Antorbital fenestrae and lizardy epipterygoids in Macrocnemus

Recent papers featuring hi-rez photos
of several  specimens attributed to Macrocnemus (Saller 2016; Jaquier et al. 2017) permit new DGS tracings and reconstructions (Figs. 1–6) that reveal, among other surprising traits:

  1. Antorbital fenestra – tentative as a tiny hole in Huehuecuetzpalli, much larger in BES SC111, Cosesaurus and other fenestrasaurs, present, but smaller in larger Macrocnemus. Sanders (2016) ignored this trait, perhaps because many protorosaurs lack it.
  2. Epipterygoids – tall, rod-like bone posterior to the orbit previously identified in all lepidosaurs, including Huehuecuetzpalli (Reynoso 1989; Fig. 1), and once tentatively identified in an x-ray of the PIMUZ T 2472 specimen (Kuhn-Schnyder E 1962) attributed to Macrocnemus (Kuhn-Schnyder 1962), but here identified in all specimens. Sanders (2016) ignored mentioning this bone, perhaps because protorosaurs lack it. The epipterygoid has been lost in fenestrasaurs, leading to their traditional misidentification as archosaurs or protorosaurs.
Figure 1. Several Macrocnemus specimens to scale alongside the ancestral taxon in the LRT, Huehuecuetzpalli, and descendant taxa in the LRT, including Cosesaurus and the fenestrasaurs Sharovipteryx, Longisquama and Bergamodactylus. The similarities in transitional taxa should be obvious.

Figure 1. Several Macrocnemus specimens to scale alongside the ancestral taxon in the LRT, Huehuecuetzpalli, and descendant taxa in the LRT, including Cosesaurus and the fenestrasaurs Sharovipteryx, Longisquama and Bergamodactylus. The similarities in transitional taxa should be obvious. In situ specimens in figures 2–5.

And now here are the in situ fossils:
So you can see for yourself.

Figure 2. Huehuecuetzpalli has a tall, narrow epipterygoid, as in other lepidosaurs, and just a pore of an antorbital fenestra in the maxilla.

Figure 2. Huehuecuetzpalli has a tall, narrow epipterygoid, as in other lepidosaurs, and just a pore of an antorbital fenestra in the maxilla, omitted by Renyoso.

Reynoso 1989 identified epipterygoids
in Huehuecuetzpalli (Figs. 2–3), and all workers accept it as a lepidosaur.

Figure 3. Huehuecuetzpalli skull insitu.

Figure 3. Huehuecuetzpalli skull insitu.

Epipterygoids and antorbital fenestrae
have not been recognized in specimens of Macrocnemus, but there they are (Figs. 4–7). Compare the in situ specimens to the DGS reconstructions. Some of these came as a surprise, but sometimes that’s what you get with higher resolution data.

Figure 4. Skull of BES SC111 specimen attributed to Macrocnemus.

Figure 4. Skull of BES SC111 specimen attributed to Macrocnemus.

Romer 1970 considered Macrocnemus a lepidosaur.
Carroll 1988 reclassified it as a member of Protorosauria, as others have prior to the advent of the large reptile tree (LRT, 1326 taxa) which tests all prior candidates for sisterhood going back to the Devonian.

Figure 6. Skull of PIMUZ T4822 specimen attributed to Macrocnemus.

Figure 5. Skull of PIMUZ T4822 specimen attributed to Macrocnemus.

The DGS method
lifts color tracings and fits them back together in Adobe Photoshop to make more precise reconstructions, as shown here. This avoids the imprecision of freehand sketches and the attending biases that inevitably are associated.

Figure 7. Skull of the T2472 specimen attributed to Macrocnemus. Epipterygoids are displaced to the orbit and anterior orbit region.

Figure 6. Skull of the T2472 specimen attributed to Macrocnemus. Epipterygoids are displaced to the orbit and anterior orbit region.

The resemblance of macrocnemids to protorosaurs
like Prolacerta is indeed striking. That’s why all candidate taxa need to be tested in a large gamut phylogenetic analysis that has been proven to weed out convergence by lumping and splitting all included taxa. Don’t be shy about this people. Dig deeper and add taxa.

So, if Macrocnemus is a tritosaur lepidosaur,
then so are Tanystropheus, Langobardisaurus, Dinocephalosaurus, and all the fenestrasaurs, including pterosaurs. Stop relying on outdated, limited gamut, suprageneric taxon lists and feel free to test taxa specified by the LRT. It works.

References
Carroll RL 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co. New York.
Jaquier VP, Fraser NC, Furrer H and Scheyer TM 2017.
Osteology of a New Specimen of Macrocnemus aff. M. fuyuanensis(Archosauromorpha, Protorosauria) from the Middle Triassic of Europe: Potential Implications for Species Recognition and Paleogeography of Tanystropheid Protorosaurs. Frontiers of Earth Science 5:91. doi: 10.3389/feart.2017.00091
Kuhn-Schnyder E 1962. Ein weiterer Schädel von Macrocnemus bassanii Nopcsa aus der anisischen Stufe der Trias des Monte San Giorgio (Kt. Tessin, Schweiz). Paläontologische Zeitschrift [Festband Hermann Schmidt zur Vollendung des 70. Lebensjahres am 3. November 1962. Sonderausgabe zur Paläontologischen Zeitschrift, 1962; 265 pp.], 110–133.
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.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.
Romer AS 1970. Unorthodoxies in Reptilian Phylogeny. Evolution 25:103-112.
Saller F 2016. Anatomia, paleobiologia e filogenesi di Macrocnemus bassanii Nopcsa 1930 (Reptilia, Protorosauria). Alma Mater Studiorum – Università di Bologna Dottorato di Ricerca in Scienze della Terra Ciclo XXVII 206pp.

http://reptileevolution.com/huehuecuetzpalli.htm

wiki/Macrocnemus

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