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).

Figure 1. Tropidosuchus and Lagerpeton compared to the new material (MCZ 101542).

Figure 1. Tropidosuchus and Lagerpeton compared to the new material (MCZ 101542).

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.

Figure 2. MCZ 101542 scapulocoracoid and humerus compared to Dromomeron humerus.

Figure 2. MCZ 101542 scapulocoracoid and humerus compared to Dromomeron humerus.

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.

Figure 2. MCZ 101542 compared to Marasuchus and Lagosuchus.

Figure 2. MCZ 101542 compared to Marasuchus and Lagosuchus.

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.

Figure 3. Ixalerpeton compared to MCZ 101542.

Figure 3. Ixalerpeton compared to MCZ 101542.

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.

Figure 4. Procompsognathus has proportions that precisely fit the MCZ 101542 material.

Figure 4. Procompsognathus has proportions that precisely fit the MCZ 101542 material.

 

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. The authors did not mention Procompsognathus. So taxon exclusion continues to be a problem here. If inappropriate, at least it should have been considered and eliminated.

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

Longisquama Pectoral Girdle and Sternal Complex Documented

In my efforts to help explain the relationship of the lepidosaur tritosaur fenestrasaur, Longisquama (Sharov 1970) to basal pterosaurs I present a pair of images (Figs. 1, 2) designed to illustrate just where the pectoral elements are. Admittedly these elements are difficult to pick out at first glance. So these are for those naysayers who often say, “I can’t see what you’re seeing!!” Here’s why:

Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

Figure 1. Click to enlarge. Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

The trouble
with seeing the bones of Longisquama is they are largely covered with scales and skin. That’s just a fact we need to embrace. Longisquama has more dermal and extra dermal tissue preserved than most other Mesozoic fossils. In such cases DGS (digital graphic segregation) really becomes a powerful tool, finding bones beneath the skin.

 

Figure 2. Click to enlarge. Longisquama ghosted out with DGS, other than the sternal complex (interclavicle+clavicles + sternum), scapula and coracoid.

Figure 2. Click to enlarge. Longisquama ghosted out with DGS, other than the sternal complex (interclavicle+clavicles + sternum), scapula and coracoid.

Here the affinities with pterosaurs are quite clear
Everyone has seen the horse-shoe-shaped clavicles, but few have recognized that they overlap medially and frame the sternum AND are surmounted by keeled interclavicle with crossbars that accommodate the quadrant-shaped ventral coracoids. (Whew!) This is the standard morphology of the classic pterosaur sternal complex (Wild 1993). Even the overall shape is identical to that in the basal pterosaur, MPUM6009 (Fig. 3). Only the fenestrasaurs, Cosesaurus, Sharovipteryx and pterosaurs share all these traits. In most derived pterosaurs the stem of the coracoid does indeed straighten out more, if you’ve noticed the difference.

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

Figure 3. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex. The quadrant-shaped coracoid is the remnant and result of the anterior fenestra of the coracoid enlarging to the posterior rim, which is the only part that remains.

Still finding it difficult to spot some of these elements?
Then I encourage you to more closely examine the specimen itself or high resolution photos and become acquainted with sister taxa that will prepare you for what you should be looking for. I know that sounds self-fulfilling, but if you know that most tetrapods have four limbs you don’t stop at three. It’s the same thing at this point. Also see this page on ReptileEvolution.com for the same images of Longisquama (Figs. 1, 2) presented on a mouseover.

Flapping
Earlier we discussed the evolution of the sternal complex and pectoral girdle in fenestrasaurs (Fig. 3) and its role in flapping on a bipedal frame. That’s why these elements are so distinctly shaped, approaching the morphology of birds by convergence. The coracoid no longer slides against the interclavicle and sternum as in more primitive quadrupedal lizards.

Front Half Only
Even if only the front half of Longisquama was known (Sharov, 1970; Fig. 1), this would still be enough to cement Longisquama to pterosaurs. Adding the back half and fingers just puts frosting on the cake.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Sharov AG 1970. A peculiar reptile from the lower Triassic of Fergana. Paleontologiceskij Zurnal (1): 127–130.
Wild R 1993. A juvenile specimen of Eudimorphodon ranzii Zambelli (Reptilia, Pterosauria) from the upper Triassic (Norian) of Bergamo. Rivisita Museo Civico di Scienze Naturali “E. Caffi” Bergamo 16: 95-120.

wiki/Longisquama