May 14 (Reuters) - Along a meandering river in a warm and arid region that is now Thailand roughly 113 million years ago, a plant-eating behemoth almost 90 feet (27 meters) long browsed on the treetops without much fear of predators due to its sheer size. This was Nagatitan chaiyaphumensis, the largest-known dinosaur from Southeast Asia.
Researchers have unearthed skeletal remains of Nagatitan, a member of the dinosaur lineage called sauropods known for having a long neck, long tail, small head and four columnar legs.
The fossils of this Cretaceous Period dinosaur were first spotted by a villager in Thailand's northeastern province of Chaiyaphum. Scientists over a period of years then dug up spine, rib, pelvis and leg bones including a front leg bone - the humerus - measuring 5.8 feet (1.78 meters) long.
Based on the dimensions of its humerus and femur, the corresponding hind leg bone, the researchers estimated Nagatitan's body mass at 25 to 28 tons. Its head and teeth were not among the fossils recovered, but the researchers have a good idea of its feeding preferences based on other sauropods.
"Nagatitan was probably a bulk browser that focused on consuming high volumes of vegetation that required little to no chewing such as conifers and possibly seed ferns," said Thitiwoot Sethapanichsakul, a University College London doctoral student in palaeontology and lead author of the research published on Thursday in the journal Scientific Reports.
The climate was probably subtropical, with some forests, but also savanna-like and shrubland habitats. Nagatitan lived alongside various other dinosaurs as well as flying reptiles called pterosaurs. The rivers were teeming with crocodiles and fish including freshwater sharks.
The ecosystem's largest predator was a relative of the giant African meat-eating dinosaur Carcharodontosaurus, probably about 26 feet (8 meters) long and around 3.5 tons.
"At that size, it was dwarfed by Nagatitan. At full size, Nagatitan likely had very little to fear in terms of predation," Sethapanichsakul said.
Predators probably avoided attacking healthy adults of any large sauropod species because of the danger of being squashed. But they may have targeted old or sick adults or vulnerable babies.
"Indeed, sauropods are known to have grown very quickly after hatching, and this probably relates to the dangers of predation. The sooner sauropods could become large, the safer they were because they would have been more difficult to tackle," University College London paleontologist and study co-author Paul Upchurch said.
Sauropods included the largest land animals in Earth's history. Nagatitan was huge by any standard, but not on the scale of some South American sauropods such as Argentinosaurus and Patagotitan that topped 100 feet (30 meters) long.
Nagatitan's name references Naga, a serpent-like being in some Asian religious traditions that is prominently depicted in various Thai temples. In all, there are 14 named dinosaurs known from Thailand.
The names of several large sauropods include the word titan. Sethapanichsakul said it might be appropriate to call Nagatitan Southeast Asia's last "titan" because the region became a shallow sea later in the Cretaceous, meaning no more sauropods would live there.
Nagatitan provides insight into sauropod diversity in the region. Not many sauropods are known from Southeast Asia, and Nagatitan is the largest and the geologically youngest of them. Nagatitan belonged to a subgroup of sauropods that possessed bones with lots of internal air sacs and thin walls, traits that lightened their skeletons.
This group originated around 140 million years ago, achieved a global distribution and, around 90 million years ago, became the only sauropods left worldwide, thriving until the dinosaur age ended 66 million years ago with an asteroid impact.
Nagatitan lived at a time when Earth's atmospheric carbon dioxide levels were climbing, corresponding to high global temperatures.
"Sauropods seem to have become particularly large at this time, with gigantic forms living in South America, China, probably North Africa, and now with Nagatitan a fairly large one in Southeast Asia," Upchurch said.
"This possible relationship between large body size and high climatic temperatures is not fully understood, but it's likely that the high temperatures had an impact on the plant fodder that was important to sauropods, which were very large-bodied herbivores. Nagatitan gives a glimpse of the period leading up to the eventual peak in body size and temperatures about 10 to 15 million years later," Upchurch said.
