Today, the land near Ban Saphan Hin in central Thailand is dusted with thin reddish soil where local farmers plant corn and tapioca. But more than 113 million years ago, this region hosted ancient floodplains that were terrorized by a fearsome dinosaur with shark-like teeth.

Described today in the journal PLOS One, the newfound predator—called Siamraptor suwati—is the most complete dinosaur of its type and age ever found in Southeast Asia. The bones of the 25-foot beast add to a string of major dinosaur finds from the region, and they reveal new insight into how a major group of predatory dinosaurs spread across the ancient world.

“It's one of the most important Thai dinosaurs ever found,” Steve Brusatte, a paleontologist at the University of Edinburgh who reviewed the study for publication, says in an email.

View Images Researchers scaled the 22 newfound fossils to reconstruct the skeleton of Siamraptor suwati. The scale bar equals one meter, or about 3.3 feet. Image courtesy of Chokchaloemwong et al., 2019

For instance, a team led by Duangsuda Chokchaloemwong, a researcher at Thailand’s Nakhon Ratchasima Rajabhat University, pored over the bones and found that the skeleton is shot through with air sacs. This would have made the dinosaur’s frame lightweight and perhaps helped it breathe faster, an idea that future scans of the bones could put to the test.

“It would have been a fierce, fast, dynamic beast,” Brusatte says.

Teeth like a shark's

Tens of millions of years before giant tyrannosaurs such as T. rex arrived on the scene, another group of large predatory dinosaurs reigned: the allosauroids. Among these meat-eating heavyweights was a group called the carcharodontosaurs (kar-KA-ro-DON-toe-SORES), which were the top predators for most of the Cretaceous.

“It was only with the decline of the carcharodontosaurs that small tyrannosaurs got big and moved into the apex predator role,” Brusatte says.

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Evidence of the group first arose from the Egyptian Sahara in 1914, when an expedition funded by German paleontologist Ernst Stromer found dinosaur teeth that were serrated like steak knives. The forbidding chompers reminded Stromer of those of Carcharodon, the shark genus that includes the great white shark, so in 1931, he named the dinosaur Carcharodontosaurus saharicus.

In the following decades, paleontologists found more relatives of Stromer’s shark-toothed dinosaur, including some of the biggest predatory dinosaurs that ever lived. But until recently, no well-preserved carcharodontosaurs had ever been found in Southeast Asia. Was this gap the sign of a true absence, or had their remains simply not yet been uncovered? To find out, scientists needed to start digging.

Digging up a dinosaur

In the last couple decades, Thai paleontologists have found a lot of fossil material from the time of the dinosaurs. Since 2007, an international team called the Japan-Thailand Dinosaur Project has found two new plant-eating dinosaurs named Ratchasimasaurus and Sirindhorna, as well as an ancient relative of alligators and crocodiles.

“This project is strikingly important to reveal evolutionary history of dinosaurs during the Early Cretaceous period,” study coauthor Soki Hattori, a paleontologist at Japan’s Fukui Prefectural Dinosaur Museum, says in an email. “The comparison of Early Cretaceous dinosaurs from Japan and Thailand enables us to understand deeply about them, such as the history of geographical radiation of dinosaurs.”

The researchers found the plant-eating Sirindhorna near Ban Saphan Hin, a village in Nakhon Ratchasima province, within a rock layer thought to have formed about 113 to 125 million years ago. Highs reached 95 degrees Fahrenheit as the team dug, and the site rang out with the constant clang of stone knocking stone.

The hard work was worth it: In addition to turning up Sirindhorna, the excavation uncovered 22 disarticulated pieces of a predatory dinosaur. The fossils were from at least four different individuals and included some backbones, parts of the limbs and hips, and fragments of the skull, including a well-preserved lower right jaw. Chokchaloemwong and her colleagues pored over the bones and found that they belonged to a carcharodontosaur.

The discovery shows that carcharodontosaurs were widespread across Earth by the early Cretaceous period. Many other dinosaur groups, including other allosauroids, also expanded their ranges by then. At the time, North America was connected to Europe and Asia, allowing the three continents’ dinosaurs to mix and mingle.