Drepanosaurs! The Strangest of the Strange

Some of the strangest, most Dr. Seussian reptiles of all time were the Late Triassic drepanosauromorphs: Hypuronector, Vallesaurus, Drepanosaurus and Megalancosaurus. Found world wide and described as chameleon-like due to their unique grasping feet, drepanosaurs almost always provide something special to gawk at, including a hooked tail tip. Due to their strangeness, drepanosaurs have been difficult to classify.

Prior Assignments
Although Drepanosaurus was first considered a lepidosaur, drepanosaurs have been difficult to nest because they have rarely been part of a large, comprehensive taxonomic inclusion set.  Prior studies mistakenly nested drepanosaurs with Coelurosauravus and Longisquama (Senter 2004), which are not related to each other. Renesto and Binelli (2004) found that the pterosaur, Eudimorphodon, nested with drepanosaurs, between archosauriforms and Prolacerta + Tanystropheidae (Tanystropheus + Macrocnemus + Langobardisaurus) which is pretty close. Unfortunately, no one attempted to include basal lizards, including Huehuecuetzpalli and Jesairosaurus, in prior studies.

Current Assignment
Here drepanosaurs were lizards and members of the previously overlooked third clade, the Tritosauria with HuehuecuetzpalliMeyasaurus and Jesairosaurus at its base. These lizards did not fuse the astragalus and calcaneum like other lizards did and do.  CosesaurusMacrocnemus and several other strange tritosaurs are sister taxa.

Jesairosaurus
Jesairosaurus
 was the first in this line to develop the higher dorsal spines over the fully erect and fully ossified scapula, perhaps most immediately comparable to that of Meyasaurus. The skull was relatively larger and may have had a slender antorbital fenestra, taller than wide. The neck was relatively longer and quite gracile. One can surmise that this was the first of the slow-moving types due to the lack of robust bones in the neck. The hind foot is unknown but probably did not have the great asymmetry of Huehuecuetzpalli, but the shorter toes of Hypuronector. The tail is also unknown.

Drepanosaurs

Figure 1. Several drepanosaurs and their ancestors, the tritosaurs Huehuecuetzpalli and Jesairosaurus.

Hypuronector
Well preserved, but lacking a skull, Hypuronector (Olsen 1979) was smaller, had a smaller mandible, shorter neck, taller neural spines, much deeper tail chevrons, narrower scapula, a taller pelvis and shorter toes.

Vallesaurus
The most primitive drepanosaur with a curled tail was Vallesaurus  (Wild 1990, Renesto and Binelli 2006). The naris and antorbital fenestra were much larger on a shorter rostrum. The tail chevrons were not so deep and the posterior tail was curled ventrally, likely a prehensile organ able to grasp branches. Pedal digit 1 was greatly enlarged, like a thumb and may have opposed the other four digits while grasping branches. The proximal ankle bones were fused.

Megalancosaurus
Originally considered a possible bird ancestor, when only the skull and neck were known, Megalancosaurus (Calvazara, Muscio and Wild 1980) had a curled tail tipped by a hook. The antorbital fenestra was smaller, the narrow snout enabled binocular vision, the neck was longer, the shoulder hump was larger, the torso and tail were deeper, the scapula was more slender, the ilium leaned anteriorly and the pedal digit 1 was smaller but still robust.

Drepanosaurus
The first drepanosaur to be described was Drepanosaurus (Pinna 1980), which was originally and correctly considered a lizard. Renesto (1994) figured out that the ulnare enlarged, replacing the ulna, and the ulna became a sort of an elbow bone. These changes were related to the hyper-enlargement of manual ungual 2, which could have been used for digging or slicing into bark, perhaps to extricate buried insects. Larger than the other drepanosaurs, headless drepanosaurus also had the shortest neck and deepest torso. The pedal “thumb” was as large as the longest digit, #2.

No drepanosaurs are known to have survived into the Jurassic.

Summary
The drepanosaurs were an arboreal clade of tritosaur lizards adapted to slow-motion, clinging to branches with four short-toed, grasping feet and a prehensile tail. Later forms either lurched out at prey with a cobra-like neck snap and binocular vision, or dug into branches with outsized claws.

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.
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.
Evans SE 1991. A new lizard−like reptile (Diapsida: Lepidosauromorpha) from the Middle Jurassic of Oxfordshire. Zoological Journal of the Linnean Society 103:391-412.
Evans, SE and Barbadillo, LJ 1996. The Early Cretaceous lizards of Montsec (Catalonia, Spain) Treb. Museo de Geol. Barcelona 5:5-13 online pdf
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
Jalil N-E 1997. A new prolacertiform diapsid from the Triassic of North Africa and the interrelationships of the Prolacertiformes. Journal of Vertebrate Paleontology 17(3), 506-525.
Olsen PE 1979. A new aquatic eosuchian from the Newark Supergroup LateTriassic-Early Jurassic) of North Carolina and Virginia. Postilla 176: 1-14.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
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. 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 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.
Renesto S, Spielmann JA, Lucas SG, and Spagnoli GT 2010. The taxonomy and paleobiology of the Late Triassic (Carnian-Norian: Adamanian-Apachean) drepanosaurs (Diapsida: Archosauromorpha: Drepanosauromorpha). New Mexico Museum of Natural History and Science Bulletin. 46:1–81.
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.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.
Senter P 2004. Phylogeny of Drepanosauridae (Reptilia: Diapsida). Journal of Systematic Palaeontology, 2(3): 257-268.
Wild R 1990. Ein Flugsaurierrest (Reptilia, Pterosauria) aus der Unterkreide (Hauterive) von Hannover (Niedersachsen). Neues Jahrbuch für Geologie und Paläontologie. Abhandlung 181241–254.

wiki/Drepanosaur
wiki/Hypuronector
wiki/Megalancosaurus
wiki/Drepanosaurus
wiki/Vallesaurus
Hairy Museum blog on Drepanosaurs

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