An examination of a tibia belonging to the most ancient known colossal tyrannosaur in North America indicates that the “king of dinosaurs,” Tyrannosaurus rex, might have possessed a remarkably sizable precursor.
The osseous relic, unearthed at the Kirtland Foundation in New Mexico, USA, during the 1980s, dates back approximately 74 million years, situating this immense tyrannosaur within the Late Campanian epoch.
This temporal placement predates the majority of T. rex fossil findings, which are typically associated with the twilight of the dinosaur era, specifically the late Maastrichtian period, although some overlap with the Campanian exists within the paleontological record.
Could this colossal shinbone, therefore, be the remnant of a formidable “elder sibling” within the tyrannosaur lineage?
Paleontologist Nicholas Longrich from the University of Bath, UK, alongside his research associates, posits that the fossil represents a close kin of Tyrannosaurus, a conclusion drawn from a meticulous scrutiny of its anatomical characteristics.
In addition to morphological resemblances, this tibia measures 84 percent of the length and 78 percent of the width of the tibia from the most substantial T. rex specimen documented—a nearly intact skeleton designated as Sue.
Based on these proportional measurements, Longrich and his team postulate that this tyrannosaur achieved a mass of around 4,700 kilograms (approximately 5.2 tons, or exceeding 10,000 pounds).
This weight is comparable to that of a particularly robust male African bush elephant.
While this mass is roughly half that of the largest Tyrannosaurids, it stands as the preeminent tyrannosaur discovered from this particular geological period.
Beyond its imposing dimensions, this fossil offers a compelling insight into the evolutionary trajectory of Tyrannosaurus. Some paleontological scholars have contended that T. rex originated in Asia.
Conversely, others propose a North American genesis—more specifically, the southernmost reaches of a landmass known as Laramidia, which subsequently formed a component of present-day North America.
“The hypothesis favoring southern origins yields a verifiable prediction regarding the fossil record: if Tyrannosaurus developed in southern North America, subsequent investigations of paleontological evidence should uncover further indications of early Tyrannosaurini within southern Laramidia,” Longrich and his colleagues articulate.
“Herein, we document a colossal tyrannosaur dating to approximately 74 million years ago, from the late Campanian of New Mexico. This specimen represents the most ancient known giant tyrannosaur from North America and may constitute the oldest identified member of the Tyrannosaurini.
The bone’s age has been ascertained through the analysis of argon isotopes preserved within the volcanic ash layer, designated the Hunter Wash, in which the fossil was entombed.
Furthermore, the geological strata situated both above and below this ash deposit possess well-defined radiometric dates, thus bolstering our confidence in the presence of large tyrannosaurids in Laramidia during the late Campanian epoch.
The precise phylogenetic placement of this novel dinosaur remains somewhat indeterminate, as our current understanding is derived from a solitary bone. While this is less informative than a complete skeletal or genetic profile, considerable deductions can still be made by the research team based on the bone’s morphology.
Similar to the tibia of a Tyrannosaurus, this shinbone exhibits robustness, a linear shaft, and a broad, triangular configuration at its distal extremity.
This latter characteristic is absent in Bistahieversor, another potential tyrannosaur relative with which the bone was compared; it is currently the sole confirmed tyrannosaurid discovered in this specific geological stratum at the Kirtland Foundation. Moreover, Bistahieversor was considerably smaller.
These combined features strongly suggest that this colossal dinosaur shares a more proximate evolutionary relationship with Tyrannosaurus than with Bistahieversor.
“Irrespective of the adopted hypothesis, the exceptional size of the Hunter Wash tyrannosaur holds considerable significance, as it signifies the hitherto unrecognized appearance of large tyrannosaurids during the late Campanian and demonstrates their earlier evolutionary emergence than previously surmised,” the research group states.
“Nonetheless, the acquisition of additional skeletal elements from the Kirtland Formation, whether isolated teeth, individual bones, or ideally, associated remains, is essential for more precisely defining both the dimensions and phylogenetic affiliations of the Hunter Wash tyrannosaur.”
