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T. rex-like dinosaur youngsters didn’t hunt like grown-ups

A juvenile Tarbosaurus dinosaur fossil from Mongolia reveals that youngsters of that species didn’t compete with adults for food.

Fossil remains of a 70 million year old juvenile Tarbosaurus bataar, the closest known relative of the infamous Tyrannosaurus rex, has given paleontologists a rare glimpse into the early years of both these dinosaur species.

Skull features of the young dinosaur, believed to be two to three years of age, showed it hadn’t yet developed powerful jaw muscles and specialized teeth that adult Tarbosaurus needed to hunt big prey. Its skeleton reveals that juvenile Tarbosaurus were fast, agile youngsters that went after smaller prey, occupying a different food niche from adults.

The Tarbosaurus specimen discovered in the Cretaceous rock of Mongolia is the youngest ever found. The entire skeleton was recovered except for the neck and part of its tail. Image Credit: WitmerLab at Ohio University and Hayashibara Museum of Natural Sciences.

The fossil, an almost-complete skeleton (it was missing its neck and a section of its tail) was found in the Gobi desert of Mongolia. It was studied by a team of paleontologists from Japan, Mongolia, and the US. They reported their findings in the May 10, 2011 issue of Journal of Vertebrate Paleontology.

Study co-authors Yasuhiro Kawahara (left) and Takehisa Tsubamoto (right), in 2006 at the Mongolia excavation site of the young Tarbosaurus. Image Credit: Hayashibara Museum of Natural Sciences.

Takanobu Tsuihiji, of the National Museum of Nature and Science in Tokyo and the paper’s lead author, commented about the 11.4 inch juvenile Tarbosaurus skull in a press release:

We knew that adult Tarbosaurus were a lot like T. rex. Adults show features throughout the skull associated with a powerful bite … large muscle attachments, bony buttresses, specialized teeth. The juvenile is so young that it doesn’t really have any of these features yet, and so it must have been feeding quite differently from its parents.

Andrew Lee of Midwestern University, a co-author, studied microstructures of the leg bones to estimate the age of the young Tarbosaurus at the time of its death. It was just two to three years old. At that age, the juvenile was about 9 feet in length, 3 feet high at its hip, and weighed around 70 pounds. In comparison, adults were 35 to 40 feet long, 15 feet high, and weighed in around 6 tons. Based on studies of T. rex, Tarbosaurus probably had a lifespan of about 25 years.

Silhouettes of an adult Tarbosaurus and the newly discovered juvenile along with a human for scale. Image Credit: Hayashibara Museum of Natural Sciences.

Lawrence Witmer, a paleontologist at Ohio University and part of the Tarbosaurus team, commented in the same press release,

It’s one of the secrets of success for tyrannosaurs – the different age groups weren’t competing with each other for food because their diets shifted as they grew.

This little guy may have been only 2 or 3, but it was no toddler … although it does give new meaning to the phrase “terrible twos.” We don’t know to what extent its parents were bringing it food, and so it was probably already a pretty capable hunter. Its skull wasn’t as strong as the adult’s, and it would have had to have been a more careful hunter, using quickness and agility rather than raw power.

This video shows details of the juvenile Tarbosaurus skull and comparisons with 14-year-old and 25-year-old adult skulls. Animation Credit: WitmerLab at Ohio University.

The different feeding niches of juvenile and adult Tarbosaurus reduced food competition within the species, perhaps contributing to their role as the dominant predators of their time. Said Tsuihiji,

The juvenile skull shows that there must have [been] a change in dietary niches as the animals got older. The younger animals would have taken smaller prey that they could subdue without risking damage to their skulls, whereas the older animals and adults had progressively stronger skulls that would have allowed taking larger, more dangerous prey.

What kind of prey would the young Tarbosaurus have pursued during this late Cretaceous period? Mahito Watabe of the Hayashibara Museum of Natural Sciences in Okayama, who led the expedition to Mongolia in 2006 that uncovered the fossil, said in the same press release,

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Tarbosaurus

is found in the same rocks as giant herbivorous dinosaurs like the long-necked sauropod Opisthocoelicaudia and the duckbill hadrosaur Saurolophus. But the young juvenile Tarbosaurus would have hunted smaller prey, perhaps something like the bony-headed dinosaur Prenocephale.

Witmer also commented that this specimen could help clinch the identity of other ambiguous T. rex-like fossils that may in fact be juveniles of known dinosaur species.

The beauty of our new young skull is that we absolutely know for many good reasons that it’s Tarbosaurus. We can use this known growth series to get a better sense of whether some of these more controversial finds grew up to be Tarbosaurus, Tyrannosaurus or some other species.

This is how the juvenile Tarbosaurus appeared, embedded in rock, when it was discovered in 2006 in the Gobi desert of Mongolia. Image Credit: Hayashibara Museum of Natural Sciences.

Fossil remains of a young dinosaur, closely related to Tyrannosaurus rex, indicate that juveniles and adults went after different kinds of prey, which reduced food competition within the species. Tarbosaurus bataar, found during a 2006 fossil-hunting expedition in the Gobi desert of Mongolia, lived about 70 million years ago. This animal was believed to be just two to three years old when it died. Its skull features reveal that juveniles were faster and more agile than the adults, going after prey that would not damage their smaller skulls. As they grew towards adulthood, their skulls grew progressively larger and stronger, allowing them to go after increasingly larger prey.

Skull of 2-year-old Tarbosaurus, shown in comparison with that of a “teenage” and adult Tarbosaurus. Image Credit: WitmerLab at Ohio University.

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