A discovery near the town of Tran has revealed that a dinosaur from the titanosaur group — the same family as the largest known dinosaur on Earth, the Argentinosaurus — once walked the territory of present-day Bulgaria. Paleontologists from the National Museum of Natural History at the Bulgarian Academy of Sciences announced that they had uncovered a fossilized bone fragment dating back to the Upper Cretaceous period, between 65 and 95 million years ago.

According to senior assistant Latinka Hristova, the identification of the fossil as a dinosaur bone was made using the method of exclusion. At the time when this sediment layer formed, the largest mammals would have been no bigger than a domestic cat, which ruled them out as possible sources of such a large bone. While the remains consist of only a few fragments, scientists have determined they belonged to a vertebrate dinosaur.

Further confirmation came through histological analysis conducted by paleontologist Vladimir Nikolov. Under a microscope, he studied thin cross-sections of the fossil and concluded that it belonged to a dwarf titanosaur. While titanosaurs as a group include the enormous Argentinosaurus — which could reach lengths of 30 to 37 meters — the Bulgarian specimen appears to be from a smaller, insular species. Despite this, the exact size of the dinosaur remains unclear due to the incomplete state of the fossil, which lacks both ends that typically offer clues about its anatomical location and dimensions.

Most titanosaurs had limb bones the size of an average human, but the specimen found near Tran is noticeably smaller. Hristova noted that without knowing which specific part of the body the bone came from, it's impossible to estimate the dinosaur’s size with confidence.

Titanosaurs were massive, plant-eating dinosaurs that roamed the Earth during the Late Cretaceous. Argentinosaurus, the largest among them, reached up to 37 meters in length and weighed between 60 and 108 tons. At birth, these giants weighed just around 5 kilograms, but by age 40, they could weigh more than 70 tons. They are believed to have swallowed vegetation whole, relying on internal bacteria to break down their food.

The discovery site near Tran was located purely by chance when a fossil enthusiast stumbled upon a bone fragment in a river. The initial find prompted a visit from geologists from Sofia University and museum paleontologists, including Hristova. Though the exact source location was not immediately evident, which sometimes leads to such finds being dismissed, further fieldwork changed the course of the investigation.

Hristova and colleague Ralitsa Konyovska returned to the area for a more thorough exploration. They discovered an outcrop where coal seams were exposed, and there they found additional bone fragments. This encouraged a larger team from the National Museum of Natural History and the Geological Institute of BAS to conduct further surveys.

Based on these results, the researchers have now mapped out a site for more systematic excavation. They are hopeful that future digs will uncover more complete fossils, including the critical epiphyses — the ends of bones that form joints — which could help identify the species and reconstruct its physical form more accurately.

This discovery is especially exciting given that in 2014, similarly well-preserved titanosaur fossils were found in Argentina, leading scientists to identify a new subspecies of Argentinosaurus as the largest dinosaur ever recorded. One of the thigh bones alone measured 2.43 meters.

That a relative — albeit a smaller one — of this colossal species lived in what is now Bulgaria is a significant find for the country. Hristova suggests the smaller size of the Bulgarian dinosaur may be due to environmental factors. During the Late Cretaceous, Europe was made up of a series of islands surrounded by shallow, warm seas. Such island environments often lead to smaller body sizes in animals, a phenomenon known as island dwarfism, due to limited space and food resources.

The climate in what is now Bulgaria was tropical, with consistently warm temperatures and no freezing conditions. These favorable conditions may have supported a variety of herbivorous species, including titanosaurs. With this discovery, Bulgarian scientists are optimistic that more fossil remains could be uncovered, allowing for more detailed reconstructions of the country’s prehistoric wildlife to be showcased in museum exhibits.