The BES SC 111 specimen of Macrocnemus – DGS helps reconstruct it

Previously considered (Renesto S and Avanzini M 2002) a juvenile due to its size, the BES SC 111 specimen of Macrocnemus (Fig. 1) sheds light on the origin of such diverse lineages as the Tanystropheidae (Langobardisaurus, Fig. 2) and the Fenestrasauria (Cosesaurus through the Pterosauria, Fig. 2). It also nests at the base of other Macrocnemus specimens including the oddly bizarre, Dinocephalosaurus (Fig. 3).

Figure 1. Click to enlarge. Stages in the DGS tracing and reconstruction of the the Macrocnemus BES SC 111 skull. I did not realize the the palatal bones were so visible. There's a palatine and ectopterygoid over the nasal and frontal, for instance. So earlier mistakes were made that are corrected here. The right mandible is traced here only along its ventral rim.

Figure 1. Click to enlarge. Stages in the DGS tracing and reconstruction of the the Macrocnemus BES SC 111 skull. I did not realize the the palatal bones were so visible. There’s a palatine and ectopterygoid over the nasal and frontal, for instance. So earlier mistakes were made that are corrected here. The right mandible is traced here only along its ventral rim.

Derived from
an early Triassic sister to Huehuecuetzpalli and/or Jesairosaurus, the BES SC 111 specimen seems to have at least a depression in the dorsal maxilla that will ultimately become an antorbital fenestra in the Fenestrasauria. Note the resemblance of this skull to that of Cosesaurus and Langobardisaurus (Fig. 2). They all share a retracted naris, large orbit, bent quadrate, short postorbital region and relatively short teeth.

The reduction of pedal digit 5 in all known Macrocnemus specimens demonstrates the BES SC 111 nests at the base of the Macrocnemus lineage. An unknown sister without this reduction would be basal to Langobardisaurus and the Fenestrasauria.

Figure 2. Macrocnemus BES SC 111 compared to sister taxa, Langobardisaurus, Cosesaurus and the basal pterosaur, MPUM 6009. Preserved loose, the orientation of the ectopterygoids could go either way, with the narrow tip contacting the maxilla instead, as in Dinocephalosaurus (Fig. 3).

Figure 2. Macrocnemus BES SC 111 compared to sister taxa, Langobardisaurus, Cosesaurus and the basal pterosaur, MPUM 6009. 

Figure 3. Dinocephalosaurus to scale with the largest Macrocnemus specimen and the smaller ones from figure 2.

Figure 3. Dinocephalosaurus to scale with a large Macrocnemus specimen, T4822, and the smaller ones from figure 2.

The take-away from this is: large odd reptiles sometimes have their origin in not-so-large, not-so-odd reptiles like the BES SC 111 specimen. At the same time, small odd reptiles may have the same origin. Make sure you add the plain, old reptiles to your cladograms. That’s where the spectacular taxa have their origin.

References
Li C, Zhao L-J and Wang L-T 2007A 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.
Nopcsa F 1931. Macrocnemus nicht Macrochemus. Centralblatt fur Mineralogie. Geologic und Palaeontologie; Stuttgart. 1931 Abt B 655–656.
Peyer B 1937. Die Triasfauna der Tessiner Kalkalpen XII. Macrocnemus bassanii Nopcsa. Abhandlung der Schweizerische Palaontologische Geologischen Gesellschaft pp. 1-140.
Renesto S and Avanzini M 2002. Skin remains in a juvenile Macrocnemus bassanii Nopsca (Reptilia, Prolacertiformes) from the Middle Triassic of Northern Italy. Jahrbuch Geologie und Paläontologie, Abhandlung 224(1):31-48.
Romer AS 1970. Unorthodoxies in Reptilian Phylogeny. Evolution 25:103-112.

wiki/Macrocnemus

This is the image (in Rieppel 1992) that led me astray in 1998.

After I went to visit the holotype of Cosesaurus (Fig. 1) in Barcelona in 1998 (the Monica Lewinsky scandal was going on at the time), I stopped in to visit the original author, Paul Ellenberger, who we discussed earlier here, here and here.  At the time I was naive, inexperienced and untutored, but I did notice the obvious strut emerging anteriorly from the anteriorly expanded ilium (Fig.1). I recognized that was an odd structure. I had never seen anything like it before on other fossils. And nothing like it since.

Cosesaurus prepubis

Figure 1. Cosesaurus prepubis in situ and reconstructed. The strut anterior to the ilium is one of the most distinct features here.  Here is is now interpreted as the stem of the nascent and displaced prepubis.

Confirmation
Ellenberger (1993) noticed the strut as well and also considered it an anterior process of the anterior ilium.

Further Confirmation
Then I noticed Rieppel (1992) described something similar in the T4822 specimen of Macrocnemus and illustrated it (Fig. 2). So, to my mind there was a sister taxon with a similar structure and that made it ‘valid.’ So I accepted and echoed that interpretation in Cosesaurus (Peters 2000). Boy, was I wrong!

Figure 1. Macrocnemus pelvis, specimens identified. Cocked to the right is the Rieppel (1992) interpretation that caused me to think there actually could be an anterior process on the ilium. The process is actually the ventral process, where the pubis connects.

Figure 2. Macrocnemus pelvis. Cocked to the right is the Rieppel (1992) interpretation that caused me to think there actually could be an anterior process on the ilium. The process is actually the ventral process, where the pubis connects. I misinterpreted his pre-actabular process as an anterior process of the ilium blade because Rieppel did not illustrate the actual acetabulum, but he did illustrate the indentation below the reinforcing lateral diagonal ridge also seen in the IVPP specimen (green arrows).

I have regretted it ever since
because that strut in Cosesaurus turned out to be the stem of the Cosesaurus prepubis (Fig. 1), a trait I overlooked in the long line of pterosaurian traits that were otherwise present there.

So I was curious
What did Rieppel (1992) actually see? Here (Fig. 2) I finally found out helped by a more recently discovered and described Macrocnemus, the IVPP V15001 specimen, in which the left pelvis is laid out perfectly in lateral view (Fig. 2) to show exactly what a Macrocnemus pelvis should look like, as opposed to the T 4822 specimen in which the ischium is largely concealed. The IVPP specimen shows the ilium includes a diagonal ridge leading toward the pubis (preacetabulur) process. Rieppel (1992) saw the same ridge in T 4822, but did not illustrate the acetabulum or the other pelvic elements. I have always found that illustration of his confusing, but now I understand it. That’s a relief.

So this is how we fix things in Science.
Correcting mistakes is what we do. I hope to publish this corrected data someday. Currently a manuscript is under review.

References
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
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.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Rieppel O 1992. The hind limb of Macrocnemus bassanii (Nopcsa) (Reptilia, Diapsida): development and functional anatomy.