DALLAS, TEXAS—Every school kid knows that Tyrannosaurus rex was one of the biggest dinosaurs that ever lived and probably the baddest, ripping into other dinos with its lacerating, bone-crunching teeth. Yet some scientists think that T. rex, which roamed the western United States between 68 million and 66 million years ago, had a smaller but equally voracious cousin, named Nanotyrannus, which lived in the same place at the same time. A new study, presented at the meeting of the Society of Vertebrate Paleontology here, disputes that claim, and concludes that the two supposed specimens of Nanotyrannus are simply juvenile versions of T. rex. If so, T. rex, as its name implies, would have been sole king of the neighborhood.

The finding has implications for how tyrannosaurids grew and also for how diverse they were. “We need to know how many dinosaurs were around during this critical time period right before they went extinct” about 66 million years ago, says Lindsay Zanno, a paleontologist at the North Carolina Museum of Natural Sciences in Raleigh.

The story of Nanotyrannus goes back to 1946, when a paleontologist at the Smithsonian National Museum of Natural History described a skull found in Montana as a member of the widespread tyrannosaur family, which includes T. rex and its relatives. But in the late 1980s, researchers including Robert Bakker, an iconoclastic paleontologist now affiliated with the Houston Museum of Natural History, studied the skull anew and concluded that it was a separate, smaller taxon. Whereas T. rex can reach up to 12 meters in length, the Montana specimen measured about 5 meters, inspiring the Bakker team to call it Nanotyrannus or “dwarf tyrant.” The team concluded that the specimen represented an adult, because the bones in its skull appeared to be fused rather than open as in immature specimens.

Some years later, Thomas Carr, a paleontologist at Carthage College in Kenosha, Wisconsin, published his own study examining features of the same specimen’s skull, teeth, and skeleton, along with comparisons with other juvenile specimens of T. rex. Carr found that the skull bones were not fused as earlier claimed, and also that the texture and microscopic structure of the skull bones were typical of an immature individual. Carr thus concluded that Nanotyrannus was simply a T. rex youngster. This conclusion was bolstered when, in 2002, another skull and partial skeleton similar to that of the claimed Nanotyrannus was found in Montana. Nicknamed “Jane,” it was better preserved than the earlier 1940s specimen, and many researchers concluded it was a juvenile based on the shape of its teeth and other skeletal features. Indeed, a few former Nanotyrannus advocates changed their minds based on the new skull. But others continued to argue that it was a separate species.

In Dallas, Carr presented a new analysis of Jane’s skull and skeleton, based on a three-dimensional computer reconstruction of the skull which filled in missing segments and allowed him to analyze the features in more detail. His team examined microscopic “growth rings” in Jane’s calf bone, which accumulate per year of life. The team found nine such rings in the bone and space for two more, leading them to conclude that Jane was indeed a juvenile, about 11 years old when she died. Moreover, close examination of the skeleton revealed that it was still undergoing “remodeling” typical of very fast growing bone. Although younger and older specimens of T. rex are known, Jane filled an important gap in researchers’ knowledge of the growth pattern of tyrannosaurs, Carr told the meeting. “She was just about to, or had already entered, the rapid phase of growth” typical of very large carnivorous dinosaurs.

In addition, Carr argued, a comparison of Jane with the 1940s Nanotyrannus skull—made possible if both are considered juveniles—kills the idea that the original Nanotyrannus skull has unique features that T. rex does not. “Jane was heralded by some as the second coming of Nanotyrannus,” Carr said. But according to his analysis, the two skulls share a number of features once thought to be unique to Nanotyrannus, including a hole in a small jaw bone and a long and low snout. Rather than being diagnostic of a separate species, Carr concluded, such features actually characterize juvenile tyrannosaurs.

Some paleontologists at the meeting found Carr’s arguments convincing. The two skulls “are the same animal, the same species,” says Stephen Brusatte, a paleontologist at the University of Edinburgh in the United Kingdom. Zanno agrees. “Tom makes a compelling argument” that there was only one top predator in western North America rather than two. “His explanation is the simplest and most parsimonious.”

But Bakker is standing his ground. “Tom has not seen the best specimen,” of Nanotyrranus, he told Science. Yet details about that specimen—a third and nearly complete skeleton found in Montana in 2006—has not yet been published. It is mired in controversy after its owners tried—and failed—to sell it at auction for $7 million or more. Most paleontologists, citing the ethics of their profession, will not study it unless it is either donated to or purchased by a museum.

Until that new specimen is available for study, says Thomas Holtz, a paleontologist at the University of Maryland in College Park, “the ball is…in the Nanotyrannus court. They need an adult specimen of Nanotyrannus to pursue [their] argument.”