Where is the Ulna on Drepanosaurus?

Drepanosaurus.

Figure 1. The unusual lepidosaur, Drepanosaurus with forelimb and elbow elements re-identfied. The carpals were poorly ossified.

Drepanosaurus unguicaudatus (Pinna 1980, 1986) Norian, Late Triassic ~210 mya was originally considered an unusual lizard. It had a fused astragalus/calcaneum and sprawling limbs.

Drepanosaurus was the first, and one of the most unusual, of all the drepanosaurs, those hook-tailed, bird-headed, arboreal, chamaeleon-like reptiles of the Triassic.

Renesto’s Reinvestigation of Pinna’s Misidentifications
In 1994 Dr. Silvio Renesto reexamined the skeleton of Drepanosaurus and clarified certain earlier errors (Fig. 2). Those plate-like bones at the elbows were originally identified as coracoids by Pinna — because they looked like coracoids. Renesto (1994) tentatively considered them ulnae.

The forelimbs of Drepanosaurus.

Figure 2. The forelimbs of Drepanosaurus. Left: According to Pinna (1986). Right: Re-identified by Renesto (1994).

Pinna (1986) considered the ulna-like bones the ulna + radius. Renesto (1994) considered them the ulnare + intermedium, essentially wrist bones replacing the ulna.

Pinna (1986) considered the medial forearm bone the scapula. Renesto (1994) identified it as the radius.

Pinna (1986) considered the tall narrow bone the interclavicle. Renesto (1994) identified it as the scapula. Pinna (1986) considered the bone between the humeri a clavicle). Renesto (1994) reidentified it as a coracoid.

Renesto (1994) correctly identified many of the strange bones of Drepanosaurus, but the result created a most unusual three-part (rather than two-part) forearm in which the tubular ulna became a plate-like disc at the elbow and the tiny disc-like ulnare became elongated and tube-like. Very unusual, but this identification was widely accepted.

The fore limb of Hypuronector

Figure 3. The fore limb of Hypuronector (from Colbert and Olsen 2001). Here the humerus is much more robust than the ulna and radius. Around the elbow there are a number of ossified elements and breaks, so the positive identification of the separate olecranon ossification, as found in sister taxa, is more difficult to ascertain.

The Evidence from Sister Taxa
Curious about the homologies of the large plate-like “elbow” bone, I looked at sister taxa recovered by the large reptile tree (Vallesaurus, Huehuecuetzpalli and Cosesaurus) to see what clues they might offer. Notably, all had an olecranon sesamoid, a distinct and separate elbow bone (Fig. 3) that typically would have been fused to the ulna, as in Sphenodon (Fig. 3).

Thus, if homologous, the bone identified as the “coracoid” by Pinna (1986) and the tentative “ulna” by Renesto (1994) was actually a greatly enlarged olecranon sesamoid that articulated with the humerus, radius and ulna. In turn, that makes the tube-like “ulnare + intermedium” tentatively identified by Renesto (1994) the ulna, located parallel to the radius as in all other tetrapods. The actual ulnare + intermedium is a small wrist bone, essentially the only bones that were ossified in the wrist.

megalancosaur elbows and wrists

Figure 3. Click to enlarge. Most sister taxa of Drepanosaurus had an olecranon sesamoid. Drepanosaurus simply had a larger one. See the Megalancosaurus olecranon below.

So what looks like the ulna is the ulna. What looks like the wrist bones are wrist bones. The big elbow bone is an elbow bone (the olecranon sesamoid). All that makes more sense, yet takes away none of the wonder from this incredible arboreal reptile.

The huge olecranon sesamoid anchored a huge muscle to drive digit 2. The ulna was “dished out” to make more room for this forearm/finger muscle complex.

The elbow of Megalancosaurus.

Figure 5. The elbow of Megalancosaurus. (UPDATED BELOW) The perfect alignment of the olecranon sesamoid with the ulna masked the separation of these two bones. Note the ulna no longer articulates with the humerus as in Drepanosaurus. Here the DGS (digital graphic segregation using Photoshop) method uncovered an overlooked trait. Personal communication from S. Renesto identifies this intriguing break as a taphonomic artifact. More on this later as the details emerge!

An Olecranon Sesamoid in Megalancosaurus
The olecranon bone was overlooked in Megalancosaurus, probably due to its perfect alignment with the ulna. Larger than in outgroup taxa, the olecranon bone separated the humerus from the ulna as in its sister taxon, Drepanosaurus.

Megalacosaurus elbow

Figure 6. The break and the broken pieces of the Megalancosaurus ulna are reidentified here. The sesamoid is prominent and crescent-shaped as in Drepanosaurus. Note that the broken part of the ulna would have stood straight up from the matrix if similar to that of Drepanosaurus, hence its destruction during crushing.

A New Interpretation of the Sesamoid in Megalancosaurus
Here the various broken pieces of the ulna are reidentified using DGS (digital graphic segregation). The results are more similar to the situation in Drepanosaurus.

A Lepidosaur?
Renesto (1994) considered the taxonomic assignment of Drepanosaurus “quite difficult,” and labeled it a Neodiapsid (all diapsids other than Araeoscelidae under the old paradigm). “Neodiapsida” is here considered a diphyletic taxon since lepidosaurs and archosaurs nest on separate reptile branches. Therefore this clade label has lost its utility.

An Atypical Tritosaur with a Fused Ankle
As Pinna (1980) surmised, Drepanosaurus indeed nested with the lepidosaurs, but it did not nest with either the Iguania or the Scleroglossa. Here Drepanosaurus nested within the Tritosauria, a third clade of squamates. And yes, the fusion of the astragalus and calcaneum came about by convergence with other members of the Lepidosauria.

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
Colbert EH and Olsen PE 2001. A New and Unusual Aquatic Reptile from the Lockatong Formation of New Jersey (Late Triassic, Newark Supergroup) American Museum Novitates, 3334: 15pp.
Olsen PE 1979. A new aquatic eosuchian from the Newark Supergroup LateTriassic-Early Jurassic) of North Carolina and Virginia. Postilla 176: 1-14.
Pinna G 1980. Drepanosaurus unguicaudatus, nuovo genere e nuova specie di Lepidosauro del trias alpino. atti Soc. It. Sc.Nat. 121:181-192.
Pinna G 1986. On Drepanosaurus unguicaudatus, an upper Triassic lepidosaurian from the Italian Alps. Journal of Paleontology 50(5):1127-1132.
Renesto S 1994. The shoulder girdle and anterior limb of Drepanosaurus unguicaudatus(Reptilia, Neodiapsida) from the upper Triassic (Norian of Northern Italy. Zoological Journal of the Linnean Society 111(3):247-264

wiki/Drepanosaurus
wiki/Hypuronector

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