Extreme Modification of the Tetrapod Forelimb in a Triassic Diapsid Reptile.
The tetrapod forelimb is one of the most versatile structures in vertebrate evolution, having been co-opted 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.
Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794, USA. Electronic address: adam.pritchard@yale.edu.
Adam C
AC
Pritchard
Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794, USA.
Alan H
AH
Turner
Natural History Museum of Utah, 301 Wakara Way, Salt Lake City, UT 84108-1214, USA; Department of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112-0102, USA.
Randall B
RB
Irmis
Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
Sterling J
SJ
Nesbitt
The Dinosaur Institute, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA.
Nathan D
ND
Smith
D000818:Animals; D005075:Biological Evolution; D005552:Forelimb / Q000033:anatomy & histology*; D005580:Fossils / Q000033:anatomy & histology*; D009516:New Mexico; D010802:Phylogeny; D012104:Reptiles / Q000033:anatomy & histology* / Q000145:classification
Diapsida; Drepanosauridae; Triassic; anatomy; evolution; functional morphology