Drepanosaurus

Drepanosaurus unguicaudatus was a small reptile from the Late Triassic, with a grasping tail hook and one of the most unusual forelimbs known among land vertebrates. A large curved claw sat on the second finger, while the ulna and wrist were reshaped around a powerful pulling motion. Three-dimensional bones from New Mexico clarified anatomy that had been flattened in the Italian type specimen. The profile belongs in the other fossil reptile catalogue, among extinct branches that are not dinosaurs.

The fossils support a specialised animal, probably adapted for climbing and opening decaying wood. They do not preserve a meal, a track of claw marks attributable to the species, or the soft tissues that would show exactly how the hand was used. A reconstruction can show a plausible action, but the motion remains an interpretation rather than a recorded behaviour.

Quick facts

The Italian holotype

Giovanni Pinna named Drepanosaurus unguicaudatus in 1980 from the associated skeleton MCSNB 5728. It came from the Calcari di Zorzino in Lombardy, northern Italy, and is preserved flattened in limestone. The fossil includes much of the body, among it the long neck, deep trunk, limbs and unusual tail tip. Its name refers to the sickle-like shape of that tail claw.

The skull is less completely preserved than the rest of the type. Full-body restorations therefore reconstruct parts of the head and other missing details by comparison with related drepanosauromorphs. A lizard-like or bird-like face in an illustration is not a direct portrait of MCSNB 5728.

Three-dimensional evidence from the Chinle Formation

The flattened Italian skeleton made it difficult to determine how the forelimb joints fitted together. Several three-dimensional bones from the Late Triassic Chinle Formation in New Mexico were assigned to Drepanosaurus and studied in 2016. They revealed an anatomy unlike that of other known four-limbed vertebrates.

The ulna was broad and crescent-shaped, with its long axis turned almost across the forearm. Two wrist bones were unusually elongated, and the second finger ended in a massive curved claw. Because similar features occur in more than one three-dimensional element, they are not readily explained as a crushing accident in the Italian slab. The American material makes the construction of the hand clearer, although its assignment and its relationship to the Italian type still depend on anatomical comparison.

How might the hand have worked?

The shape of the claw and the range permitted by the joints fit a hook-and-pull action. One proposed use is to tear into decaying wood and reach hidden invertebrates. Comparisons with living mammals can help describe that mechanical possibility, but they do not make Drepanosaurus an ecological equivalent of an anteater.

Small teeth and the animal's specialised anatomy are compatible with feeding on arthropods. No stomach contents identify particular insects, however, and there are no pieces of wood with unambiguous traces left by this species. The food and the exact sequence of movements remain inferred from anatomy, not directly observed.

Tail, climbing and body size

The tail was prehensile and ended in an enlarged claw-like structure. Together with curved digits, it may have provided extra contact points while the animal moved among branches. The combination is consistent with an arboreal lifestyle, but it does not by itself show how often the animal climbed or whether it spent most of its time above the ground.

Estimates place the animal at less than about a metre long. The available individuals are incomplete, so the maximum adult length is not known precisely. Missing tail and head proportions also make an exact measurement inappropriate. The striking body outline in a complete reconstruction combines preserved anatomy with restored regions.

Two kinds of deposits and a changing classification

The Italian holotype was buried in marine limestone after its carcass was transported from land. The American forelimb bones came from continental Chinle deposits. Different settings of preservation can add complementary anatomical information without implying that Drepanosaurus lived in the sea.

Drepanosauromorphs form an early branch of diapsid reptiles, but their precise position shifts among evolutionary analyses. The Triassic Period saw many specialised reptile lineages evolve side by side. Unlike the differently specialised Rhynchosaurus, which had a beak and tooth plates for plant processing, Drepanosaurus is notable for its grasping hand and hooked tail. It is listed with other extinct reptile branches in the catalogue of other fossil reptiles, a navigation group rather than a single close-knit lineage.