A small, bipedal Macrocnemus: PIMUZ T4823

It’s a bipedal, but folded specimen
with a skull and neck resting against its own spine (like Langobardisaurus).

The PMUZ T4823 specimen
of Macrocnemus (Peyer 1937; Figs. 1, 2) had such short forelimbs that it foreshadowed one of its more famous fully bipedal relatives, Sharovipteryx (Fig. 3). I even wondered if they were somehow sisters, but the LRT said, ‘no’, they were only convergent.

Figure 1. 'Macrocnemus' specimen PIMUZ T4832 in situ. Having the skull and neck bent back against the spine makes this a good problem for DGS to attempt.

Figure 1. ‘Macrocnemus’ specimen PIMUZ T4832 in situ. Having the skull and neck bent back against the spine makes this a good problem for DGS to attempt. Even colorized, this specimen still needs to be unfolded to make proper sense of its morphology and proportions. Photo from Saller 2016.

This specimen is hard to figure out
without unfolding that long neck (Fig. 2). When that happens, using DGS methods, the T4823 specimen starts to make sense. If you’re like me, sometimes the brain just needs to see things in vivo, not as if it was tucked into an eggshell.

Figure 2. 'Macrocnemus' specimen PIMUZ T4832 lifted from the in situ figure 1 at right, and unfolded at left. Not everything is guaranteed correct here, but it's pretty close. At 72 dpi screen resolution, this image is full scale.

Figure 2. ‘Macrocnemus’ specimen PIMUZ T4832 lifted from the in situ figure 1 at right, and unfolded at left. The skull is shown in situ and reconstructed. Not everything is guaranteed correct here,  The lower pelvis is a big guess because the elements may be tucked under the sacrum. At 72 dpi screen resolution, this image is full scale.

The anterior dorsal ribs of the T4832 specimen were also extra long,
perhaps creating a wide, aerodynamic, pancake-like torso, again, as in Sharovipteryx (Fig. 3) or Draco.

Note the five sacrals that helped support this sprawling lepidosaur (according to the LRT) while bipedal.

The pectoral girdle is tiny with small, disc-like coracoids. Thus, the T4832 specimen was not flapping, like Sharovipteryx (Fig. 3).

There was a soft tissue rostral crest. Soft tissue is impressed everywhere else, too.

Like Sharovipteryx, a pair of large hyoids extend neck skin, creating an aerodynamic strake or throat sac.

That is a very slender set of cervicals for such a large skull. Perhaps most of the bone was preserved below the surface. Remember, this is a cast of the destroyed original. In any case, this was a gracile specimen. If like all other Macrocnemus specimens, it had hollow bones, too.

Figure 2. Cosesaurus was experimenting with a bipedal configuration according to matching Rotodactylus tracks and a coracoid shape similar to those of flapping tetrapods. Long-legged Sharovipteryx was fully committed to a bipedal configuration.

Figure 3. Cosesaurus was experimenting with a bipedal configuration according to matching Rotodactylus tracks and a coracoid shape similar to those of flapping tetrapods. Long-legged Sharovipteryx was fully committed to a bipedal configuration.

This is not the first time
someone has suggested that Macrocnemus was facultatively bipedal. Nopcsa 1931 and Rieppel 1989 thought so, too.

This is not the first time
that a member of the Macrocnemus family became bipedal (Fig. 3). Actually most of the descendants of Macrocnemus were bipedal, whether on land or in the water.

Figure 5. Subset of the LRT focusing on the Tritosauria. Note the separation of one specimen attributed to Macrocnemus.

Figure 5. Subset of the LRT focusing on the Tritosauria. Note the separation of one specimen attributed to Macrocnemus.

Saller writes (translated by Google form Italian):
PIMUZ T4823: cast of the holotype, originally kept at the Civic Museum of Natural History of Milan (Museum Civico de Storia Naturale in Milano) was destroyed during the Second World War. Exemplary in a bad state of  conservation, described by Peyer (Peyer, 1937). Includes skull, neck, trunk, parts of the limbs and the front portion of the tail.”

Rieppel (1989) writes: 
T2473: Specimen “Besano III” (Peyer, 1937). The specimen was collected in the “Sciti bituminous” of Besano and turned over to the Museum Civico de Storia Naturale in Milano after its description by Peyer (1937),, where it was destroyed during World War II. A cast of the specimen is preserved in Zurich. The specimen is fragmentary, but includes a well-preserved hind limb.”

A renumbered specimen?
Rieppel (1989) makes no mention of PIMUZ T 4822, T4823, T4833 or T4834, but his description of the well-known specimen, A III/208. is listed first and matches this description, so it is likely renumbered in Saller 2016,

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.
Rieppel, O 1989. The Hind Limb of Macrocnemus bassanii (Nopcsa) (Reptilia, Diapsida): Deverlopment and Functional Anatomy. Journal of Vertebrate Paleontology. 9 (4): 373–387.
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.

PIMUZ – Palaeontologisches Institut und Museum, University of Zuerich, Zurigo, Switzerland.

2 thoughts on “A small, bipedal Macrocnemus: PIMUZ T4823

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