A fossil tyrannosaur pulled from the badlands of southern New Mexico has forced paleontologists to rethink when giant predatory dinosaurs first reached extreme body sizes in North America. The specimen, formally named Tyrannosaurus mcraeensis and cataloged as NMMNH P-3698, lived roughly 6 to 7 million years before the famous Tyrannosaurus rex, yet it appears to have been just as large. That timeline gap challenges the long-held assumption that apex tyrannosaur size was a late evolutionary development, confined to the final stretch of the Cretaceous period.
A New Species From New Mexico’s Deep Past
The fossil comes from the Hall Lake Formation in south-central New Mexico, a geological unit that has received far less attention than the Hell Creek and Lance formations of Montana and Wyoming where most T. rex material has been recovered. Researchers described the new tyrannosaur species in Scientific Reports, diagnosing it on the basis of anatomical features in the skull that distinguish it from its younger relative. The specimen includes portions of the lower jaw, braincase, and other cranial elements, enough to establish both its identity and its remarkable size.
What makes the dating especially persuasive is the radiometric evidence. A volcanic ash layer, or tuff, sitting below the fossil horizon returned a U/Pb radiometric age of 73.2 plus or minus 0.7 million years. That places T. mcraeensis firmly in the Campanian stage of the Late Cretaceous, well before the Maastrichtian interval when T. rex dominated western North America. The precision of that date is significant because it rules out the possibility that this animal was simply an early T. rex population; it lived in a distinctly older geological window.
The Hall Lake Formation itself has emerged as an important but still underexplored archive of Late Cretaceous life in southern Laramidia. Unlike the better-known northern formations, its fossil content suggests a somewhat different mix of dinosaurs and other vertebrates, hinting at regional endemism along the western shoreline of the interior seaway. In that context, the presence of a giant tyrannosaur underscores how little is yet known about predator evolution in these southern ecosystems.
Rivaling the Biggest T. Rex on Record
Size comparisons in paleontology always carry caveats because most dinosaur skeletons are incomplete, and body-mass estimates depend on which bones survive and which scaling equations researchers apply. Still, the dimensions of NMMNH P-3698 place it in the same weight class as the largest known T. rex individuals. The most famous of those is a specimen nicknamed “Scotty,” cataloged as RSM P2523.8, which was described as an exceptionally massive adult in a peer-reviewed study published in The Anatomical Record. A corresponding press announcement from the publisher confirmed the “largest” designation at the time of that paper’s release.
The fact that a tyrannosaur living millions of years earlier could match Scotty’s proportions is the core surprise. Conventional thinking held that tyrannosaurids grew progressively larger over geological time, reaching their peak only in the final 2 to 3 million years before the end-Cretaceous extinction event 66 million years ago. T. mcraeensis breaks that neat narrative. If an animal of this size already existed around 73 million years ago, then the evolutionary pressure to become enormous was operating much earlier than most researchers had assumed.
To estimate the body size of NMMNH P-3698, scientists compared preserved skull elements with corresponding bones in well-known T. rex specimens. The proportions of the lower jaw and braincase, when scaled up, imply an overall skull length and body mass on par with the very largest tyrannosaurs. Although the skeleton is far from complete, the preserved pieces are precisely the ones most useful for gauging size, because skull dimensions in tyrannosaurids correlate closely with overall body length and weight.
What Southern Fossils Reveal About Northern Bias
One reason this discovery is so disruptive is geographic. The bulk of well-studied tyrannosaur material comes from northern formations in Montana, Alberta, and Saskatchewan. Southern North America, particularly New Mexico and Texas, has been sampled less intensively, partly because exposures of the right-aged rock are smaller and harder to access. That sampling gap has skewed the evolutionary picture. When scientists built family trees of tyrannosaurids, the data was dominated by northern species, and the resulting models naturally suggested that gigantism tracked the northern lineage leading to T. rex.
A 2019 study in Nature Ecology and Evolution developed a widely used evolutionary framework for tyrannosauroids, tracing how these predators diversified and moved across ancient North America. That work highlighted a complex biogeographic history, with lineages dispersing and sometimes becoming isolated in different parts of Laramidia. T. mcraeensis slots into that broader picture by demonstrating that southern populations were not evolutionary backwaters. They were producing apex predators of extraordinary size on their own schedule, possibly driven by different prey bases and ecological conditions than their northern cousins.
The new species also dovetails with earlier research showing that Late Cretaceous dinosaur faunas were strongly regional. Studies of Campanian-age communities have documented distinct northern and southern assemblages, suggesting that climate gradients, sea levels, and mountain-building all helped partition habitats. In such a patchwork landscape, it is plausible that giant tyrannosaurs evolved independently in more than one area, rather than following a single, linear path toward gigantism.
Early Gigantism and Its Ecological Implications
Why would a tyrannosaur become enormous so early? The answer likely involves the ecosystems of Campanian-age southern Laramidia, the western landmass created when an interior seaway split North America in two. Southern Laramidia supported a different suite of herbivorous dinosaurs than the north, including large ceratopsians and hadrosaurs whose own body sizes may have created selective pressure for bigger predators. If prey animals were already large and abundant, then a tyrannosaur lineage that could grow to 9 or 10 tons would have had a clear competitive advantage.
This ecological reasoning is speculative but consistent with what the cranial anatomy of NMMNH P-3698 reveals. The skull elements indicate an animal built for the same bone-crushing bite mechanics that made T. rex the dominant predator of its time. Jaw structure, tooth placement, and braincase morphology all point to a creature that occupied the same ecological role, just millions of years earlier and hundreds of miles farther south.
The implication is that giant tyrannosaurs may have evolved their extreme body sizes under a broader set of environmental conditions than previously recognized. Rather than being a late, northern innovation tied only to the very end of the Cretaceous, gigantism appears to have been an option that tyrannosaur lineages could exploit whenever ecosystems offered sufficient resources and ecological space. That flexibility may help explain why tyrannosaurids ultimately became the top predators across much of western North America.
Other lines of research support the idea that dinosaur communities were more dynamic and regionally varied than once thought. Work on Late Cretaceous biodiversity has shown that species turnover and local extinctions were common, with different regions experiencing their own evolutionary trajectories long before the final asteroid impact. A landmark analysis of dinosaur diversity patterns, published in Science and accessible via a widely cited DOI record, argued that many groups were undergoing complex changes in abundance and distribution in the last 10 million years of the Cretaceous. The emergence of an early giant like T. mcraeensis fits comfortably within that picture of shifting, regionally distinct ecosystems.
Rewriting the Tyrannosaur Timeline
Together, the anatomical, geological, and biogeographic evidence from New Mexico forces a revision of the standard tyrannosaur story. Instead of a gradual, north-centered march toward ever-larger body sizes culminating in T. rex, the new fossil suggests a more branching, experimental pattern. In this revised view, multiple tyrannosaur lineages may have flirted with gigantism at different times and in different places, with only some of those experiments leaving a robust fossil record.
For now, T. mcraeensis stands as a reminder of how incomplete that record remains. A few bones eroding from a remote outcrop have been enough to shift the timeline of tyrannosaur evolution by several million years and to highlight the importance of underexplored regions like southern Laramidia. As more fossils emerge from these areas, paleontologists expect further surprises, perhaps even older or differently specialized giant tyrannosaurs that will complicate the story again.
What is clear already is that the path to apex predation in the Late Cretaceous was neither simple nor confined to one corner of the continent. With each new discovery, the tyrannosaur family tree looks less like a straight ladder leading to T. rex and more like a tangled, regionally branching network of predators adapting in parallel to a rapidly changing world.
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*This article was researched with the help of AI, with human editors creating the final content.
