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Piksi: Look! It’s a bird! No it’s a pterosaur!

Saturday, December 29, 2012 15:51

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(Before It's News)

Varricchio (2002) described a new Cretaceous bird from the western US based on a lower humerus and upper radius and ulna, basically an elbow, plus a piece identified as a distal ulna (Figs. 1, 2). Just recently Agnolin and Varricchio (2012) reinterpreted the material as pterosaurian, most likely ornithocheirid and excluded from the Azhdarchidae.

Unfortunately
Except for the images of Piksi itself, the comparable evidence presented by Agnolin and Varricchio (2012, Fig. 1) was not of a high caliber. While Piksi was shown in high detail, the comparables were not (see comments in Fig. 1 caption).

Upper segment: Piksi compared to various pterosaurs and birds

Figure 1. Click to enlarge. Upper two rows: Piksi compared to various pterosaurs and birds as interpreted by Agnolin and Varricchio (2012). Lower two rows and then some: the same only rearranged with rotations to distal humerus view added (why were some inverted?). The capitulum is homologous with the dorsal condyle. Note the three condyles in Piksi, but only two were labeled. Lines were used for both bumps and foramina (not good). Did all of the pterosaur humeri have large distal foramina? Apparently so. Like birds, Piksi did not.

Guilt by association
Agnolin and Varricchio (2012) considered Piksi a type of ornithocheirid. They illustrated the humerus (Fig. 1) in a lineup with other pterosaurs, but it’s really not a good match for any of them with those three large condyles, the lack of distal foramina and those shorter distal, longer ventral dimensions. Unfortunately a lateral view was not presented. That might have shown larger trochlear joints that Piksi had, but ornithocheiroids don’t have, but Dimorphodon and Titanopteryx did. An olecranal fossa was present on Piksi (Fig. 2). A comparable deep depression is not seen in Anhanguera (Fig. 1).

As an experiment, I moved all the example pterosaur humeri together in one line and moved Piksi down to the birds line (Fig. 1) to see if part of the problem was “guilt by association.” What do you think?

Figure 2. Comparing the elbow of Piksi with the same bones in Anhanguera. It’s not a good match. However, a better match among pterosaurs appears in Titanopteryx, an azhdarchid, but not a large one. These photos don’t closely match what is portrayed in the Agnolin and Varricchio (2012) illustrations.

Confession time
Agnolin and Varricchio (2012) report that the Piksi bones are not like those of other contemporary pterosaurs, from the outline shape to the shape of the trochlea. They considered the distal view of the humerus “sub-triangular” as in Anhanguera (Fig. 1), but the two are quite distinct from each other in shape (so, perhaps this is wishful thinking?).

Is it really a bird?
As an experiment I restored bird-like elements and rearranged the bones of Piksi to a bird-like configuration (Fig. 3). The results are not too far off those of other birds. I’m no bird expert, but it looks like there is some variation in the olecranon process of the ulna and elsewhere on the skeleton.

Figure 3. Piksi as a bird. Some birds have an extended olecranon, others do not (pink arrows). The ulna appears to have a distinct curve, which pterosaurs never have. The distal ulna identified by Agnolin and Varracchio (2012) is a flattened circular disk, not the expected deeper shape. Who knows how long the actual bones were…

So, what is Piksi?
I suppose the final answer depends on who is interpreting the bones and what comparables are available. None of the above reconstructions are so close to pterosaurian or avian patterns to call it a walk-off home run. No wonder there was confusion. It didn’t help that Agnolin and Varracchio (2012) had some inverted illustrations and that the holes and hills could not be segregated.

I wonder if Piksi was flightless, which might have affected its morphology.

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
Agnolin FL and Varricchio D 2012. Systematic reinterpretation of Piksi barbarulna Varricchio, 2002 from the Two Medicine Formation (Upper Cretaceous) of Western USA (Montana) as a pterosaur rather than a bird. Geodiversitas 34 (4): 883-894. http://dx.doi.org/10.5252/g2012n4a10.

Varricchio DJ 2002. A new bird from the Upper Cretaceous Two Medicine Formation of Montana. Canadian Journal of Earth Science 39: 19-26.