Tyrannosaurids, evolving from smaller ancestors, dominated Late Cretaceous Laurasian predator niches. The emergence of *Tyrannosaurus rex*, the largest and last tyrannosaurid, remains a significant puzzle, particularly concerning its origins. Competing hypotheses suggest Asian or western North American origins for Tyrannosaurini (*T. rex* and its kin). This study addresses this uncertainty by presenting a new tyrannosaurine species discovered in New Mexico's Hall Lake Formation. Understanding the evolutionary history of tyrannosaurids, especially the emergence of gigantism, is crucial for comprehending Late Cretaceous ecosystems and the factors driving dinosaur evolution. The discovery of a new species, potentially providing insight into the lineage leading to *T. rex*, necessitates a reassessment of existing hypotheses about the timing and location of tyrannosaurin origins. This new species is particularly important because it offers the potential to bridge the apparent gap in the fossil record between earlier tyrannosaurids and the abrupt appearance of *T. rex* in the latest Maastrichtian, clarifying the evolutionary history of this iconic predator and its relationship to other tyrannosaurids. This study focuses on resolving the phylogenetic position of the new species within the Tyrannosauridae family and exploring the biogeographic implications of its discovery. Additionally, it examines the broader evolutionary context of tyrannosaurid gigantism, including the role of environmental factors and interactions with other megafauna.

Previous research on tyrannosaurid origins focused on two main hypotheses: an Asian origin, supported by the presence of closely related species like *Tarbosaurus bataar* and *Zhuchengtyrannus magnus* in Asia, and a North American origin, arguing for an endemic Laramidian lineage. The discovery of earlier Asian taxa posed a challenge to the North American origin hypothesis, leading to theories involving dispersal between Asia and North America via Beringia. However, the relatively sudden appearance of *T. rex* in the late Maastrichtian without close relatives in the preceding North American fossil record leaves significant questions unanswered. Studies on tyrannosaurid phylogeny, biogeography, and body size evolution have been conducted, but there's been a lack of crucial data from the late Campanian and early Maastrichtian of southern Laramidia. The scant fossil evidence from this period limited the ability to fully assess the evolutionary trajectories leading to *T. rex*. Existing studies often focused on later Maastrichtian faunas and lacked the fine-grained temporal resolution needed to track the evolution of gigantism within tyrannosaurids. Earlier studies, while important for establishing foundational knowledge about tyrannosaurid anatomy and relationships, were constrained by incomplete fossil records and lacked the advanced phylogenetic and geochronological tools available today. The present study incorporates recent advances in these fields to address lingering uncertainties.

The research began with the re-examination of NMMNH P-3698, a specimen initially identified as *T. rex*. New skeletal elements were discovered and analyzed, leading to the recognition of a distinct species. This involved detailed morphological comparisons with existing tyrannosaurid specimens, focusing on subtle differences in skull bone shapes and articulations to differentiate *T. mcraeensis* from *T. rex*. High-resolution images, including photographs and computed tomography (CT) scans, were crucial for analyzing bone structure. Phylogenetic analyses were conducted using two approaches: a parsimony-based analysis and a Bayesian tip-dated analysis. These analyses incorporated existing character-taxon matrices, augmented with new characters identified in *T. mcraeensis*. The data included morphological characters from the postorbital, squamosal, dentary, splenial, prearticular, and articular bones. A total-evidence approach was used in the Bayesian tip-dated analysis, combining morphological characters with fossil ages. This total-evidence analysis also used a fossilized birth-death model in conjunction with a Bayesian MCMC framework to estimate divergence times. Ancestral range reconstruction was conducted to infer the biogeographic history of tyrannosaurins. Multiple biogeographic models were assessed using RASP software, and the best-fitting model (DEC+J) was selected to estimate ancestral ranges and diversification patterns. Radiometric dating of volcanic tuffs in the Hall Lake Formation provided age constraints for the fossil. The age determination for *T. mcraeensis* was also informed by the age of associated fossils and the stratigraphic context. The researchers used a “diversity” strategy for selecting fossil age estimates from sampled taxa to improve the robustness of divergence time estimates. The data were optimized using RASP software and utilized a DEC+J model to analyze ancestral range (Fig. 5B). Comparing the morphology of *T. mcraeensis* to existing *T. rex* specimens helped determine if the differences are diagnosable or due to ontogenetic or individual variation within the same species. Statistical methods may have been used to assess the significance of observed differences between species.

The research definitively established *Tyrannosaurus mcraeensis* as a new species distinct from *T. rex*. The key differentiators were subtle yet consistent morphological differences across multiple cranial bones, including the postorbital, squamosal, dentary, splenial, prearticular, and articular. These differences were found to be outside the range of natural variation observed in *T. rex* specimens. Phylogenetic analysis consistently placed *T. mcraeensis* as the closest known relative of *T. rex*, supporting a sister group relationship. The age of *T. mcraeensis* was estimated to be approximately 73.2 ± 0.7 Ma based on radiometric dating and stratigraphic context. This indicates that *T. mcraeensis* predates *T. rex* by around 6-7 million years. The size of *T. mcraeensis* was comparable to *T. rex*, suggesting that gigantism in Tyrannosaurini arose earlier than previously thought. The biogeographic analysis indicated that Tyrannosaurini originated in southern Laramidia, with *T. mcraeensis* belonging to a distinct southern fauna, including giant ceratopsians, hadrosaurs, and titanosaurs. This southern Laramidian fauna showed little overlap with the contemporary fauna found in northern Laramidia, highlighting high endemicity in the southern region. The analysis suggests that gigantism in tyrannosaurids may not be directly tied to increasing land area or sea-level changes; rather, other factors were responsible for the evolution of giant theropods within geographically limited ranges. The coexistence of *T. mcraeensis* and other giant herbivores in southern Laramidia suggests a possible ecological link between predator and prey body size evolution.

The discovery of *T. mcraeensis* significantly alters our understanding of tyrannosaurid evolution and biogeography. The species' age and close phylogenetic relationship to *T. rex* push back the timeline of tyrannosaurin gigantism to the late Campanian. This finding challenges previous hypotheses that emphasized a later origin in the Maastrichtian or an Asian origin for the lineage. The distinct southern Laramidian fauna to which *T. mcraeensis* belonged highlights regional endemism and supports the hypothesis of a southern origin for Tyrannosaurini. The study suggests that the evolution of gigantism in tyrannosaurids may be more complex than previously thought, not solely determined by landmass size or sea-level fluctuations. Further research is needed to explore other factors driving gigantism, possibly including climate change or changes in the diversity or abundance of potential prey species. This finding necessitates reevaluation of previous biogeographic hypotheses concerning tyrannosaurid dispersal and speciation. Future research should focus on improving the fossil record of the late Campanian-early Maastrichtian, particularly in southern Laramidia, to refine our understanding of the evolutionary relationships and biogeographic patterns in tyrannosaurids.

This paper establishes *Tyrannosaurus mcraeensis* as a new species, profoundly impacting our understanding of tyrannosaurid evolution. The discovery pushes back the origin of tyrannosaurin gigantism to the late Campanian, refutes previous hypotheses, and highlights southern Laramidia as the origin of Tyrannosaurini. The study demonstrates that gigantism in tyrannosaurids was likely influenced by factors beyond landmass size or sea-level changes, underscoring the need for further investigation. Future research should focus on expanding the fossil record of the region and refining phylogenetic analysis to enhance our understanding of tyrannosaurid evolution and biogeography.

While the study presents compelling evidence for *T. mcraeensis* as a distinct species, the analysis is based on a single, albeit relatively complete, specimen. The discovery of additional fossils would greatly strengthen the conclusions. Additionally, the radiometric dating is reliant on a limited number of samples. Obtaining additional radiometric dates from the Hall Lake Formation could provide more precise age estimates. The current phylogenetic analyses are limited by the completeness of the fossil record and the availability of relevant data for comparison. Future research incorporating additional fossil discoveries, more comprehensive character sets, and potentially advanced analytical methods could further refine the results and test their robustness.