A New Elbow for Megalancosaurus

Just reviewing some old posts, I noticed an autapomorphy in my tracings of the elbow of Megalancosaurus. What I thought was a suture that shortened the ulna relative to the radius was in reality a break (also noted by S. Renesto who sent me some other closeups to support his observation, see below) and the two bones were actually similar in length, as in sister taxa. Here I correct that error and recover a more crescent-shaped sesamoid, more like the giant sesamoid in Drepanosaurus, plus lots of little broken bones that would have represented the ulnar crest.

Megalacosaurus elbow

Figure 1. The break and the broken pieces of the Megalancosaurus ulna are reidentified here. The sesamoid is prominent and crescent-shaped as in Drepanosaurus. The broken portion of the ulna would have stood straight up from the matrix,  if similar to that of Drepanosaurus, which is why it broke during crushing.

More Specimens
Other specimens of this genus also demonstrate a sesamoid present at the elbow of Megalancosaurus.

Elbow sesamoid in another specimen of Megalancosaurus, MPUM 8437.

Figure 2. Elbow sesamoid in another specimen of Megalancosaurus, MPUM 8437.

Another Megalancosaurus elbow, MPUM 6008.

Figure 3. Another Megalancosaurus elbow, MPUM 6008.

These are further data supporting the hypothesis that the odd bone at the elbow of Drepanosaurus was a sesamoid, not the ulna, as Renesto (1994) proposed, nor the coracoid, as Pinna (1980) proposed.

Note, I have not seen the specimens close up, but have relied on photographs using DGS, digital graphic segregation. That’s not an excuse. That’s further support for the method.

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
Calzavara M, Muscio G and Wild R 1980. Megalancosaurus preonensis n. gen. n. sp., a new reptile from the Norian of Friuli. Gortania 2: 59-64.
Feduccia A and Wild R 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Natur Wissenschaften 80:564–566.
Geist NR and Feduccia A 2000. Gravity-defying Behaviors: Identifying Models for Protoaves. American Zoologist 4):664-675. online pdf
Pinna G 1980. Drepanosaurus unguicaudatus, nuovo genere e nuova specie di Lepidosauro del trias alpino. atti Soc. It. Sc.Nat. 121:181-192.
Pinna G 1986. On Drepanosaurus unguicaudatus, an upper Triassic lepidosaurian from the Italian Alps. Journal of Paleontology 50(5):1127-1132.
Renesto S 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of Northern Italy. Journal of Vertebrate Paleontology 14(1):38-52.
Renesto S 1994. The shoulder girdle and anterior limb of Drepanosaurus unguicaudatus(Reptilia, Neodiapsida) from the upper Triassic (Norian of Northern Italy. Zoological Journal of the Linnean Society 111(3):247-264
Renesto S 2000. Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–179.

wiki/Megalancosaurus
wiki/Drepanosaurus

2 thoughts on “A New Elbow for Megalancosaurus

  1. Dear David Peters,

    First and foremost, thank you for presenting your interpretations alongside your observations obtained through your DGS-technique on this website. However, I am somewhat reluctant to accept your observations regarding the Megalancosaurus ulnar sesamoid bone, at least as based on your Figure 2 and 3.

    In Figure 2, you have clearly outlined the extend of the supposed ulnar sesamoid bone in green. According to your interpretation, the sesamoid (indicated in green) extends slightly onto, what appears to be, a crescentic (?) bone fragment located in close approximation to the distal humerus. However, when I examine the photograph underneath your interpretation (still Figure 2), there does not appear to be a difference in texture / coloration, or any other indication of a discontinuation of the crescentic (?) bone fragment that would support the partial sesamoid excursion that you have interpreted at this location. Would you be able to provide a less closely zoomed-in photograph on your website that could allow further interpretation of this region of the skeleton?

    In Figure 3, the supposed ulnar sesamoid bone appears to be a continuation of the ulna instead of a separate ossification. For example, I do not see a distinct “break” that would justify your interpretation that a separate ossification is present. Instead, I observe a continuation of the irregular surface features between the ulnar shaft and the proximal ulna preserving the articulation with the distal humerus. A similar surface texture is present on all the other bones laying in close proximity to the elbow-portion of this Megalancosaurus-skeleton (e.g. the humerus). A similar interpretation can be made for the distal extend of the supposed sesamoid bone in your Figure 2.

    Many thanks for taking the time to consider my alternative interpretations.

    • Thank you for your comment, Rutger. The images are courtesy of Silvio Renesto and I will pass your request for additional photos on to him. We have an ongoing discussion on this. Of course you know the sesamoid in Sphenodon becomes co-ossified to the ulna with just a slight suture to betray its presence. That may be what is happening here in these specimens (figures 2 and 3). Given the odd elbow bone in Drepanosaurus, I searched for similar morphologies in sister taxa and found several smaller sesamoids. The situation in Megalancosaurus may be complicated by at least partial coossification, as you noted.

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