Largest dinosaur in Southeast Asia discovered in Thailand

Thitiwoot Sethapanichsakul grew up with a passion for dinosaurs that he's never shaken off.

"I always told my high school teachers, 'I'm going to name a dinosaur and it's going to be from Thailand,'" he said.

Today, Mr Sethapanichsakul is studying for a PhD in palaeontology at University College London — and is the co-lead author of a paper in Scientific Reports, describing the largest dinosaur ever discovered in South-East Asia.

Based on about a dozen bones, including two enormous upper limbs, Mr Sethapanichsakul and his colleagues have calculated that the creature weighed about 27 tonnes and stretched 27 metres from head to tail.

When it was alive, more than 100 million years ago, it would have had the classic shape of a sauropod dinosaur, with thumping column-shaped legs and a long neck for munching high branches of conifers.

The team has dubbed the species Nagatitan chaiyaphumensis after the serpent "Naga" from South-East Asian mythology, and the Chaiyaphum province where the bones were found.

Pelvis by a pond

The fossil first came to light in 2016 when a villager, Thanom Luangnan, saw something unusual sticking out of the ground by a lake, exposed by the dry season.

"The story goes that it was a local charity event at the temple and they had gone out fishing at the pond," Mr Sethapanichsakul told Lab Notes on ABC Radio National.

"Mr Luangnan saw some bones peeking out from the edge of the pond and he immediately realised that there's something odd about this.

"He spotted the pelvis first; it's one of the largest blocks that we have of the bones."

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In consultation with a local teacher, Mr Luangnan — who has since died, but is acknowledged in the research paper — called the Department of Mineral Resources to report what he'd found.

Stephen Poropat, a palaeontologist at Curtin University who was not involved in the research, said it was "so characteristic of palaeontology" that a major discovery started with the sharp eyes of a passer-by.

"They're found by amateurs or they're found by people just out for a walk, or people who are in a certain area and notice differences from day to day, week to week, month to month.

"You cannot overstate the value of people like that."

Several years of excavation followed the initial discovery until funding ran out in 2020, Mr Sethapanichsakul said.

"All of the specimens that had been dug out were shelved."

It was in 2024 that he and co-author Sita Manitkoon from Mahasarakham University secured a National Geographic grant to finish the dig, study the bones and start engaging local communities with the work.

'I was told it was big'

While even larger sauropod specimens have been unearthed in places like China and Argentina, Mr Sethapanichsakul and his collaborators knew very quickly that they had found something significant for the region.

"I was told it was big and then I finally saw the humerus — the upper [front] leg bone — and it was taller than myself," Mr Sethapanichsakul said.

"I had to stop and go, 'OK. What have I gotten myself into?'

Detailed scans of the humerus and an upper back leg bone (femur), several vertebrae and ribs, plus most of a giant pelvis, allowed the team to reconstruct a computerised 3D skeleton for the Nagatitan, filling in the missing bones based on similar dinosaurs.

They also got a good fix on its size.

"We use the circumference of the humerus and the femur," Mr Sethapanichsakul explained.

"There's a kind of a statistical correlation between those measurements and an animal's body mass, especially for quadrupeds."

Estimated at 25 to 28 tonnes — about as heavy as six elephants, or three Tyrannosaurus rex — Nagatitan eclipses the Tangvayosaurus from Laos that weighed roughly 23 tonnes.

The dino comes from an era, 120 to 100 million years ago during the Cretaceous Period, when the world was warming and these sauropods were ballooning in size.

"Nagatitan actually gives us a little snapshot into that changing world where dinosaurs like sauropods were being pushed to get bigger and bigger," Mr Sethapanichsakul said.

"You're able to access resources that other dinosaurs couldn't, by reaching up to the tops of trees. As you get bigger, you get hunted less because [carnivorous] dinosaurs can't go after you.

"And it creates this feedback loop where evolution is driving sauropods to just get bigger and bigger, because they're more successful when they're bigger and taller."

Dr Poropat said this new addition to the sauropods shows just how large they got, even at low latitudes near the equator, which was "really cool to see".

"Twenty-eight tonnes is a pretty considerable animal."

A Thai dino boom?

Mr Sethapanichsakul wants the thundering footsteps of Nagatitan to clear a path for more palaeontologists in Thailand to follow their passion.

"A lot of young researchers are finishing their degrees … more research papers are being cranked out on palaeontology."

Dr Poropat also sees this as an exciting prospect — and a refreshing change from the past.

"For the longest time … teams from Europe would come in and excavate material and describe it. They might name it after people in Thailand, but they wouldn't necessarily include Thai researchers in the process."

Mr Sethapanichsakul and Dr Manitkoon worked hard to communicate their work on Nagatitan to the public, throughout the process.

Even before the paper was published, the brand new Thainosaur Museum in Bangkok opened with a life-sized reconstruction of their discovery as its centrepiece. Mr Sethapanichsakul said the name-plate was unveiled last week.

"Finally being able to put the dinosaur name on the plaque and go, yes, this is Nagatitan chaiyaphumensis really is the cherry on top," he said.

"Hopefully we'll get to see a few more sauropods, a few more dinosaurs come out in the next couple of years here."