Back in 2001, a team of paleontologists spotted bones protruding from rock in the Hell Creek Formation, in Ekalaka, Montana. Subsequent excavations uncovered what is now being called the most complete skeleton of an adolescent Tyrannosaurus rex ever found. In a new study of the specimen, dubbed “Jane,” scientists say the young T. rex fills in a critical gap in their understanding of how tyrannosaurs grow and develop. In addition, the study’s authors are using their findings to argue that the supposed specimens of a smaller, related species of carnivorous dinosaur (Nanotyrannus) are actually juvenile versions of T. rex.
It was another skull discovered in Ekalaka that first sparked the debate over how many species of meat-eating dinosaurs roamed North America during the Cretaceous period (more than 66 million years ago). Unearthed in 1942, the skull was identified as a juvenile member of the tyrannosaur family, which includes T. rex and its relatives, and went on display in the Cleveland Museum of Natural History for some 50 years.
In the late 1980s, the famed paleontologist Robert Bakker analyzed the specimen and claimed it belonged not to a young T. rex but to another species altogether. To support his conclusion, Bakker pointed to the fact that the skull’s bones appeared to be fused rather than open, as they would be in less mature skeletons. As the skull’s owner would have measured only 5 meters in length, as compared to the 12 meters achieved by a typical T. rex, Bakker called his new species Nanotyrannus, or “dwarf tyrant.”
Flash forward to 1999, when Thomas Carr, an associate professor of biology at Carthage College in Wisconsin, published his own study. Arguing that the bones in the skull in question were not in fact fused, and that their texture and microscopic structure were typical of an immature dinosaur, Carr concluded Nanotyrannus was simply a young T. rex. Those who doubted his theory had trouble believing a dinosaur could increase in size and change in shape so dramatically over the course of its life.
Enter “Jane.” This week, Carr and his fellow authors presented a new, as yet unpublished study at the 75th annual Society of Vertebrate Paleontology conference in Dallas, Texas. According to their analysis, which included a 3-D computer reconstruction that filled in missing fragments of the skull, the 20-foot-long skeleton is the most complete adolescent T. rex yet found. On Jane’s calf bone, the scientists examined a series of microscopic “growth rings,” each of which correspond to one year of life. Finding nine such rings, with space for two more, they concluded that Jane was around 11 years old when she died. According to Carr’s presentation in Dallas, she “was just about to, or had already entered, the rapid phase of growth” characteristic of giant carnivorous dinosaurs.
After comparing Jane’s skull with that of the 1940s specimen—the supposed Nanotyrannus—the researchers found a number of common features, including a long, low snout and a hole in the jawbone. According to Carr and his colleagues, these commonalities are not characteristics of a different species, but of immature tyrannosaurs. For them, Jane represents a key intermediate step in size and shape between juvenile and adult T. rex specimens, making the skeleton an invaluable source of information about one of the best-known dinosaur species.
While some paleontologists who attended the meeting said the new study’s arguments convinced them, Bakker still contends that Nanotyrannus is a separate species. He told Science magazine that he believes Carr “has not seen the best specimen” of Nanotyrannus, referring to a nearly complete skeleton discovered in Montana in 2006. That particular specimen sits at the center of its own heated controversy: Its owner, commercial fossil hunter Clayton Phipps, tried to sell the skeleton at auction in 2013, but withdrew it after the bidding failed to reach his desired $7 million or more. Although Phipps was legally entitled to sell the skeleton because he found it on private land, the ethical guidelines of the Society of Vertebrate Paleontology prohibit selling to a private collector. Similarly, most paleontologists won’t study a specimen unless it is donated to or purchased by a museum—which means no details have been published about the 2006 find.