Scapulocoracoid and humerus ‘assigned’ to Lagerpeton might belong to Procompsognathus

McCabe and Nesbitt 2021
assigned a disarticulated Late Triassic scapulocoracoid and humerus (MCZ 101542) to Lagerpeton (Fig. 1) in the absence of any pervious similar bones for the Lagerpeton holotype.

Gutsy.
Workers have been trying to rebuild a chimaera of Lagerpeton from disassociated parts for several years now, hoping it will somehow shed some insight into dinosaur and pterosaur origins.

This is all for naught because Lagerpeton is a bipedal chanaresuchid that ran on two toes, not an archosaur (dinosaurs + crocs) or fenestrasaur (pterosaurs and their ancestors).

How can McCabe and Nesbitt assign that pectoral girdle?
The holotype of Lagerpeton lacks any pectoral girdle material. So we can only imagine missing elements based on phylogenetic bracketing and comparative anatomy.

Given that,
does the MCZ 101542 material closely resemble comparable bones in closely related taxa? In the large reptile tree (LRT, 1810+ taxa) Lagosuchus nests with Tropidosuchus (Fig. 1), not with dinosaurs or pterosaurs.

A problem arises.
Tropidosuchus (Fig.1) has a larger, hourglass-shaped scapula with a short ‘waist’. By contrast the MCZ 101542 scapula (Fig. 1) has a smaller, straighter, narrower, more rectangular shape. So, maybe we should look for a better match… if there is one.

Is material from another taxon a little more similar?
Marasuchus (Fig. 2; PVL 3871) has a more robust, but otherwise similarly straight scapulocoracoid with a dinosaurian deltopectoral crest located about a third the way down the slender humerus, and more similar in scale. Lagosuchus (Fig. 2; UPLR 090) has a similarly gracile scapulocoracoid (at least what’s left of it). It’s all iffy.

McCabe and Nesbitt also make comparisons
when they note, “Compared to Lagosuchus talampayensis (PVL 3871), the scapular blade of MCZ 101542 is much more strap-like (near parallel anterior and posterior side) and the distal end expands more in Lagosuchus talampayensis.” 

Their table 2 lists ‘species’ Marasuchus‘ with specimen number PVL 3871. So their Marasuchus (PVL 3871) is not Lagosuchus (UPLR 090; Fig. 2).

McCabe and Nesbitt also write
“The glenoid of MCZ 101542 is directed posteroventrally like that of other avemetatarsalians (e.g., lagerpetids, Lagosuchus talampayensis, silesaurids, dinosaurs).”

In the LRT Avemetatarsalia is a junior synonym for Reptilia because it also include pterosaurs. Lagerpetids are proterochampsids, not dinosaur relatives. And, once again the authors’ Table 2 does not match their text with regard to nomenclature and specimen numbers.

The protorosaur, Ixalerpeton, 
(Fig. 3) is similar in size to MCZ 101542, but the shapes are slightly different.

The authors note,
“Within Lagerpetidae, the humerus of Ixalerpeton polesinensis (ULBRA-PVT059) is more robust than MCZ 1010541 (Fig. 4), with proportionally much larger proximal and distal expansions. The proportions of the humerus of Lagosuchus talampayensis (PVL 3871) matches that of MCZ 101541, with overall weakly expanded articular ends.”

Would you like to see a ‘Hail Mary’ pass based on taxon exclusion?
The authors report, “Overall, the gracile proportions of MCZ 101541 (= MCZ 101542 = the humerus) are unlike early archosaurs and their close relatives.”

When workers give up like this,
it’s usually due to taxon exclusion, whether intentional or not.

In this case there is a close match for the gracile proportions
of MCZ 101542 and it comes from a taxon that happens to be missing the scapulocoracoid and humerus, the Late Triassic theropod from Germany, Procompsognathus (Fig. 4), a taller relative of Marasuchus in the LRT. Like a lock and a key, a Yin and a Yang, the MCZ material is a perfect fit including the narrow, but deep anterior torso required to fit the narrow but deep scapula and coracoid. Still, this is only a gutsy guess. See how reconstructions can help?

The LRT uses more complete taxa
whenever possible. To assign two bones to a specific genus is getting close to “Pulling a Larry Martin.” Be careful when you go there. It’s worth a shot (Fig. 4), but it’s easy to be wrong.

References
McCabe MB and Nesbitt SJ 2021. The first pectoral and forelimb material assigned to the lagerpetid Lagerpeton chanarensis (Archosauria: Dinosauromorpha) from the upper portion of the Chañares Formation, Late Triassic. Palaeodiversity, 14(1) : 121-131.

wiki/Procompsognathus