The research stems from a decade-long project collecting Late Triassic fossils, including those of Drepanosaurus, from the Hayden Quarry in Ghost Ranch, northern New Mexico. Drepanosaurs are an extinct group of reptiles that although lizard-like are not lizards nor are they dinosaurs. Most were small (only 1 to 2 feet in length) and known to have lived exclusively during the Late Triassic (235 to 201 million years ago). Prior to the team’s discoveries at Ghost Ranch, Drepanosaurus was known from decades-old, badly crushed skeletons from Italy.
In nearly all four-limbed vertebrates, including in diverse species such as dolphins, bats, or birds, the forelimb consists of two elongate and parallel bones—the radius and ulna. These bones contact a set of wrist bones that form the bridge between the forearm and the hand. However, the researchers found that in Drepanosaurus the radius and ulna are no longer parallel, and the ulna is flat and retracted toward the elbow. Additionally two of the wrist bones have effectively moved into the forearm.
“Many of the Drepanosaur fossils we collected are preserved in three dimensions but are very fragile”, said co-author Alan H Turner, PhD, Associate Professor in the Department of Anatomical Sciences at Stony Brook University School of Medicine. “Because of this we were able to leverage high resolution imaging, such as microCT, to reveal aspects of the forelimb and claw morphology previously unattainable from the Italian fossils. This proved critical in unraveling the strange anatomy of these animals.”
“Drepanosaurus really stretches the bounds of what we thought were the evolutionary constraints on limb evolution,” added Dr. Adam Pritchard, lead author and a postdoctoral fellow at Yale University. “Ecologically, the animal seems to be a sort of chameleon-anteater hybrid, which is really bizarre for the Triassic Period. It possesses a totally unique forelimb among vertebrate animals.
“When you combine this with a claw nearly as large as the arm, you get an arm well adapted for powerful hook-and-pull digging. Think of anteaters digging into insect mounds.”
In addition to the forelimb specialization, Drepanosaurus had grasping feet and a claw-like structure at the end of the tail that might have assisted a life spent in the trees.
The research was funded by the National Science Foundation, the Stony Brook Research Foundation, University of California Museum of Paleontology, University of Utah, Field Museum Women’s Board, and the National Geographic Society Committee for Research & Exploration.
Co-authors of the paper are Randall Irmis of the University of Utah, Sterling Nesbitt of Virginia Polytechnic Institute and State University, and Nathan Smith of the Dinosaur Institute at the Natural History Museum of Los Angeles County.