It dates back 92 million years, about 20 million years before the T. rex stomped the Earth.
The paper details how the dinosaur called Suskityrannus hazelae provided missing clues about how T. rex evolved into the massive predator that ruled the later Cretaceous period.
The new dinosaur, named Suskityrannus hazelae, was a tiny relative of Tyrannosaurus rex, about 9 feet (2.7 m) long and 3 feet (0.9 m) tall at the hip. The typical weight for a full-grown Tyrannosaurus rex is roughly 9 tons.
However, despite obvious differences, its diet likely consisted of the same as its larger meat-eating counterpart, with Suskityrannus hazelae likely hunting small animals, although what it hunted is unknown.
"Suskityrannus gives us a glimpse into the evolution of tyrannosaurs just before they take over the planet", said Sterling Nesbitt, study author and assistant professor in the Virginia Tech College of Science's Department of Geosciences, in a statement. That discovery led to a lengthy investigation, and Nesbitt - now a Virginia Tech paleontologist - is part of a team publishing the findings in the journal Nature Ecology and Evolution. What makes this new dinosaur special, however, is that it's more closely related to T. rex and other tyrannosaurids than to other early Cretaceous tyrannosauroids.
Nesbitt found the remains of this incredible dinosaur when he was a pipsqueak himself - just a 16-year-old high-school student who was on a dig expedition in western New Mexico in May 1998.
In fact, the first partial skull of the mini tyrannosaur was found in 1997 by Robert Denton, now a senior geologist with Terracon Consultants, an engineering consulting firm in New Jersey.
This illustration shows a Suskityrannus hazelae, foreground, a cousin of the Tyrannosaurus rex. It was only until other small tyrannosauroids were discovered that they began re-examining this important find, which Smithsonian Institution paleobiologist Hans Sues believes is "the first really good record of the early tyrannosaurs in North America".
The name Suskityrannus hazelae is derived from "Suski", the Zuni Native American tribe word for "coyote", and from the Latin word "tyrannus" meaning king and "hazelae" for Hazel Wolfe, whose support made possible many successful fossil expeditions in the Zuni Basin.
Armita Manafzadeh, a PhD candidate from Brown University who is not affiliated with the study, is particularly interested in the Suskityrannus' arctometatarsalian foot where the three long bones of the foot's sole are pinched together.
The specimen Nesbitt found is so complete, it's helping researchers learn about this wee tyrannosaur, which predates T. rex by about 25 million years. "Clearly the most complete individual skeleton we had found in the entire basin and we had not even started to collect it".
At the time, experts thought he might have found a dromaeosaur such as a Velociraptor.
The Arizona Museum of Natural History will house a replica of the dinosaur. Dr Nesbitt then took them with him through all his postings as student and researcher.
Nesbitt's find marked the second specimen.