Friday, September 30, 2016

[Paleontology • 2016] Extreme Modification of the Tetrapod Forelimb in a Triassic Diapsid Reptile, Drepanosaurus unguicaudatus

In this illustration set 212 million years ago in what is today New Mexico, a Drepanosaurus rips away tree bark with its massive claw and powerful arm.
Painting by Victor Leshyk  @VictorLeshyk 

- Three-dimensional fossils of the Triassic diapsid Drepanosaurus are described
- Fossils support hypothesis that the forelimb is unique among tetrapods
- The radius and ulna are unequal in length, and two carpals are longer than the radius
- Forelimb range of motion and large claws suggest specialized hook-and-pull digging

In Brief
Pritchard et al. describe new fossils of the Triassic diapsid Drepanosaurus. Previously known from a single crushed skeleton, the new three-dimensional fossils show a unique forelimb with asymmetric radii and ulnae and elongate carpals. The forelimb and enlarged second manual ungual suggest a digging similar to modern anteaters.

The tetrapod forelimb is one of the most versatile structures in vertebrate evolution, having been coopted for an enormous array of functions. However, the structural relationships between the bones of the forelimb have remained largely unchanged throughout the 375 million year history of Tetrapoda, with a radius and ulna made up of elongate, paralleling shafts contacting a series of shorter carpal bones. These features are consistent across nearly all known tetrapods, suggesting that the morphospace encompassed by these taxa is limited by some sort of constraint(s). Here, we report on a series of three-dimensionally preserved fossils of the small-bodied (<1 m) Late Triassic diapsid reptile Drepanosaurus, from the Chinle Formation of New Mexico, USA, which dramatically diverge from this pattern. Along with the crushed type specimen from Italy, these specimens have a flattened, crescent-shaped ulna with a long axis perpendicular to that of the radius and hyperelongate, shaft-like carpal bones contacting the ulna that are proximodistally longer than the radius. The second digit supports a massive, hooked claw. This condition has similarities to living ‘‘hook-and-pull’’ digging mammals and demonstrates that specialized, modern ecological roles had developed during the Triassic Period, over 200 million years ago. The forelimb bones in Drepanosaurus represent previously unknown morphologies for a tetrapod and, thus, a dramatic expansion of known tetrapod forelimb morphospace.

Adam C. Pritchard, Alan H. Turner, Randall B. Irmis, Sterling J. Nesbitt and Nathan D. Smith. 2016. Extreme Modification of the Tetrapod Forelimb in a Triassic Diapsid Reptile.  Current Biology. 

212-Million-Year-Old Reptile Had Anteater-Like Arms | Popular Science via @PopSci

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