How a bizarre Triassic reptile could change what an arm looks like

Most forearms look the same. Whether the animal is a human, a cat, or even a Tyrannosaurus rex, the same bones attach in about the same places to form roughly the same shaped limb.

But not in the Triassic reptile Drepanosaurus.

The strange Drepanosaurus forelimbs might actually have been particularly well adapted for digging, according to new research published Thursday in the journal Current Biology. And this weird reptile could change scientists' ideas of what a forelimb can look like.

"Across a huge sample of tetrapod animals, you have a very consistent, strict sort of pattern of how the forelimb is put together," study lead author Adam Pritchard, a postdoctoral researcher at Yale University tells The Christian Science Monitor. "You could almost call it a rule of how the forearm of tetrapod animals develops and is manifested in a wide sample of animals," he says.

But, Dr. Pritchard adds, "What's remarkable about Drepanosaurus is that it violates those patterns of how the forearm is put together."

In almost every four-limbed animal from a dog to a wimpy-armed T. rex, the forearm has the same structure. Two long shaft-like bones, the radius and the ulna, are parallel and connect to a bunch of short, stocky wrist (carpal) bones. But the 212-million-year-old Drepanosaurus forearms look nothing like that.

Instead, while "the radius is somewhat shaft-shaped," Pritchard describes, "the ulna is this weird, flattened crescent bone that basically sticks out perpendicular to the radius." And those little stubby wrist bones aren't so stubby. They're actually fairly long and slender.

"This is a huge departure from the conventional" structure, Nick Fraser, a vertebrate paleontologist at the National Museums Scotland who was not part of the research, tells the Monitor. "This tells us that it's probably not as much of a straightjacket as we thought. So then the question becomes, why hasn't this happened more often?"

Courtesy of Adam Pritchard et al.

Examples of forelimb anatomy in a number of extinct and living reptiles, showing the consistency in the paralleling of forearm bones and the shortness of the carpal bones. The forelimb of Drepanosaurus violates all of these patterns.

Drepanosaurus actually isn't the only animal scientists know about with these strange arms. Silky anteaters have similar arms. 

Most notably, the anteaters and the extinct reptiles both have humongous claws at the tip of one particularly large digit, while the surrounding ones are significantly smaller. 

Silky anteaters use this unique feature to perform "hook-and-pull" digging. The animal hooks its massive claw into a substrate, be it an ant's nest, a tree branch, or some other material containing a tasty morsel. Then, with their claw still hooked, the animal pulls its entire arm back, ripping open whatever material they are targeting.

The similar structure in Drepanosaurus suggests that the Triassic reptile also was well adapted to this behavior, Pritchard says.

The Triassic period was full of bizarre creatures that were remarkably similar to animals that arose during very different time periods, Pritchard says. For example, body shapes like those of the big carnivorous dinosaurs that dominated the Earth in the late Cretaceous (over 100 million years later) arose along with crocodile-like animals. 

"Now Drepanosaurus adds to that," he says, in that "we have an analogue to anteaters present almost 200 million years before they actually show up in the fossil record in the Cenozoic."

The Triassic period, which spanned from about 252 million years ago to 201 million years ago, saw the first dinosaurs – although they didn't rise to dominance until the following period, the Jurassic. Strange crocodile ancestors roamed the single continent, Pangea, and ichthyosaurs, marine reptiles that resembled modern dolphins, swam the seas during the Triassic period. 

It wasn't just a plethora of bizarre animals, says Dr. Fraser. "It was the beginning of the age of the dinosaurs, but it also sees the origin of most of the groups of vertebrates living today," he says. The first mammals, the first frogs, the first lizards, and the first crocodiles roamed Earth during that period. "It's a strange mix of ancient and modern."

"The Triassic was really a time of experimentation for the vertebrates" in terms of body shapes and sizes, Fraser says.