Ubirajara jubatus: Shoulder rods? Or long skinny leg bones?

Smyth et al. 2020
brings us a new, articulated, partial, crushed skeleton of a small Aptian (Early Cretaceouse) compsognathid theropod with interesting soft tissue. The authors compared the integumentary structures of Ubirajara jubatus to those of the standard wing bird-of-paradise. A reconstruction (Fig. 1) shows four “stiff rod-like structures projecting from its shoulders,” according to Karina Shah, writing for NewScieintist.com.

We’ve never seen anything like this,
which makes it newsy. But is it real?

This taxon will not go into the LRT
because too little is known of the skeleton (Figs, 2, 3).

Figure 1. Ubirajara illustration showing proposed four "stiff rod-like structures projecting from its shoulders."

Figure 1. Ubirajara illustration showing proposed four “stiff rod-like structures projecting from its shoulders.”

The specimen reconstruction (above) was restored
from a plate and counter plate (Fig. 2) with bones at the periphery and a big glob in the middle.

Figure 2. Plate and counter plate image and tracing from Smyth et al. 2020. The tracings were combined by Smyth et al. here in figure 3.

Figure 2. Plate and counter plate image and tracing from Smyth et al. 2020. The tracings were combined by Smyth et al. here in figure 3. Sorry for the low resolution. This is just for display.

Fortunately, Smyth et al. provided a combined tracing
(Fig. 3). Note both legs are missing.

Or are they?
Instead Smyth et al. identify two pairs of straight 15 cm rods, which you can see in their illustration above (Fig. 1). Their diagram shows BMFIs directed outside the blob, aiming toward the top of the scapula.

Occam’s Razor suggests
those paired rods emanating from the shoulders may instead be long, straight legs, knees flexing near the shoulders, splitting posteriorly as shown on the overlays (Fig. 3) toward an absent pelvis for the femur and an absent foot for the tibia. This alternate restoration is a guess based on the scant evidence shown here and an aversion to completely new structures.  But somebody has to say it, just to open this discussion. If I’m wrong, I’m wrong.

Figure 3. From Smyth et al. 2020 with overlays suggesting the possibility that the paired rods growing from the shoulders may instead just be legs with knees near the shoulders. Just a hypothesis awaiting confirmation or refutation. Here the vertebrae are also renumbered.

Figure 3. From Smyth et al. 2020 with overlays suggesting the possibility that the paired rods growing from the shoulders may instead just be legs with knees near the shoulders. Just a hypothesis awaiting confirmation or refutation. Here the vertebrae are also renumbered and the hand is reconstructed.

Anyone can make a mistake.
Even if there are four co-authors. We’ve seen this sort of thing before in Yi qi and Ambopteryx where the authors mistook a displaced ulna or radius for a novel bone, their styliform. The important thing is to not perpetuate the myth of an entirely new structure, if it is a myth. This Ubirajara example is not so clear (based on indistinct impressions) so I could be wrong. Let’s figure this out. This is the loyal opposition talking, building on the tenth man rule (from World War Z).

Figure 4. Ubirajara rough reconstruction from diagram in Smyth et al. 2020.

Figure 4. Ubirajara rough reconstruction from diagram in Smyth et al. 2020 (Fig. 3).

Has anyone else
come up with this novel hypothesis? Let me know if this leg idea can be readily refuted.


References
Smyth RSH, Martill DM, Frey E Rivera-Silva HE and Lenz N 2020. A maned theropod dinosaur from Brazil with elaborate integumentary structures. Cretaceous Research. doi:10.1016/j.cretres.2020.104686

wiki/Ubirajara_jubatus

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