A giant tyrannosaur from the Campanian-Maastrichtian of southern North America and the evolution of tyrannosaurid gigantism

The study addresses the unresolved origin and timing of the Tyrannosaurus lineage (tribe Tyrannosaurini). Two competing hypotheses place the origin either in Asia with subsequent dispersal into North America or within western North America (Laramidia) with possible later dispersal to Asia. The authors re-examine a large tyrannosaurid specimen (NMMNH P-3698) from the Hall Lake Formation of New Mexico, previously assigned to Tyrannosaurus rex, to test whether it represents a distinct, earlier tyrannosaurin and to infer biogeographic origins and the evolution of gigantism in tyrannosaurids.

Prior work documents tyrannosaurid dominance in Late Cretaceous ecosystems and the late Maastrichtian appearance of T. rex as the largest apex predator. Close relatives of T. rex have been described from Asia (e.g., Tarbosaurus bataar, Zhuchengtyrannus magnus), supporting an out-of-Asia hypothesis via Beringian dispersal with later back-dispersal to Laramidia. Alternatively, some have proposed a North American endemic origin of Tyrannosaurus. The New Mexico specimen (NMMNH P-3698) was initially referred to T. rex decades ago but later questioned. Biostratigraphic and geochronologic studies in the region, including identification of the chasmosaurine Sierraceratops and occurrences of titanosaurs and hadrosaurids, refine the local faunal context and age, challenging a late Maastrichtian assignment. Previous phylogenetic frameworks for Tyrannosauridae, biogeographic models of Laramidian endemism, and debates over intraspecific versus interspecific variation within T. rex set the stage for reassessing the specimen and testing biogeographic scenarios.

The authors restudied specimen NMMNH P-3698, including newly collected cranial and mandibular elements, and conducted detailed comparative osteology against tyrannosaurids (e.g., T. rex, Tarbosaurus, Zhuchengtyrannus, Daspletosaurus, Lythronax, albertosaurines). Diagnostic characters were identified across postorbital, squamosal, dentary, splenial, angular, prearticular, and articular. Phylogenetic analyses included both parsimony and Bayesian tip-dated approaches. They expanded an existing tyrannosaurid character–taxon matrix by adding Tyrannosaurus mcraeensis, 10 new characters, and additional taxa. Total-evidence divergence-time estimation used the fossilized birth–death clock model under diversified sampling; tip ages were chosen using a diversity strategy to span internal branches. Biogeographic history was optimized on the summary tree by coding taxa into six areas (Europe, South America, Asia, Appalachia, Northern Laramidia, Southern Laramidia) with Utah/Colorado as the north–south Laramidian boundary. Multiple models were tested in RASP, with DEC+J selected as best fit, and most-likely ancestral ranges reported. Stratigraphic context was constrained by U–Pb CA-ID-TIMS zircon ages: a tuff 33 m below the site dated 73.2 ± 0.7 Ma; multiple Late Campanian dates in the underlying Jose Creek Formation (75.2 ± 1.3, 74.6 ± 0.6, 74.9 ± 0.7, 75.0 ± 1.1 Ma). Sediment thickness above the site and overlying occurrences (e.g., titanosaur 108 m above) bracket the fossil’s age. Biostratigraphic comparisons to regional dinosaur faunas further informed age and biogeography.

- New species: Tyrannosaurus mcraeensis sp. nov., from the uppermost Campanian–lower Maastrichtian Hall Lake Formation, McRae Group, near Kettle Top Butte, Sierra County, New Mexico. LSID: urn:lsid:zoobank.org:pub:F1658ACA-60DB-442E-AA04-015C050205BD. - Holotype: NMMNH P-3698, partial skull including right postorbital and squamosal, left palatine, fragmentary maxilla, and lower jaws (left dentary, right splenial, prearticular, angular, articular), isolated teeth, and associated chevrons. - Diagnosis: Large tyrannosaurin distinguished from T. rex by a suite of cranial and mandibular characters (e.g., low posteriorly positioned cornual process on postorbital; downturned quadratojugal process of squamosal; concave medial squamosal margin; strong ridge at squamosal ventral pneumatic fossa; unusually shallow, upturned posterior dentary margin; anteriorly positioned triangular splenial apex with shelf-like dentary overlap; weakly bowed prearticular; T-shaped articular; deep, quadrangular retroarticular process). - Size: Dentary measures 645 mm from tip to coronoid level (≈894–900 mm along preserved long axis). This equals or exceeds the holotype T. rex dentary measurements (589 and 855 mm) and overlaps large T. rex individuals (e.g., RSM P2523.8 ≈650 mm), indicating T. mcraeensis rivaled T. rex in size. - Tooth count: Dentary with 13 alveoli, a low count uniquely shared with T. rex; higher counts in other tyrannosaurids (e.g., Tarbosaurus 14–15, Zhuchengtyrannus 15, Daspletosaurus 17). - Age: Radiometric constraint of 73.2 ± 0.7 Ma (tuff 33 m below). Stratigraphic arguments and sediment accumulation suggest an age window of ~72.7–70.9 Ma (latest Campanian to earliest Maastrichtian), predating T. rex by approximately 5–7 million years. - Phylogeny: Recovered as sister taxon to T. rex in both parsimony and Bayesian tip-dated analyses, united by reduced dentary tooth count, deep dentary symphysis, posteriorly shallow lingual bar, large size, and additional tyrannosaurin synapomorphies. Alternative matrices still place T. mcraeensis as closest to T. rex, with Tarbosaurus and Zhuchengtyrannus as successive outgroups. - Biogeography: Ancestral-range reconstruction supports a southern Laramidian origin for Tyrannosaurini, with subsequent dispersal northward and to Asia. The Hall Lake fauna is highly endemic and includes giant herbivores (Sierraceratops, cf. Alamosaurus, large hadrosaurid), suggesting coevolutionary pressures favoring gigantism in southern Laramidia. - Macroevolution: Tyrannosaurin gigantism originated by late Campanian in southern Laramidia, preceding end-Maastrichtian marine regression; gigantism appears not driven by increased land area. Regional patterns indicate larger body sizes in southern Laramidia versus smaller northern forms (e.g., Nanuqsaurus).

The authors demonstrate that NMMNH P-3698 is morphologically distinct from all known T. rex specimens beyond intraspecific variation or ontogeny, justifying a new species, Tyrannosaurus mcraeensis. Differences are subtle but consistent across multiple cranial elements, and outside the observed range of T. rex variation. Stratigraphic, radiometric, and biostratigraphic evidence indicates a latest Campanian to earliest Maastrichtian age, making T. mcraeensis older than T. rex by ~5–7 Myr. Phylogenetic analyses place T. mcraeensis as the sister to T. rex, implying that Tyrannosaurini originated in southern Laramidia. Biogeographic reconstructions and the composition of the co-occurring fauna highlight strong Laramidian provinciality and suggest that giant tyrannosaurins evolved in the southern province before dispersing northward. Patterns of body size evolution (giant ceratopsians, hadrosaurids, titanosaurs co-occurring with giant tyrannosaurins) support a southern locus of dinosaur gigantism, seemingly independent of land-area changes associated with sea-level fluctuations. The presence of some derived features unique to T. mcraeensis indicates it was not directly ancestral to T. rex, implying at least two giant tyrannosaurids co-existed in North America prior to the origin of T. rex. Alternative placements of Asian taxa (notably the wildcard Zhuchengtyrannus) can affect inferences of geographic origin, underscoring the need for improved sampling and character data from Asian tyrannosaurids.

This study identifies and diagnoses Tyrannosaurus mcraeensis, a giant tyrannosaurin from the latest Campanian–earliest Maastrichtian of New Mexico, predating and rivaling T. rex in size. Phylogenetic and biogeographic analyses recover T. mcraeensis as the closest known relative of T. rex and support a southern Laramidian origin of Tyrannosaurini, with later dispersal northward and into Asia. The results imply that tyrannosaurin gigantism evolved earlier than previously recognized and was centered in southern Laramidia, paralleling patterns in herbivorous dinosaur clades. This refines the timeline and geography of tyrannosaurid evolution and highlights strong provincialism in Late Cretaceous North America. Future research should refine radiometric constraints on the Hall Lake Formation, expand sampling of Asian tyrannosaurids to resolve phylogenetic ambiguities, and further test drivers of dinosaur gigantism across latitudes and ecosystems.

- Publication reports no radiometric dates above the fossil horizon; the age is bracketed indirectly by underlying dates and overlying fossils, requiring additional radiometric ages and palynostratigraphy for precise placement. - Phylogenetic inference of biogeographic origins is sensitive to incomplete and problematic taxa (e.g., Zhuchengtyrannus acting as a wildcard), necessitating better sampling and character data for Asian tyrannosaurids. - Diagnostic differences from T. rex are subtle and based on a partial skull and mandible; although argued to exceed intraspecific variation, broader sampling of southern Laramidian tyrannosaurids could further test species boundaries and character polarity. - Biogeographic reconstructions rely on area codings and model assumptions (DEC+J), which may influence inferred ancestral ranges.