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| Tyrannosaurus mcraeensis Dalman, Loewen, Pyron, Jasinski, Malinzak, Lucas, Fiorillo, Currie & Longrich, 2024 Artwork by Sergei Krasinski. |
Abstract
Tyrannosaurid dinosaurs dominated as predators in the Late Cretaceous of Laurasia, culminating in the evolution of the giant Tyrannosaurus rex, both the last and largest tyrannosaurid. Where and when Tyrannosaurini (T. rex and kin) originated remains unclear. Competing hypotheses place tyrannosaurin origins in Asia, or western North America (Laramidia). We report a new tyrannosaurin, Tyrannosaurus mcraeensis, from the Campanian–Maastrichtian Hall Lake Formation of New Mexico, based on a fossil previously referred to T. rex. T. mcraeensis predates T. rex by ~ 6–7 million years, yet rivaled it in size. Phylogenetic analysis recovers T. mcraeensis as sister to T. rex and suggests Tyrannosaurini originated in southern Laramidia. Evolution of giant tyrannosaurs in southern North America, alongside giant ceratopsians, hadrosaurs, and titanosaurs suggests large-bodied dinosaurs evolved at low latitudes in North America.
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| Locality and stratigraphy of Tyrannosaurus mcraeensis sp. nov., NMMNH P-3698. (A), type locality in Sierra County, New Mexico; (B), stratigraphy of fossil and the Hall Lake Formation (C), recovered skull elements. Scale = 10 cm. Map by Ron Blakey. |
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| Mandibular elements of Tyrannosaurus mcraeensis sp. nov. (NMMNH P-3698). Left dentary in (A), medial view; (B), lateral view; (C), dorsal view; (D), right splenial, medial view; (E), right angular, medial view; (F), right prearticular, medial view. ang angular contact, de shelf for dentary, mec Meckelian canal, pre prearticular facet, a1, a5, a15 alveoli 1, 5, and 15, sp splenial, sym symphysis. Scale = 20 cm. |
Dinosauria Owen, 1842.
Theropoda Marsh, 1881.
Tetanurae Gauthier, 1986.
Coelurosauria von Huene, 1914.
Tyrannosauridae Osborn, 1905.
Tyrannosaurinae Currie, 2003.
Tyrannosaurini Olshevsky, 1995.
(Tyrannosaurini is here defined as the last common ancestor of Tarbosaurus baatar and Tyrannosaurus rex and all its descendants).
Tyrannosaurus mcraeensis sp. nov.
Diagnosis: Large tyrannosaurin distinguished from Tyrannosaurus rex (Fig. 4; SI) by the following characters (*autapomorphies): postorbital with low, posteriorly positioned cornual process; postorbital with anteriorly projecting prefrontal/frontal articular surfaces; squamosal with ventrally projecting quadratojugal process; squamosal with a concave medial margin; strong ridge bounding the anterior margin of the squamosal ventral pneumatic fossa; dentary very shallow posteriorly and with a convex posteroventral margin*; splenial with anteriorly positioned apex*; splenial with shelf-like dentary overlap*, splenial with deep, posteriorly directed angular process; prearticular weakly bowed*; small ventral prearticular-angular contact; articular T-shaped in dorsal/ventral view; retroarticular process deep and quadrangular in posterior view.
Holotype: NMMNH P-3698, partial skull including right postorbital and squamosal (Fig. 2), left palatine, fragment of maxilla, and lower jaws (Fig. 3) including left dentary, right splenial, prearticular, angular and articular, isolated teeth and associated chevrons.
Horizon and locality: Uppermost Campanian or lower Maastrichtian of the Hall Lake Formation, McRae Group, NMMNH locality 343, near Kettle Top Butte, Sierra County, New Mexico21 (Fig. 1A). The site lies 43 m above the base of the Hall Lake Formation. A tuff 33 m below the tyrannosaur site has a U/Pb age of 73.2 ± 0.7 Ma22 (Fig. 1B).
Etymology: The species name, mcraeensis, refers to the McRae Group of western New Mexico.
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| Reconstruction of Tyrannosaurus mcraeensis. Artwork by Sergei Krasinski. |
Sebastian G. Dalman, Mark A. Loewen, R. Alexander Pyron, Steven E. Jasinski, D. Edward Malinzak, Spencer G. Lucas, Anthony R. Fiorillo, Philip J. Currie and Nicholas R. Longrich. 2024. A Giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the Evolution of tyrannosaurid gigantism. Scientific Reports. 13: 22124. DOI: 10.1038/s41598-023-47011-0
