The Antorbital Fenestra of Drepanosaurs

The drepanosaurs are a rag-tag clade of slow-moving arboreal tritosaur lizards derived from Jesairosaurus in the large reptile tree. Drepanosaurs include Hypuronector, Vallesaurus, Megalancosaurus and Drepanosaurus in order of greater derivation.

The skull of Vallesaurus in situ.

Figure 1. The skull of Vallesaurus in situ. Difficult to interpret, to be sure, but I don’t see any large maxillary plates here. Perhaps palatal elements, though.

There has been some debate as to whether or not drepanosaurs had an antorbital fenestra. If so, this would have been the fifth (or perhaps the fourth since this clade is very close to Langobardisaurus and Cosesaurus) origin of the antorbital fenestra. Unfortunately only two drepanosaurs include skulls, Vallesaurus and Megalancosaurus. Here we’ll trace and reconstruct both. It’s no easy task.

The skull of Vallesaurus as interpreted by Renesto

Figure 2. The skull of Vallesaurus as interpreted by Renesto (2006). He found no evidence of an antorbital fenestra. The stem-like nasal and lacrimal are both present here. The question is, what, if anything, filled the space between them? Was it folded and flattened palatal bones? Renesto did not note any palatal elements here, yet they must have been present.

The palatal elements of Jesairosaurus,

Figure 3. The palatal elements of Jesairosaurus, the outgroup for the drepanosaurs. Fairly solid-looking. This contrasts with the lighter airier palates of fenestrasaurs and Megalancosaurus.

The palate
When skulls are crushed in lateral view, something has to happen to the transverse palatal elements. Typically they flip up or down, like playing cards, presenting their broadest surface to the matrix plane. It is difficult to discern the shape of the palatal elements in Vallesaurus, but if they were more solid, like the elements in Jesairosaurus, then this may account for the bone that Renesto found behind and through the rostrum (= antorbital fenestra).

In the more derived drepanosaur, Megalancosaurus (fig. 8) these solid elements were reduced to more gracile elements.

Figure 4. Interpretation of the skull elements of Vallesaurus. Note the slender ascending process of the maxilla in green.

Figure 4. Interpretation of the skull (sans palatal) elements of Vallesaurus. Note the slender ascending process of the maxilla in green and the slender lacrimal stem in magenta.

Interpretation of the skull of Vallesaurus based on figure 4.

Figure 5. Interpretation of the skull of Vallesaurus based on figure 4.

Vallesaurus
Talk about difficult skulls to trace (Figs. 1, 4), reconstruct and restore (Fig. 5)!! And remember, if we are looking at a light and airy skull, we’re looking through a light and airy skull to the right side elements obscured by intervening palatal elements. I did not attempt to outline the palatal elements in Vallesaurus, but considering the size of the eyes, the palate had to be nearly as wide as the height of the skull, as in Megalancosaurus.

Both part and counterpart of Megalancosaurus superimposed.

Figure 6. Both part and counterpart of Megalancosaurus superimposed using Photoshop.

Interpretation of figure 6, the skull of Megalancosaurus.

Figure 7. Interpretation of figure 6, the skull of Megalancosaurus. Struts of bone surround antorbital fenestra here. The slender ascending process of the left maxilla is convincing evidence for an antorbital fenestra in Megalancosaurus. The huge size of the naris is further evidence for skull lightening as a selective influence.

Megalancosaurus
The skull of Megalancosaurus was split in two, the part and counterpart (Figs. 6, 7). Reassembling them with software helps to realign the two crushed halves together for interpretation of the elements. What I see there is a slender ascending process of the maxilla. That can only occur if there is an antorbital fenestra. If that bone is another bone, then this will have to be reconsidered. At this point an antorbital fenestra seems likely. And I’m open to new data if anyone out there has some.

Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

Figure 8. Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

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
Calzavara M, Muscio G and Wild R 1980. Megalancosaurus preonensis n. gen. n. sp., a new reptile from the Norian of Friuli. Gortania 2: 59-64.
Feduccia A and Wild R 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Natur Wissenschaften 80:564–566.
Geist NR and Feduccia A 2000. Gravity-defying Behaviors: Identifying Models for Protoaves. American Zoologist 4):664-675. online pdf
Renesto S 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of Northern Italy. Journal of Vertebrate Paleontology 14(1):38-52.
Renesto S 2000. Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–179.
Renesto S and Binelli G 2006. ’Vallesaurus Cenensis“’ Wild, 1991, A Drepanosurid (Reptilia, Diapsida): From the Late Triassic of Northern Italy”, Rivista Italiana di Paleontologia e Stratigrafia 112: 77–94, Milano.
Wild R 1990. Ein Flugsaurierrest (Reptilia, Pterosauria) aus der Unterkreide (Hauterive) von Hannover (Niedersachsen). Neues Jahrbuch für Geologie und Paläontologie. Abhandlung 181,241–254.

wiki/Vallesaurus
wiki/Megalancosaurus

3 thoughts on “The Antorbital Fenestra of Drepanosaurs

  1. Brief interjection here: thanks to supplemental and very detailed photos of the material of virtually all megalancosaurid specimens known to date, including the holotype of Megalancosaurus preonensis but especially Renesto’s analysis of the skull, it is obvious, if not blisteringly so, that the skull is not preserved in any even split down the side, but irregularly through the bones, especially one sides at once (indicating severe flattening of the mateerial during transformation to dolomite). Both maxillae are thus preserved on both main and counter slabs, but due to skew are misaligned, so that one is slightly dorsal to the other, and is shown in your reproduction of the photo as the “ascending process” of the self-same maxilla. It is, however, the OTHER maxilla and not a feature of just one element. This problem with the specimen is not only pervasive in the specimen, it is pervasive in other dolomitic specimens from Preone, including most other megalancosaurid skeletons. This makes identification of the cranial features difficult (Renesto, 2000). A solution to bone identification is made more difficult by understanding that both halves of each maxilla are present on each side of the slab, as to obscure “simple” explanations. I suggest taking the “casual” appearance of the material with a hefty shaker of salt. More complicated, but more accurate.

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