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Diapsida : Archosauromorpha : Prolacertiformes

Drepanosauromorpha
The "monkey lizards"

Imagine an animal the size of a cat, with a long, heavy body like an iguana, grasping hands and feet like a chameleon, a bony ridge over its shoulders, a thin neck and a small, pointed head, like a plucked chicken, and a long prehensile tail, like a spider monkey—only with a claw on the end. With features seemingly drawn at random from evolution's spare parts box, drepanosaurs look more invented than real. But these tree-climbing insectivores actually existed in the Late Triassic, and although their fossils are rare, they show that the group was widespread in the northern supercontinent of Laurasia.

Drepanosaurs are still relatively new to science. The first specimens were described in 1980, but these strange animals were not recognized as a distinct group until 1992. There are currently five known drepanosaurs:

Drepanosaur Described From

Drepanosaurus

Megalancosaurus

Dolabrosaurus

Hypuronector

Vallesaurus

1980

1980

1992

2001

2010

Italy

Italy

New Mexico

New Jersey

Italy

Megalancosaurus

A restoration of Megalancosaurus illustrates the prehensile tail and birdlike head.
 
In addition, fragmentary drepanosaur remains were described from another site in New Mexico in 2002 and from Great Britain in 2003. In general, small arboreal animals do not produce sturdy fossils. It is not surprising that so few drepanosaurs have been found; if anything, it is surprising that so many have been found in such a short span of time.

In general, drepanosaurs are characterized by:

  • a lightly built, triangular skull that superficially resembles those of birds
  • a long, gracile neck
  • a heavy body, often with vertebrae fused into a ridge over the shoulders
  • chameleon-like limbs with grasping hands and feet
  • long, tall, narrow tails. In the most derived forms, the tail-tip is modified into a claw, and the whole tail was probably prehensile, like the tails of New World monkeys.

Drepanosaurus itself is unusual in having a huge claw on the index finger of each hand. The skull of Drepanosaurus is unknown, but if it was built like that of Megalancosaurus, then each hand claw was probably about the same size as the animal's entire head! Hypuronector differs from other drepanosaurs in having an extremely deep, narrow tail that was almost certainly not prehensile. All of the drepanosaurs are small, less than 50 cm long; and at a mere 18 cm (~7 inches), Hypuronector could sit in the palm of your hand. Skull material has only been described for Megalancosaurus, Hypuronector and Vallesaurus. Megalancosaurus has very small teeth in its otherwise birdlike jaws. Only the lower jaw is known from Hypuronector. It had large teeth toward the back of its mouth, but the front half of the jaw is modified into a pointed, toothless beak.

Drepanosaurus

A skeletal restoration of Drepanosaurus (the skull has not been found) clearly illustrates the oversize claw on the hand.

Systematics
From the beginning, drepanosaurs have been a challenge to classify. Most specimens are incomplete and flattened into two dimensions, and their anatomy is so bizarre that paleontologists sometimes disagree about which bones are which. The first problem was recognizing that drepanosaurs represent a distinct clade of organisms. As more drepanosaurs were discovered, it became easier to recognize the similarities between them. The most striking similarities are in the tall, narrow tails and unusual vertebrae. Two names have been proposed for the clade that unites all known drepanosaurs: Simiosauria, the "monkey lizards," and Drepanosauromorpha.

Vallesaurus

This reconstruction pictures Vallesaurus as an arboreal insectivore.
 
The second problem has been to determine what sorts of animals drepanosaurs are: where do they fit in the Tree of Life? For most of their history, the various drepanosaurs were thought to belong to Lepidosauromorpha, the diapsid group that includes Sphenodon, lizards and snakes. However, Megalancosaurus has often been regarded as an archosaur, mainly on the basis of skull characters, and one phylogenetic analysis suggested that it belonged to the Prolacertiformes, a group of superficially lizard-like reptiles that are themselves related to archosaurs. In the most recent and most complete analysis of the group, drepanosaurs were found to be Prolacertiformes.

Ecology
The life habits of drepanosaurs have been almost as contentious as their relationships. Based on the huge hand claws, Drepanosaurus was originally characterized as a digger. All drepanosaurs have fairly deep tails, and the tail of Hypuronector is especially paddle-like, so they have often been thought to be semiaquatic swimmers. However, the grasping feet and claw-tipped tails are much more similar to those of arboreal animals. Hypuronector's deep tail was probably not very flexible and would have made a poor paddle, and its long limbs are very different from those of animals that are specialized for a life in water. Drepanosaurs probably lived in trees and ate insects. Drepanosaurus may have used its big claws to rip open bark to find grubs and beetles. The deep tail of tiny Hypuronector is roughly leaf-shaped, and may have been used for camouflage.

Dr. Silvio Renesto has done more work on more drepanosaurs than anyone else. You can find photos and drawings of the Italian taxa at his webpage. The Hairy Museum of Natural History has a cool page on drepanosaurs, with skeletal diagrams, life restorations, and links to other resources.

Sources

  • Colbert, E.H. and P.E. Olsen. 2001. A new and unusual aquatic reptile from the Lockatong Formation of New Jersey (Late Triassic, Newark Supergroup). American Museum Novitates 3324:1-24.
  • Feduccia, A., and R. Wild. 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Naturwissenschaften 80:564-566.
  • Harris, J.D., and A. Downs. 2002. A drepanosaurid pectoral girdle from the Ghost Ranch (Whitaker) Coelophysis Quarry (Chinle Group, Rock Point Formation, Rhaetian), New Mexico. Journal of Vertebrate Paleontology 22:70-75.
  • 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 111:247-264.
  • Renesto, S., and N.C. Fraser. 2003. Drepanosaurid (Reptilia: Diapsida) remains from a Late Triassic fissure infilling at Cromhall Quarry (Avon: Great Britain). Journal of Vertebrate Paleontology 23:703-705.
  • Renesto, S., J.A. Spielmann, S.G. Lucas, and G.T. Spagnoli. 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.
  • Senter, P. 2004. Phylogeny of Drepanosauridae. Journal of Systematic Palaeontology 2:257-268.

Text by Matt Wedel, 5/2007. Megalancosaurus, Drepanosaurus and Vallesaurus illustrations courtesy of Silvio Renesto, University of Insubria, Italy.