Science News - May 21, 2011

Dino rise During the late Triassic, dinosaurs showed less range in morphology and body types, called disparity, than the crocodile-like crurotarsans. This changed in the Jurassic.

Dinosaur disparity over time

also where most of the Triassic dinosaur fossils have been found. So dinosaurs and their close relatives may have originated in Gondwana before spreading to other parts of the world, says Irmis.

That concept was bolstered in 2009, when Nesbitt, Irmis and colleagues described a theropod that lived 213 million years ago in what is now New Mexico (SN Online: 12/11/09). Despite coming more than 15 million years after the “dawn” dinosaurs of Argentina, this creature — called Tawa after the Hopi name for the Pueblo sun god — still has a lot to tell paleontologists.

Degree of disparity

That’s because the rocks that contain Tawa look quite a bit like those from Argentina 230 million years ago. The New Mexican fossil might represent a creature that evolved in an environment and landscape similar to the home of the earliest dinosaurs known, or it might be the descendant of a long-distance migration. “Although Tawa was found in the Northern Hemisphere, it probably was a lineage that originated in the Southern Hemisphere,” Irmis says.

Tawa’s skeleton also shares many features with Herrerasaurus and other primitive dinosaurs, and so it helps paleontologists classify those earlier creatures as true theropods. Tawa mixes earlier dinosaur characteristics with later ones, allowing researchers to see when certain features, such as air sacs within the spinal column, evolved.

Another dinosaur unearthed recently in New Mexico is similarly helping

Crurotarsi Dinosauria Carnian

NorianAnisian-
Ladinian
SOURCE: S.L. BRUSATTE E T AL/EARTH-SCIENCE REVIEWS 2010

paleontologists understand the relationships among early theropods. At 205 million years old, this animal, called Daemonosaurus, is younger than Tawa yet still retains certain characteristics from its ancestors.

Known from a single buck-toothed skull, Daemonosaurus has relatively large bones at the tips of its jaws, as Herrerasaurus did, mixed with the vertebral air sacs that generally characterize later dinosaurs. Once again, paleontologists can use such findings to untangle which features appeared when in theropod history. A team led by Hans-Dieter Sues of the Smithsonian National Museum of Natural History in Washington, D.C., described Daemonosaurus online in April in the Proceedings of the Royal Society B.

Discovered in 230-million-year-old rocks
in Argentina, Eodromaeus (skull shown)
ate meat while a similar-sized dino from
the same time and region ate plants.

Dino dominance

For all the recent insight into the dawn of the dinosaurs, one question still looms: How did such specimens come to dominate the planet? Somehow these runty chickenlike creatures managed to give rise to the formidable T. rex, Triceratops, Stegosaurus and more.

One clue may lie in yet another mass
extinction. Around 200 million years
ago, at the end of the Triassic period,
something traumatic happened on
Earth. As Pangaea began splitting apart,
great quantities of magma poured out
along the planetary seam that would
become the middle of the Atlantic
Ocean. Atmospheric carbon dioxide
levels spiked, and the chemistry of the
oceans shifted dramatically.