Suture or Crack? Reinterpreting the skull of Macrocnemus fuyuanensis.

Updated May 26, 2020
with new DGS tracings of the M. fuyuanensis specimen, GMPKU-P-3001.

Macrocnemus fuyuanensis (Li et al. 2007 and Jiang et al. 2011) was described as a new species of Macrocnemus, and the first from China. The skull (Figs. 1-3) was crushed, but largely articulated. Even so, the cracks looked like sutures and the sutures looked like cracks. With few other reconstructions to work from Jiang et al. (2011) got most of the skull right, but some sutures were misidentified. Their identification of a few skull elements in the rostrum was unlike those of other Macrocnemus and unlike those of other sister taxa like Huehuecuetzpalli, Tanystropheus, Cosesaurus and Jesairosaurus. Let’s correct them.

Macrocnemus fuyuanensis.

Figure 1. The Jiang et al. (2011) identification of the skull bones of Macrocnemus fuyuanensis. This specimen was touted as the first to clearly show the rostrum but the recovered reconstruction was unlike any other Macrocnemus and unlike sister taxa, like Tanystropheus, Jesairosaurus, Cosesaurus and Huehuecuetzpalli. Their external naris (exn) is actually the split between the two premaxillae with the left dentary visible between them (see figure 2).

Figure 2. Macrocnemus fuyuanensis skull bones reidentified using the DGS method. I have not seen the fossil, but I have reconstructed several less well-articulated Macrocnemus skulls. Note the elongated ascending process of the premaxilla, as in other Macrocnemus and sister taxa. These elements are reconstructed in figure 3. The tiny lacrimal, in purple, is common to this clade. The inside of the left maxilla was originally misidentified as the left premaxilla because the original describers did not understand and look for the expected morphology based on sister specimens.

Figure 2. Macrocnemus fuyuanensis skull bones reidentified using the DGS method. I have not seen the fossil, but I have reconstructed several less well-articulated Macrocnemus skulls. Note the elongated ascending process of the premaxilla, as in other Macrocnemus and sister taxa. These elements are reconstructed in figure 3. The tiny lacrimal, in purple, is common to this clade. The inside of the left maxilla was originally misidentified as the left premaxilla because the original describers did not understand and look for the expected morphology based on sister specimens. Figure 3. Using the DGS method the tracings of Macrocnemus fuyuanensis in figure 2 were refit to create this reconstruction. It helps to know what other Macrocnemus and other sister taxa look like in order to identify elements and sutures and the cracks that tend to confuse matters.

Not So Different After All
The resulting reconstruction of the Chinese Macrocnemus is not so different from the European specimens. That should come as no surprise. The post-crania were also quite similar. Even so, there were distinct morphologies that suggest speciation, as originally described. The many cracks in the skull bones and an apparent lack of experience in reconstructing Macrocnemus skull bones produced certain mistakes in the Jiang et al. (2011) report that were corrected here.

Figure 2. M. fuyuanensis overall + skull.

Figure 2. M. fuyuanensis overall + skull.

Macrocnemus: a Tritosaur lepidosaur, not a protorosaur 
Just as a reminder, Macrocnemus nests with that new third clade of lizards, the Tritosauria. Despite convergent appearances, Macrocnemus was not related to Protorosaurus and Prolacerta.


References
Li C, Zhao L-J and Wang L-T 2007. A new species of Macrocnemus (Reptilia: Protorosauria) from the Middle Triassic of southwestern China and its palaeogeographical implication. Science in China D, Earth Sciences 50(11)1601-1605.
Jiang D-Y, Rieppel O, Fraser NC, Motani R, Hao W-C, Tintori A, Sun Y-L and Sun Z-Y 2011. New information on the protorosaurian reptile Macrocnemus fuyuanensis Li et al., 2007, from the Middle/Upper Triassic of Yunnan, China. Journal of Vertebrate Paleontology 31: 2011-1237, DOI:10.1080/02724634.2011.610853

wiki/Macrocnemus

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