Tarbosaurus

About Tarbosaurus

Characteristics and Biomechanics of Tarbosaurus

Tarbosaurus, found in Mongolia, is an apex predator representing the Asian region during the Late Cretaceous period (approx. 75 to 70 million years ago) and is very closely related to the North American Tyrannosaurus.

Its forelimbs were extremely reduced, with only two functional digits. Although the third metacarpal bone remained, it had become a very thin vestigial organ. This reduction of the forelimbs is thought to be the result of the evolution of its primary weapon for catching prey from "grasping forelimbs" to a "massive skull and powerful bone-crushing jaws."

Compared to Tyrannosaurus, the skull of Tarbosaurus was narrower and had a more elongated snout. The most distinctive feature was the structure of its lower jaw, where the main bones making up the jaw (such as the dentary and angular bones) interlocked like a jigsaw puzzle, overlapping tightly without any gaps (interlocking mechanism). This eliminated the flexibility needed to absorb the shock of biting prey, but allowed it to directly transmit the immense power required to crush its prey's bones.

According to the latest biomechanical simulations, the bite force of Tarbosaurus reached up to approximately 24,000 Newtons, overwhelmingly surpassing other carnivorous dinosaurs of the same size. Inside its thick, nearly conical teeth was a specialized shock-absorbing layer with a fine mesh-like structure (mantle dentin), which protected the roots from the massive impact of crushing bones. Furthermore, recent studies suggest the possibility of a large connective tissue called "exoparia" around its cheeks, which mechanically stabilized the lower jaw against the skull and supported its powerful chewing motions.

In addition, examining the shape of its brain cavity (endocast) revealed that Tarbosaurus had excellent sensory organs. Its olfactory bulbs, which control the sense of smell, were very large, indicating it could detect the scent of distant prey or carrion with high sensitivity. The structure of its inner ear also showed that it had an excellent ability to hear low-frequency sounds.

Growth Changes from Juvenile to Adult (Allometry)

Tarbosaurus fossils are found in abundance from juveniles to adults, making it an extremely important subject for understanding how large theropods grew.

A juvenile's skull is long and narrow, with large orbits (eye sockets) and shallow jaws. As it grows, however, the height of each bone increases rapidly, transforming the entire skull into a robust structure.

Recent research on a 2- to 3-year-old juvenile specimen with a 290mm-long skull revealed that it had 13 teeth in the upper jaw and 14 to 15 teeth in the lower jaw, which is the same as in a giant adult Tarbosaurus. This means the number of teeth did not decrease to increase jaw strength as it grew.

When young, their bite force was weak, and they were suited for slicing and hunting small, agile prey. However, as they became adults, biomechanical simulations confirm that they shifted their diet (niche shift) to hunting large prey with thick skeletons, crushing them bones and all.

Moreover, healed bite marks are often found on the skull fossils of adult Tarbosaurus. Since these scars are not present on juveniles and suddenly start appearing on subadults, they are considered evidence of ritualistic combat involving biting each other's faces during territorial disputes or mating behavior accompanying sexual maturity.

Asian Apex Predator or Another Species of Tyrannosaurus?

Since its discovery, the classification of Tarbosaurus has been one of the biggest controversies in paleontology. The question was, "Is Tarbosaurus an independent genus, or is it an Asian species of the North American genus Tyrannosaurus (Tyrannosaurus bataar)?"

Today, many researchers agree to treat Tarbosaurus as an independent genus (Tarbosaurus) due to minute morphological and biomechanical differences in the skull.

• Decisive differences from Tyrannosaurus:
  • Tyrannosaurus had a wide expansion at the back of its skull, providing it with a broad stereoscopic vision with both eyes facing forward. Tarbosaurus, on the other hand, had a gentler expansion, and its entire snout was elongated.
  • In Tyrannosaurus, the nasal bones fused complexly with surrounding bones to disperse the impact of its bite force. In Tarbosaurus, while the nasal bones were domed, their connections were loose, meaning they lacked this force-dissipating structure.
  • Tarbosaurus's lower jaw had a rigid interlocking mechanism, an independent evolutionary path that increased rigidity by eliminating jaw flexibility.

Another small tyrannosaurid called "Alioramus" also inhabited the Nemegt Formation in Mongolia during the same period. Alioramus had a slender snout, lacked bone-crushing power, and specialized in slashing bites for small prey. Tarbosaurus, however, used its robust skull and lower jaw to hunt large herbivorous dinosaurs, crushing them bones and all. It is believed that differences in their physique and jaw functions allowed them to coexist without competing for prey in the same environment.

Did Tarbosaurus Have Feathers? Latest Research on Integument

For many years, there was active debate over the hypothesis that "Tarbosaurus and Tyrannosaurus might have been covered in dense feathers." This was because feather fossils had been discovered in basal, small tyrannosauroids that were their ancestors.

However, recent investigations of fossilized skin impressions have led to a major revision of this feather hypothesis. Only polygonal, mesh-like "tiny scales" have been found in the skin fossils of several large tyrannosaurids, including Tarbosaurus, with no traces of feathers.

If a massive body weighing several tons were covered in highly insulating feathers, it would be unable to release the heat generated during strenuous hunting activities or in warm climates, increasing the risk of a fatal overheat (heatstroke). Therefore, it is thought that as tyrannosaurids evolved to have gigantic bodies, they lost their feathers to efficiently dissipate excess body heat and reacquired scaly skin through "reverse evolution."

Discovery and Description

In 1946, a joint Soviet-Mongolian expedition was conducted in the Gobi Desert of Mongolia. They discovered a large skull and vertebrae in the Nemegt Formation, which is thought to have been a rich river system and floodplain.

In 1955, Soviet paleontologist Evgeny Maleev described it as a new species of Tyrannosaurus, Tyrannosaurus bataar, based on specimen PIN 551-1. Initially, unable to recognize growth stages or individual variations, he established multiple taxa.
Later, a study in 1965 by Soviet paleontologist A.K. Rozhdestvensky revealed that these represented the ontogeny (growth stages) of a single taxon. Because it had characteristics different from the North American Tyrannosaurus, it began to be treated as an independent genus, Tarbosaurus.

Auction and Smuggling Controversy

In June 2012, an almost complete skeleton put up for auction at Heritage Auctions in New York as "Tyrannosaurus bataar" was sold for about $1.05 million.

However, appraisals by paleontologists clearly showed that the fossil had been illegally excavated and smuggled from the Nemegt Formation in Mongolia, leading the US government to intervene and seize the fossil. Under Mongolian law, dinosaur fossils excavated within the country are national cultural property, and their export for commercial purposes is strictly prohibited.

The smugglers were arrested, and this Tarbosaurus bataar was officially repatriated to Mongolia in 2013. This incident became a precedent that impressed upon the international community that fossils are not merely commercial goods, but the scientific and cultural heritage of their countries of origin. Today, it is passed down as a major turning point that raised awareness of the importance of a fossil's provenance.

Tarbosaurus Stamp and Fossil Gallery