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.
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.
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.
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.
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.
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.
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.