Another lepidosaur with a big antorbital fenestra

Quick backstory and summary:
Pterosaurs and their Middle Triassic precursors with a big antorbital fenestra are lepidosaurs (LRT 2020, Peters 2007). Macrocnemus is one of those Middle Triassic precursors, but this one is the only one has a large antorbital fenestra…by convergence.

Skull details on this specimen have been overlooked since 2007.
Macrocnemus fuyuanensis (Li, Zhao and Wang 2007; < 1 m in length; GMPKU P3001, Fig. 1), was the first and only member of this genus not considered conspecific by its authors (actually, no two are alike, see Fig. 3). Earlier we looked at the GMPKU specimen. Today the GMPKU specimen enters the large reptile tree (LRT, 1694+ taxa) today nesting with the T2472 specimen from Europe (Fig. 2).

Figure 1. Macrocnemus fuyuanensis (GMPKU-P-3001) in situ and as traced by the original authors, (middle) flipped with colors applied to bones, and (above) bone colors moved about to form a reconstruction. Darker yellow and darker green are medial views of premaxilla and maxilla. Note the long ascending process of the premaxilla and the palatal elements seen through the various openings all overlooked by those with firsthand access to the fossil. Epipterygoids are lepidosaur synapomorphies not present in protorosaurs.

Figure 1. Macrocnemus fuyuanensis (GMPKU-P-3001) in situ and as traced by the original authors, (middle) flipped with colors applied to bones, and (above) bone colors moved about to form a reconstruction. Darker yellow and darker green are medial views of premaxilla and maxilla. Note the long ascending process of the premaxilla and the palatal elements seen through the various openings all overlooked by those with firsthand access to the fossil. Epipterygoids are lepidosaur synapomorphies not present in protorosaurs.

This referred GMPKU specimen was brought to mind
when Scheyer et al. 2020 discussed in detail the larger holotype M. fuyuarnensis with the skull preserved in ventral view (IVPP V15001, Fig. 4). Scheyer et al. 2020 mistakenly considered it an archosauromorph due to taxon exclusion. Jiang et al. mistakenly considered it a protorosaurian due to taxon exclusion.

All prior workers also overlooked the twin epipterygoids
in the referred specimen (Fig. 1). This is a trait not found outside the Lepidosauria and is lost in several subclades of the Lepidosauria (e.g. Fenestrasauria).

All prior workers overlooked the tiny supratemporals,
which are easy to overlook unless you are looking for them based on phylogenetic bracketing. Taxon exclusion is, once again, the chief problem here. A poor tracing (e.g. Li et al. 2007; Jiang et al. 2011) is the secondary problem.

Figure 2. M. fuyuanensis GMPKU-P-3001 overall. This specimen nests with T2472 in figure 3.

Figure 2. M. fuyuanensis GMPKU-P-3001 overall. This specimen nests with T2472 in figure 3.

The antorbital fenestra
was previously (Li et al. 2007; Jiang et al. 2011) and recently (Scheyer et al. 2020) overlooked because earlier workers considered palatal bones to be rostral bones. That is repaired here (Fig. 1) using DGS methods.

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 3. 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.

The larger holotype IVPP V15001 specimen
(Fig. 4) preserves the skull upside down (mandible in ventral view). Other elements clearly show the pectoral girdle, pelvic girdle, manus and pes and other elements, more or less in articulation. These are typically scattered in European fossils of Macrocnemus.

Figure 7. The IVPP V15001 specimen of Macrocnemus fuyuanensis in situ. Colors and reconstructions added. Some disagreement here with the pectoral elements.

Figure 4. The IVPP V15001 specimen of Macrocnemus fuyuanensis in situ. Colors and reconstructions added. Some disagreement here with the pectoral elements. Note how the coracoids slide along the interclavicle bound by the sternum reidentified here from the original coracoid. The skull and mandibles are in the center in ventral view.

For those who forget how important the pectoral girdle is
in Macrocnemus and its descendants, others of you might remember the migration of the sternum to the interclavicle, the erosion if the anterior coracoid rim, the elongation of the scapula, the wrapping of the clavicles and the development of the anterior process of the interclavicle that gradually evolves to become the sternal complex in pterosaurs and their flapping precursors, the fenestrasaurs (Fig. 5). This is why it is vitally important to include more taxa in your analyses in order to keep the specimen you are describing in a proper phylogenetic context. All prior workers who studied Macrocnemus lack this context.

Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

Figure  5. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

The Tritosauria (“third lizards”)
is a new squamate clade, now all extinct. The Tritosauria flourished in the Triassic, was reduced to only the Pterosauria during the Jurassic and Cretaceous, and became extinct thereafter. Several members have an antorbital fenestra, most in the lineage of pterosaurs. The GMPKU specimen has an antorbital fenestra convergent with those taxa.

In 2020 pterosaur experts
still have not presented a better hypothesis for the origin of pterosaurs, but prefer to follow their professors who taught them pterosaurs belong with dinosaurs (e.g. Avemetatarsalia, Ornithodira). When will the first one of them break away from promoting this myth?


References
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
Li C, Zhao L and Wang L 2007.
A new species of Macrocnemus (Reptilia: Protorosauria) from the Middle Triassic of southwestern China and its palaeogeographical implication. Sci China Ser D: Earth Sci, 50(11): 1601–1605.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Scheyer TM, Wang W, Li C, Miedema F and Spiekman SNF 2020. Osteological re-description of Macrocnemus fuyuanensis (Archosauromorpha, Tanystropheidae) from the Middle Triassic of China. Vertebrata PalAsiatica. DOI: 10.19615/j.cnki.1000-3118.200525

wiki/Macrocnemus

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