust width and overall size, the bone is a good match for the ulna in the more completely known Triebold specimen of Pteranodon, NMC41-358 (Fig. 3). The overall size and relative length vs. width of RMA 22574 identifies this as a large Pteranodon (Fig. 2), perhaps the largest by a few percent, rather than a small azhdarchid (Fig. 4).

So, contra Farke 2021,
this specimen can be precisely constrained rather precisely beyond Pterosauria. It just takes a little comparative anatomy and taxon inclusion. Farke employed only one Pteranodon specimen (FHSM 184) for comparison, perhaps not realizing that no two known taxa are identical, even in the post-crania (Fig. 2), and others demonstrate a wide variation in size and morphology. By the way, FHSM 184 is a large solitary metacarpal 4.

Figure 2. The largest Pteranodon post-crania compared to RMA 22574, slightly larger than the largest.

Figure 2. The largest Pteranodon post-crania compared to RMA 22574, slightly larger than the largest.

A little repair work
to the broken proximal end (elbow) helps complete the match between RMA 22574 and NMC41-358 (Fig. 3).

Figure 2. Comparing RMA 22574 with the smaller and more gracile NMC41-358.

Figure 3. Comparing RMA 22574 with the smaller and more gracile NMC41-358 scaled to the same length.

A selection of Pteranodon post-crania
can be seen to scale here and one of the largest is shown here (Fig. 2). Note the relatively shorter, broader antebrachia (= radius + ulna) in the largest, latest Pteranodon species relative to the humerus (Fig. 2 upper right). The relatively shorter, more robust, largest antebrachium with the characters of RMA 22574 is restricted to large, late Pteranodon specimens.

Figure 3. RMA 22754 compared to Quetzalcoatlus sp. which has a more slender radius.

Figure 4. RMA 22754 compared to Quetzalcoatlus sp. which has a more slender radius.

Comparisons to appropriately sized azhdarchids
(Fig. 4) do not match as well. These tend to have a more gracile, hourglass appearance.


References
Farke AA 2021. A large pterosaur limb bone from the Kaiparowits Formation (late Campanian) of Grand Staircase-Escalante National Monument, Utah, USA. PeerJ 9:e10766 https://doi.org/10.7717/peerj.10766

reptileevolution.com/pteranodon-skulls.htm
reptileevolution.com/pteranodon-postcrania.htm

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