SVP abstracts 24: Macrocnemus revisited

Scheyer et al. 2020 bring us
a review of the tanystropheid, tritosaur, lepidosaur, Macrocnemus (Fig. 1), a genus known from several specimens, not all of which are congeneric in the large reptile tree (LRT, 1752+ taxa; subset Fig. 2).

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

Figure 1. M. fuyuanensis GMPKU-P-3001 overall. This specimen nests with T2472.

Figure 2. Subset of the LRT focusing on the relatives of Macrocnemus.

Figure 2. Subset of the LRT focusing on the relatives of Macrocnemus.

From the abstract:
“Over the past decades, an increasing number of mostly marine reptiles have been described from the Triassic of southern China. Many of these taxa had a Tethys-wide distribution, whereas terrestrial reptile taxa known from both western and eastern margins of the Tethys are exceptionally rare.”

“One such terrestrial animal is the small to medium sized tanystropheid archosauromorph Macrocnemus from the Middle Triassic. The genus is represented by two European species, the well-known M. bassanii and M. obristi, known only from posterior postcranial remains, and the slightly younger M. fuyuanensis, known from two complete specimens from southwestern China.”

The LRT finds the several members of the traditional genus, Macrocnemus (Fig. 2), are not congeneric. One tested specimen from China (Fig. 1, GMPKU-P-3001) is congeneric with several European specimens. The BES SC 111 specimen is small, leading to the clade Fenestrasauria and Pterosauria. It is not a juvenile as workers assume prior to running a phylogenetic analysis.

“Species recognition in the genus classically relied on proportional differences of the limb bones.”

The LRT scores the entire specimen and proportional differences of the limb bones are not  relied upon. That would be “Pulling a Larry Martin.” Scheyer et al. should reconstruct the various specimens to discover the diversity in the shapes of skull, pedal and all other skeletal elements.

“M. fuyuanensis was recently tentatively proposed to be present also in Europe, based on a single specimen from the Besano Formation of Monte San Giorgio, southern Switzerland/northern Italy. Further analysis, however, was hampered due to limited original description of the holotype specimen of M. fuyuanensis and insufficient understanding of the cranial anatomy of M. bassanii, despite being known from several complete and well-preserved specimens in European collections.”

As noted above in the LRT.

“To clarify the relationships among Macrocnemus species, we re-described both the holotype of M. fuyuanensis and a well-preserved skull of M. bassanii, the latter using high-resolution synchrotron micro-computed tomography, which allowed us to reconstruct and describe the configuration of the skull, including the braincase for the first time, in high detail. Our findings reveal that the osteology of both species is very similar and no clear differences were found in the cranium.”

As noted above in the LRT without benefit form µCT scans.

“The skull of Macrocnemus has a rigid, tightly fitting squamosal-quadrate joint allowing little, if any, cranial kinesis. The configuration of the palatal bones with the tooth-bearing pterygoids, palatines and vomers could be reconstructed for the first time.”

No, Kuhn-Schnyder 1962 did this earlier (Fig. 3). So did Miedema et al. 2020 (Fig. 4), but that specimen, PIMUZ T 2477, is not congeneric with other European Macrocnmeus specimens. That is something you only find out by running a wide gamut phylogenetic analysis, evidently missing from Scheyer et al. 2020.

Figure 3. Palate of Macrocnemus from Kuhn-Schnyder 1938.

Figure 3. Palate of Macrocnemus from Kuhn-Schnyder 1962.

Figure 2. The PIMUZ T 2477 specimen wrongly traditionally assigned to Macrocnemus.

Figure 4. The PIMUZ T 2477 specimen wrongly traditionally assigned to Macrocnemus.

“In the postcranium, besides the limb ratios, we confirmed the identification of the interclavicle as the most important bone for species recognition.”

This is, by definition, “Pulling a Larry Martin.” Don’t do that! Run the analysis.

“The interclavicle of M. fuyuanensis can be distinguished from M. bassanii, among other features, by its short and fusiform posterior process and anterior facing rod-like processes that extend from a common base enclosing a narrow V-shaped median notch. The presence of M. fuyuanensis at Monte San Giorgio confirms its widespread distribution over the entire Tethys realm.”

You heard it here first (Fig. 2) using phylogenetic analysis. The same analysis nests Macrocnemus within Lepidosauria, far from Archosauromorpha. Add taxa to resolve this issue for yourself. That’s all you have to do.

Kuhn-Schnyder E 1962.
Ein weiterer Schädel von Macrocnemus bassanii Nopcsa aus der anisischen Stufe der Trias des Monte San Giorgio (Kt. Tessin, Schweiz). Palaeontologische Zeitschrift 36:110-133.
Scheyer T, Miedema F, Wang W, Li C, Spiekman S, Fernandez V and Reumer J 2020. Standard osteological and virtual 3D anatomical re-investigation of Macrocnemus (Tanystropheidae Archosauromorpha), a rare Middle Triassic terrestrial reptile with a Tethys-wide distribution. SVP abstracts 2020.

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